What supplements should I take while on a calorie deficit?

Reviewed and updated: 20 October 2026

If you are wondering what supplements to take on a calorie deficit, the honest answer is that you are not shopping for a fat-loss pill, you are covering the nutrients your smaller plate no longer delivers. Cut your food intake by a third and you cut roughly a third of your vitamins and minerals with it, and some of them run short far faster than others. I am Dr. Miron, and I founded Pure Vitamins UK. This is the question I am asked more than almost any other, usually by someone whose scales are moving in the right direction while they themselves feel steadily worse.

So this guide is about the gaps. Which nutrients thin out first when the volume of food comes down, what the UK actually recommends you take in, how much is too much, how to read a label so you can tell a real dose from a decorative one, which of the popular fat-burner ingredients are worth your money and which are not, and what to do when your appetite has all but disappeared and simply eating more is not realistically on the table.

What Supplements to Take on a Calorie Deficit: The Short Answer

• Start with the four that thin out fastest: vitamin B12, iron, magnesium and vitamin D.
• Protein is a nutrient priority, not an optional extra. Protein contributes to the maintenance of muscle mass, and a deficit is exactly when that matters.
• Food first, always. Fortifying a small plate so each mouthful carries more will do more for you than most of what is sold in this category.
• Do not spend your budget on fat burners. The best current systematic review of the category found only a small minority of trials showed a meaningful weight difference, and the differences were mostly under a kilogram.
• A sensible multivitamin is a safety net, not a substitute for food. It is there for the days your intake is genuinely low.
• Learn the NRV column. It is the fastest way to tell whether an ingredient is on the label for you or for the marketing.
• Who this is for: anyone eating meaningfully less than usual, whether by choice, by circumstance, or because appetite has dropped away.
• The one honest caution: supplements cover shortfalls. They do not make an inadequate diet adequate, and persistent fatigue deserves a blood test rather than a bigger tub of tablets.

Illustrative image

Why eating much less creates nutrient gaps

Calories and micronutrients travel together. Your body has a daily requirement for iron, B12, magnesium and the rest that barely changes when you eat less, but the food carrying those nutrients has shrunk. That is the whole mechanism, and it is why the problem is arithmetic rather than bad luck.

It has been measured. Researchers took 21 suggested menus from three commercial diet plans and ran the nutrient analysis on them. Vitamin D came out inadequate in every single plan. B12, selenium and zinc were low on the reduced-calorie vegan menus, and magnesium and thiamine were low on the low-carbohydrate ones (Engel and colleagues, 2018, Nutrients). An earlier analysis of four popular plans had found much the same pattern across 27 micronutrients.

Read those as a map of where the holes tend to appear rather than as a diagnosis of you personally, because they are analyses of menus on paper rather than measurements of blood levels in real people. But the pattern is consistent enough to plan around.

Speed matters too. When gradual and rapid weight loss have been compared for the same total weight lost, the gradual approach produced more fat loss and better preservation of resting metabolic rate. Worth being honest about the limits of that work: it found no significant difference in fat-free mass between the two approaches, so the popular claim that slow dieting automatically protects muscle was not borne out.

The nutrients that run low first

Across the research and across the questions that land in my inbox, the same short list comes up again and again:

• Iron. The most common nutritional shortfall in the UK, and it falls disproportionately on menstruating women.
• Vitamin B12. Concentrated in animal foods, so it drops quickly if meat, fish, eggs and dairy are the first things cut.
• Magnesium. Spread thinly across wholegrains, nuts, seeds and leafy greens, which are often the volume foods that go first.
• Vitamin D. Barely available from British food at all, and from October to March barely available from British sunlight either.
• Calcium. Follows dairy down, and dairy is one of the first categories people cut when they are counting.
• Iodine. The quietest one on the list, and the one almost no guide mentions. It arrives in the British diet mainly through milk, dairy and white fish, so cutting dairy hard takes iodine with it.
• Omega-3 fats. Most of us were not hitting two portions of fish a week before the deficit started.
• Zinc. Follows protein foods down.
• Vitamin C and folate. Follow fresh fruit and vegetables down.

Three symptoms send people looking for answers more than any others: persistent tiredness that sleep does not fix, hair that feels thinner or sheds more in the shower, and a general sense of being cold, foggy or flat. None of those is specific to one nutrient, which is exactly why a blood test beats guesswork.

What are the best vitamins to take while dieting?

If I had to rank them for someone eating meaningfully less than usual, I would go in this order, and I would base the order on two things: how fast the nutrient runs short, and whether topping it up does something you can actually feel.

Vitamin B12 and iron come first because both are directly involved in getting oxygen around your body and both carry authorised UK function claims for the reduction of tiredness and fatigue. Magnesium comes next for muscle and nervous system function. Vitamin D comes next because the UK gap is seasonal and near universal. Everything after that is genuinely a case of covering bases rather than fixing something urgent.

Vitamin B12 when your intake drops

The UK reference nutrient intake for B12 is 1.5 micrograms a day, which sounds tiny until you notice it comes almost entirely from animal foods. Cut those hard and the supply stops quickly, though the liver holds a reserve that can mask the change for months.

Under GB rules, vitamin B12 contributes to normal energy-yielding metabolism, contributes to the reduction of tiredness and fatigue, contributes to normal red blood cell formation and contributes to normal function of the nervous system. Those function claims apply where B12 is present at 15 percent or more of the nutrient reference value.

How real is the shortfall in people who cut animal foods? Pooled research across seventeen studies found vegans had significantly lower serum B12 and significantly raised homocysteine compared with omnivores, and that supplementing normalised those markers. The honest caveat is that the two most specific functional markers, methylmalonic acid and holotranscobalamin, did not reach statistical significance. If you want the fuller picture on forms and absorption, our vitamin B12 guide goes into the detail, and our vitamin B12 supplement uses methylcobalamin with adenosylcobalamin.

Iron when you are eating less

The UK reference nutrient intake is 14.8 mg a day for women aged 19 to 49 and 8.7 mg for men and post-menopausal women. Meeting 14.8 mg from food is not trivial at a normal intake, and it becomes considerably harder on a reduced one, particularly if red meat is among the first things to go.

Iron contributes to normal formation of red blood cells and haemoglobin, contributes to normal oxygen transport in the body and contributes to the reduction of tiredness and fatigue. Vitamin C, usefully, increases iron absorption, which is why the two are so often formulated together.

There is a decent trial here worth knowing about. In 198 non-anaemic menstruating women with ferritin below 50 micrograms per litre and unexplained fatigue, twelve weeks of oral iron reduced fatigue scores by 47.7 percent against 28.8 percent on placebo. Being straight about the limits: there was no significant benefit for quality of life, mood or anxiety, and fatigue was self-reported (Vaucher and colleagues, 2012, CMAJ). We cover the label numbers in is 14mg of iron a day enough and the stores question in how to increase ferritin. Our iron tablets pair ferrous fumarate with vitamin C, B12 and folic acid.

Illustrative image: nutrient-dense foods

Magnesium and vitamin D

Magnesium is the quiet one. The UK reference nutrient intake is 300 mg a day for men and 270 mg for women, and it is spread thinly across wholegrains, nuts, seeds, beans and dark leafy greens. Those are bulky foods, and bulky foods are what disappear when portions shrink.

Magnesium contributes to normal muscle function, contributes to normal functioning of the nervous system, contributes to electrolyte balance and contributes to the reduction of tiredness and fatigue. If you have had night cramps or twitchy eyelids since cutting your intake, that combination is usually where people start looking.

One caution on how magnesium status is judged. A normal serum magnesium reading does not rule out a genuine shortfall, because the reference range in common use was derived from a population distribution in the 1970s rather than from clinical outcomes, and roughly half the adult population takes in less than the estimated average requirement. The fair reading of that is that magnesium status is difficult to measure, not that everyone is deficient.

Vitamin D is the simplest of the lot. NHS advice is that everyone should consider a daily supplement containing 10 micrograms of vitamin D during autumn and winter, because UK sunlight between October and March is too weak for your skin to make it, and food contributes very little. Vitamin D contributes to the maintenance of normal bones, contributes to normal absorption and utilisation of calcium, contributes to the maintenance of normal muscle function and contributes to the normal function of the immune system. Our vitamin D3 and K2 tablets pair D3 with MK-7, and if you are choosing a magnesium form, glycinate or bisglycinate clears up the most common labelling confusion. Our magnesium glycinate complex uses three bioavailable forms. If you want the full form comparison in the context of a smaller plate, which magnesium is best when you're eating less weighs glycinate, citrate, malate and oxide against each other on absorption, gut comfort and elemental dose.

Electrolytes and hydration when your intake drops

This is the section almost nobody writes and almost everybody needs, because the first week of eating much less is the week people feel worst, and it is usually fluid and electrolytes rather than calories doing it.

Here is what actually happens. A meaningful chunk of your daily fluid arrives in food rather than in a glass, so a smaller plate quietly means less water. At the same time, when carbohydrate intake drops, your body uses up its stored glycogen, and glycogen holds water with it, roughly three grams of water for every gram of glycogen. That water leaves, and sodium leaves with it. That is the whoosh on the scales in week one, and it is also why the headache, the light-headedness on standing, the flat legs in the gym and the calf cramps in bed all cluster in the same few days.

The four that matter, in the order they usually cause trouble:

• Sodium. The one people wrongly slash to zero at the same time as everything else. If you have cut processed foods hard and you now feel dizzy standing up, adding a little salt back to your cooking is more likely to help than another supplement. UK advice is to keep total salt to no more than 6 g a day, which is about a level teaspoon, and that is a ceiling, not a target to fall far below without thinking.
• Potassium. Comes from the fruit, vegetables, potatoes, beans and dairy that shrink with the plate. Food is the sensible route here, and I would be cautious with high-dose potassium supplements, which are restricted for good reason.
• Magnesium. Covered above, and the one most associated with night cramps.
• Fluid itself. If a third of your food volume has gone, a couple of extra glasses of water a day is not fussiness, it is replacement. Spread it across the day rather than drinking it in two large hits, and keep most of it away from mealtimes if appetite is already low.

A practical note on electrolyte sachets. Most of them are useful mainly for the sodium and are otherwise expensive flavoured water, so read the label before assuming the price reflects the content. If you are getting through more than one a day, my guide to whether two electrolyte drinks a day is too much works through the sodium arithmetic on a UK label, and if you are trying to decide when in the day to have them, are electrolytes better morning or night works through the timing. If you have not yet decided whether you need them at all, should I take electrolytes during weight loss takes that question first, and if you are standing in the aisle deciding which one to buy, what makes for the healthiest electrolyte drink works through the four label numbers that decide it. If the question is which of the minerals themselves matter most once your plate has shrunk, the best electrolytes to take when you're eating much less goes through all four in turn. And if you feel faint, notice a racing heart, or get cramps that are severe or persistent rather than the ordinary night-time twinge, that is a GP conversation rather than a shopping decision.

Calcium, iodine and omega-3, the three that get quietly forgotten

Calcium. The UK reference nutrient intake for adults is 700 mg a day, and most of it arrives from dairy, fortified plant drinks, tinned fish with the bones in, and leafy greens. Dairy is very often among the first things cut, which makes calcium one of the easiest nutrients to lose without noticing. Calcium contributes to normal muscle function, contributes to normal neurotransmission and is needed for the maintenance of normal bones. There is a second reason to care in this context: weight loss itself is associated with some loss of bone mineral, particularly when it is fast, so the period when you are eating least is not the period to be casually short of calcium and vitamin D. Two things worth knowing practically. Calcium absorbs better in split doses of around 500 mg than in one large hit, and it competes with iron, so if you take both, keep them a couple of hours apart.

Iodine. This is the nutrient nobody writes about and I think that is a mistake, because in Britain iodine arrives overwhelmingly through milk, yoghurt, cheese and white fish. Cut dairy sharply, or swap to a plant milk that has not been fortified, and iodine goes with it almost silently. The UK reference nutrient intake for adults is 140 micrograms a day. Iodine contributes to normal thyroid function, contributes to the normal production of thyroid hormones, contributes to normal energy-yielding metabolism and contributes to normal functioning of the nervous system. Two practical points. Most unfortified plant milks contain very little iodine, so check the back of the carton rather than assuming the swap was like for like. And iodine is one where more is genuinely not better: high intakes can disturb thyroid function just as low ones can, so treat kelp and seaweed supplements with real caution, since their iodine content varies enormously and can be far above anything you need. If you are pregnant, planning a pregnancy, or have a known thyroid condition, this is a GP conversation rather than a shelf decision.

Omega-3. UK advice is two portions of fish a week, one of them oily, and national survey data has consistently shown most adults nowhere near that even before a deficit narrows the diet further. The authorised wording is specific and worth quoting properly: EPA and DHA contribute to the normal function of the heart at 250 mg a day, and DHA contributes to the maintenance of normal brain function and normal vision at the same daily amount. Omega-3 is also a fat-soluble nutrient in a diet that has probably reduced its fat, which is a second reason it slides. If you eat no fish at all, an algal oil covers DHA without the fish, and our guides on EPA versus DHA and how much you need and omega-3 for heart and brain health set out the numbers.

Protein and holding on to muscle

This is the part people underrate, and it is the part with the strongest evidence behind it. The UK reference nutrient intake for protein is about 0.75 g per kg of body weight a day, which is a floor for an average adult in energy balance, not a target for someone in a deficit.

Protein contributes to the maintenance of muscle mass and contributes to a growth in muscle mass. Those are authorised claims, and they are exactly the outcome at stake when your intake drops. If you are weighing up a collagen powder, I have looked at whether collagen is worth taking when you're eating much less separately.

In a controlled trial, young men on a roughly 40 percent deficit training six days a week gained 1.2 kg of lean mass on 2.4 g/kg/day of protein against 0.1 kg on 1.2 g/kg/day. The honest footnote is that performance improved equally in both groups, and it was a four-week supervised study in 40 men, not real life (Longland and colleagues, 2016, American Journal of Clinical Nutrition). Pooled work across 29 studies points the same way, finding a strong probability of a dose-response relationship between higher daily protein and more favourable fat-free mass change during energy restriction, though those authors labelled their own findings exploratory because the studies varied so much.

Here is the arithmetic in everyday terms, because grams per kilogram means nothing until you put food against it. A 70 kg adult aiming for roughly 1.6 g/kg needs about 112 g of protein a day. Spread across four eating occasions that is around 28 g each, which looks like: a 150 g pot of Greek yoghurt with a tablespoon of peanut butter, a tin of tuna, a 120 g chicken breast, or 200 g of cottage cheese. Hitting that on a small appetite is the real challenge, which is why the density trick below matters so much.

The practical version: prioritise protein at every meal, and pair the deficit with resistance training if you can. In older adults with obesity, resistance training during calorie restriction retained the great majority of the lean mass that would otherwise have been lost. If you want the nutrient side of that picture in detail, which vitamins are involved in holding on to muscle goes through vitamin D, magnesium, iron and B12 one at a time. If a shake is the easier route to your protein on a small appetite, our guide to protein shakes when you are eating much less covers the formats and how to read the label. If you are rebuilding after weight loss, our guide on how to regain muscle mass after weight loss covers timelines, training and protein per meal.

Food first: how to fortify a small plate

Before any of this becomes a shopping list, there is a step that costs almost nothing and that most guides skip entirely. If your plate has got smaller, the answer is not only to add tablets, it is to make each mouthful carry more. Dietitians call this food fortification, and it is the standard first-line approach in the UK precisely because it works without a single supplement.

What it looks like in an ordinary kitchen:

• Fortified milk. Whisk four tablespoons of dried skimmed milk powder into a pint of whole milk and keep it in the fridge. Use it on cereal, in tea, in sauces and in soups. It roughly doubles the protein and calcium of the milk without adding any volume you have to get through.
• Grate cheese into things. Into mashed potato, scrambled eggs, soup, beans on toast. A 30 g handful is about 7 g of protein and a useful slug of calcium.
• A spoonful of oil or butter stirred into vegetables or potatoes at the end. This one matters twice over, because vitamins A, D, E and K need some fat present to be absorbed properly, and a very low-fat small plate quietly undermines the very supplements you have just bought.
• Nut butters and ground nuts. Into porridge, yoghurt, smoothies, or simply on toast. Dense in calories, protein, magnesium and zinc all at once.
• Drink your nutrition when chewing is the barrier. Milk, a yoghurt-and-banana smoothie with a spoon of nut butter, or a homemade milkshake will go down on days when a plate will not.
• Eggs, tinned fish and full-fat dairy are the highest-return foods on a small plate. Tinned sardines in particular bring protein, omega-3, calcium from the bones and vitamin D in one tin.
• Keep drinks away from mealtimes so you are not filling up on water or tea before you have eaten.

My honest view is that if you do only two things from this whole guide, make it the fortified milk and the spoon of oil. They cost pennies and they raise the nutritional floor of every meal you eat.

Fibre, digestive comfort and glucomannan

Fibre is worth protecting when portions shrink, both for digestive comfort and because it slows a meal down. The UK recommendation is 30 g a day for adults, and most of us do not reach it even at a normal intake. If you are also wondering about a live culture, I have written about what to look for in a probiotic when you're eating much less.

The digestive side of eating much less deserves more attention than it gets. Two things tend to happen at once. Less food and less fluid means less material moving through, so constipation is common in the first few weeks and is usually a fluid and fibre problem rather than anything more complicated. At the same time, some people find that when they do eat, they feel uncomfortably full very quickly, which makes the fibre advice harder to follow. The way through that is quality rather than quantity: oats, ground linseed, beans stirred into an existing dish, and fruit with the skin on, all of which bring fibre without much volume. Increase it gradually, and increase your fluid at the same time, because fibre without fluid makes constipation worse rather than better.

Glucomannan comes up constantly in this context, so here is the balanced view. It is a soluble fibre from konjac root, and GB rules do permit a weight-related claim for it, but only at 3 g a day taken in three 1 g doses with water before meals as part of an energy-restricted diet. That condition is not optional, and the wording attaches to glucomannan specifically, not to fibre in general.

What the trials show is less flattering than the marketing. Pooled across eight randomised trials, glucomannan produced no statistically significant weight difference against placebo, a mean difference of about minus 0.22 kg with a confidence interval crossing zero, substantial variation between studies, and reported side effects of abdominal discomfort, diarrhoea and constipation. An eight-week randomised trial similarly found no effect on weight, body composition, or even on ratings of hunger and fullness. We do not include glucomannan in our range, and that evidence is why. Protein, vegetables and whole foods do the satiety job more reliably.

A realistic look at fat burners and why most do not work

This is where most of the money in this category goes, and it is where I would spend none of yours. Let me be specific rather than vague about it.

The most thorough recent assessment screened over 20,000 citations, reviewed 1,743 full papers and ended up with 315 randomised controlled trials covering 14 categories of supplement or alternative therapy marketed for weight loss. Only 52 of those 315 trials, about one in six, were judged low risk of bias with enough data to support a conclusion at all. Of those 52, just 16 showed a significant difference in weight between groups, and the range of those differences was 0.3 to 4.93 kg. The authors concluded that the category has a limited high-quality evidence base (Batsis and colleagues, 2021, Obesity). Note what that means in practice: even among the trials that worked, the typical result sits at the bottom of that range, and the whole thing rests on a small fraction of a large literature.

Ingredient by ingredient, here is the honest ledger:

• Green tea extract. A Cochrane review of randomised trials of at least twelve weeks concluded that green tea preparations produce a small, statistically non-significant weight loss in overweight or obese adults, too small to be clinically important, and no significant effect on keeping weight off afterwards (Jurgens and colleagues, 2012, Cochrane Database of Systematic Reviews). There is also a safety point that gets buried under the marketing: concentrated green tea catechin extracts, as opposed to brewed tea, have been associated with rare liver problems, which is why European assessments have flagged high daily EGCG intakes from supplements. Brewed green tea is a pleasant drink. A high-dose extract is a different proposition.
• Caffeine. The one with genuine, measurable pharmacology behind it. It does modestly raise energy expenditure and it does reduce how hard exercise feels, which is a real and useful thing. What it does not do is produce meaningful fat loss on its own, and tolerance to the metabolic effect builds. My honest view is that caffeine is worth having as coffee before a training session and is not worth buying as a fat-loss tablet. Keep the total under 400 mg a day, which is roughly four mugs of coffee, and lower if you are pregnant.
• Garcinia cambogia and hydroxycitric acid. Repeatedly tested, and the pooled effects are small and inconsistent enough that most reviewers have concluded the case is not made.
• Conjugated linoleic acid. Long-running trials have generally failed to show additional body weight or fat mass change against placebo, and gastrointestinal side effects are common.
• Raspberry ketones, forskolin, chromium picolinate, L-carnitine, bitter orange. Grouped together because they share a pattern: strong claims, thin human evidence, and in the case of bitter orange a stimulant profile that deserves more caution than it usually gets.
• Glucomannan. Covered above, because unlike the rest it does carry an authorised GB claim, with strict conditions, and the trial record still does not match the marketing.

If you take one thing from this section, take this: the products in this category compete on the promise, not on the evidence. Spend the money on protein and on covering your actual nutrient gaps and you will get more from it.

Illustrative image

When appetite itself is the problem

Every article on this subject assumes you are choosing to eat less. Plenty of people are not. Appetite can fall away because of illness, stress, medication, low mood, age, or recovery from something. If you physically cannot face a plate of food, being told to eat more vegetables is useless advice, and if that is your situation rather than a chosen deficit, we have written a companion guide on what vitamin to take if you don't eat enough.

What actually helps in that situation is shifting from volume to density, using the fortification tricks in the section above, eating small and often rather than facing three large plates, and covering the micronutrients with a supplement, because a genuinely small intake will not get there from food alone.

Zinc deserves a mention here. It contributes to normal macronutrient metabolism and contributes to the maintenance of normal hair, skin and nails, and low zinc is also associated with a dulled sense of taste and smell, which can itself make eating less appealing. The UK reference nutrient intake is 9.5 mg for men and 7 mg for women, and we look at that link and the rest of the nutrients involved in more detail in which vitamins can help increase appetite.

If appetite loss is new, unexplained, or lasting more than a couple of weeks, please see your GP rather than reaching for a supplement. That is a symptom worth investigating properly.

Hair, skin and nails during rapid weight loss

This one deserves its own section because it worries people more than almost anything else on the list, and because the internet's answer to it is usually to sell a biotin gummy.

Here is the honest picture. Increased hair shedding a couple of months after a sharp drop in intake is a well-recognised pattern, and the timing is the giveaway: hair follicles respond to a physiological stress by moving into the resting phase together, and the shedding shows up roughly two to three months later, not on the week it started. That delay is why people so often connect it to the wrong cause. It is typically diffuse, all over the head rather than in patches, and it usually settles once the underlying trigger eases. I have gone deeper into which vitamins matter for hair when you are eating much less.

What tends to sit underneath it in this context:

• Too little protein. Hair is largely protein, and it is a low priority when supply is short.
• Low iron. The single most common one in menstruating women, which is why ferritin is usually the first thing a GP checks.
• Low zinc. Follows protein foods down. Zinc contributes to the maintenance of normal hair, skin and nails.
• The pace of the deficit itself. A steep, fast cut is a bigger stressor than a modest, gradual one.

Skin and nails follow similar logic. Skin can get drier and slower to settle when fat intake and overall nutrition drop, and nails can turn brittle or develop ridges. Biotin contributes to the maintenance of normal hair and normal skin, but I would be straight with you: biotin shortfall is uncommon on a normal varied diet, and the evidence for biotin helping hair in people who are not short of it is thin. Iron, zinc and protein are far more likely to be the actual story here. Our honest guide to vitamins for hair, skin and nails goes through what each nutrient does and does not do.

Two practical points. Give it time, because hair grows about a centimetre a month and any recovery is measured in seasons rather than weeks. And if shedding is patchy rather than diffuse, or it carries on for more than about six months, see your GP rather than buying a stronger supplement.

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A multivitamin as a safety net

A multivitamin is the least glamorous item on this list and often the most sensible one. It is not there to do anything dramatic. It is there so that on the days your intake is genuinely low, the shortfall is smaller than it would otherwise have been.

The one thing to check is dose. Plenty of multivitamins carry twenty impressive-sounding ingredients at doses too small to matter. Our guide to comparing multivitamin formulas walks through how to read one properly, and what vitamins help with tiredness covers the fatigue side specifically. You can browse everything we have written by nutrient on our supplement guides hub.

How to read the NRV percentages properly

The percentage column is the fastest quality check on any label, and almost nobody uses it. Four rules cover it.

• NRV means nutrient reference value, the reference amount used for labelling across the whole UK market. It is a common yardstick for comparison, not a personal target, and it is deliberately generous enough to cover most adults.
• Fifteen percent is the threshold that matters. A nutrient has to be present at 15 percent or more of its NRV before any authorised function claim can be attached to it. So if a formula lists a nutrient at 5 percent, that ingredient is on the label for the marketing, not for you.
• Check the per-serving column, not the per-tablet one. If the serving is three capsules, the impressive number is for three capsules.
• Very high percentages are normal for some nutrients and not for others. Water-soluble B vitamins routinely sit in the hundreds or thousands of percent because absorption is inefficient and surplus is excreted. A fat-soluble nutrient such as vitamin D or vitamin A at a very high percentage is a different matter and deserves a look at the upper limits below.

What 100 percent NRV actually buys you

This is the bit the label never explains, and it changes how you read every multivitamin on the shelf. A stated 100 percent NRV is not the same as 100 percent of what you personally need, and for a few nutrients it is well short of the UK figure.

• Iron. 100 percent NRV is 14 mg. That is close to the 14.8 mg UK reference intake for women aged 19 to 49, but well above the 8.7 mg for men and post-menopausal women. So the same tablet is roughly right for one person and generous for another.
• Vitamin D. 100 percent NRV is 5 micrograms, which is only half the 10 micrograms the NHS suggests for autumn and winter. A multivitamin at 100 percent NRV therefore does not, on its own, cover the UK winter recommendation.
• Calcium. 100 percent NRV is 800 mg, but a multivitamin almost never contains it, because 800 mg of calcium will not physically fit in a tablet alongside everything else. If your dairy intake has dropped, assume your multivitamin is not covering calcium.
• Magnesium. 100 percent NRV is 375 mg, and again most multivitamins carry a fraction of it for the same reason of bulk. Check the actual milligrams, not the percentage alone.
• Iodine. 100 percent NRV is 150 micrograms, which sits comfortably above the UK reference intake of 140 micrograms. This is one where a standard multivitamin genuinely does the job.
• Vitamin B12. 100 percent NRV is 2.5 micrograms against a UK reference intake of 1.5 micrograms, and most B12 products carry many times that, which is normal and not a cause for concern.

The practical upshot: a multivitamin is a good broad safety net for the trace nutrients and the B group, and a poor one for the three bulky minerals, calcium, magnesium and sometimes iron. That is not a flaw in the product, it is physics. It is also why the four I listed at the top are often worth taking as separate, properly dosed products rather than hoping a single tablet covers them. Run those checks across the products you already own and you will usually find one you can stop buying.

How much is too much: upper limits worth knowing

More is not better, and a few of these nutrients have real ceilings.

• Vitamin D: NHS guidance is not to take more than 100 micrograms (4000 IU) a day for adults.
• Iron: supplemental doses above roughly 20 mg a day commonly cause stomach pain, constipation and nausea. Do not take iron speculatively without a blood test, because excess iron is stored, not excreted.
• Zinc: keep supplemental zinc to 25 mg a day or less. Sustained high intakes interfere with copper.
• Magnesium: supplemental magnesium at 400 mg a day or less is unlikely to cause harm beyond loose stools, which some forms cause more readily than others.
• Calcium: keep total intake from food and supplements to around 1,500 mg a day or below, and split supplemental doses rather than taking one large one.
• Iodine: around 500 to 600 micrograms a day is the ceiling most European assessments land on for adults. Kelp and seaweed products can carry many times that in a single serving, which is the main reason people accidentally overshoot.
• Selenium: 350 micrograms a day is the UK ceiling, and too much selenium is associated with hair loss, which is the opposite of what most people are taking it for.
• Caffeine: 400 mg a day for most adults, and 200 mg in pregnancy. Remember that a pre-workout, a fat burner and your coffee all count towards the same total.
• Vitamin A (retinol): treat with care if you are pregnant or already taking a supplement containing it.

Timing and what not to combine

• Iron and vitamin C together, iron and tea apart. Vitamin C increases iron absorption; tea, coffee and calcium reduce it. Leave an hour either side.
• Iron and levothyroxine need separating by at least four hours, as do calcium and magnesium.
• Fat-soluble vitamins with a meal containing fat. Vitamin D, A, E and K absorb better that way, which matters more, not less, when meals are small. This is where that spoonful of oil earns its place twice over.
• Magnesium suits the evening for most people. B vitamins suit the morning.
• Split calcium rather than stacking it, since absorption is better in doses of around 500 mg.
• High-dose biotin interferes with blood tests, including thyroid and cardiac assays. Tell whoever takes your blood what you are taking.

When to involve a GP or a dietitian

There is a point where this stops being a shopping question, and it is worth knowing where that line sits.

See your GP if:

• You are losing weight without meaning to, or faster than you intended.
• Tiredness is persistent and out of character, or you are breathless, dizzy or unusually pale.
• You have pins and needles, numbness or any change in sensation.
• Hair loss is patchy rather than diffuse, or is still going after about six months.
• Your appetite has dropped away and you cannot explain why.
• Your periods have become irregular or stopped.
• You take prescribed medication, are pregnant or breastfeeding, or have a diagnosed condition, and want to add supplements.

Ask about a registered dietitian if:

• You have been eating much less for months and want a proper look at whether the diet is actually adequate.
• You are combining a deficit with a restricted diet, such as vegan, gluten-free or low FODMAP, where the gaps stack up.
• You have a history of disordered eating, in which case structured support matters more than any supplement.
• You have a condition such as coeliac disease, inflammatory bowel disease or diabetes that changes what a sensible deficit looks like for you.
• You have had bariatric surgery, where lifelong supplementation is a clinical protocol rather than a personal choice.

In the UK a dietitian is a protected, regulated title, and your GP can refer you. That distinction is worth knowing, because anyone at all can call themselves a nutritionist.

A few cautions worth knowing

Speak to your GP or pharmacist before starting supplements if you take blood thinners such as warfarin, if you have kidney disease, if you are pregnant or breastfeeding, or if you take thyroid medication. Iodine in particular deserves a conversation rather than a guess if your thyroid is already being monitored. If you are eating much less under medical supervision for any reason, your clinical team should be the ones guiding what you take alongside it, and nothing here replaces their advice.

And a plain word on the category as a whole. No supplement makes an inadequate diet adequate. If you are persistently exhausted, losing hair, or losing weight without meaning to, that is a reason to ask for a blood test, not a reason to buy more tablets.

Illustrative image: a small, nutrient-dense plate

What to realistically expect

If you were genuinely short of something, topping it up can feel noticeable within a few weeks. Iron is the slowest of the group; ferritin takes months to rebuild even when you are taking it consistently. If you were not short of anything, you will most likely feel nothing at all, and that is a perfectly good outcome. It means the gap was covered.

What will not happen is a supplement making the deficit itself easier or faster. That part is food, movement, sleep and time.

Frequently asked questions

Do I need vitamin supplements while dieting?

Not automatically. If you are eating a varied diet at a modest deficit, you may well be fine. The steeper the cut, the narrower the range of foods, and the longer it goes on, the stronger the case becomes, because the arithmetic works against you on all three counts.

Can I take B12, vitamin D and magnesium together?

Yes, and they are a sensible trio. Vitamin D absorbs better with a meal containing some fat. Magnesium suits the evening for many people. B12 is fine at any time and is often easiest with breakfast.

Does 100 percent NRV mean I am covered?

Not always. For iodine and the B vitamins, 100 percent NRV comfortably covers the UK reference intake. For vitamin D it does not, since 100 percent NRV is 5 micrograms against the 10 micrograms the NHS suggests for autumn and winter. And calcium and magnesium are too bulky to fit into a multivitamin at anything near 100 percent, so check the actual milligrams rather than trusting the percentage alone.

Do I need to worry about iodine if I have cut out dairy?

It is worth checking. In Britain iodine comes mainly from milk, dairy and white fish, so cutting dairy sharply removes a large part of the supply, and most unfortified plant milks contain very little. The UK reference nutrient intake is 140 micrograms a day and a standard multivitamin usually covers it. Be careful with kelp and seaweed supplements, because their iodine content varies enormously and high intakes can disturb thyroid function just as low ones can.

Am I eating too little to lose weight?

Very low intakes are hard to sustain, tend to cost you lean mass, and make the nutrient gaps discussed above much wider. If you are permanently cold, exhausted, not sleeping, or your hair is shedding, those are signals to ease the deficit rather than deepen it, and to speak to your GP.

What are the early signs of low B12?

Commonly reported ones are unusual tiredness, pins and needles or numbness in hands and feet, a sore or smooth tongue, breathlessness, and difficulty concentrating. None of these is specific to B12, so a blood test is the only way to know.

How do I fortify food if I can only manage small portions?

Add density rather than volume. Whisk dried milk powder into whole milk and use it everywhere, grate cheese into mashed potato, eggs and soup, stir a spoonful of oil or butter through vegetables at the end, and add nut butter to porridge or yoghurt. On days when chewing is the barrier, drink your nutrition as a milky smoothie instead. Keep drinks away from mealtimes so you are not filling up before you eat.

Do fat burners actually work?

Mostly not. A systematic review of 315 randomised trials across 14 categories of weight-loss supplement found only about one in six trials was of good enough quality to draw a conclusion from, and of those only 16 showed a significant weight difference, mostly under a kilogram. Green tea extract in particular produced a small, statistically non-significant effect in a Cochrane review. Caffeine has real pharmacology behind it but does not produce meaningful fat loss on its own.

Why do I feel dizzy and get cramps in the first week?

That is usually fluid and electrolytes rather than calories. Cutting carbohydrate empties your glycogen stores, and glycogen holds water, so water and sodium leave together. Drinking a little more, not cutting salt to zero, and keeping magnesium and potassium-rich foods in the diet usually settles it. Persistent dizziness or a racing heart is a GP conversation.

Why is my hair shedding since I started eating less?

Increased shedding two to three months after a sharp drop in intake is a recognised pattern, and it is usually diffuse rather than patchy. Low protein, low iron and low zinc are the most common things sitting underneath it, and the pace of the deficit matters too. It usually settles once the trigger eases, but patchy loss, or shedding still going after about six months, deserves a GP appointment.

Should I take calcium and omega-3 as well?

Both are worth checking. Calcium slides when dairy is cut, and 700 mg a day is the UK adult reference intake. Omega-3 slides because most people were not eating two portions of fish a week to begin with, and 250 mg a day of EPA and DHA is the amount tied to the authorised heart-function wording. Cover them from food first, and supplement whichever one your diet genuinely is not delivering.

How this guide is researched and reviewed

A word on where this comes from, because you should not have to take a supplement company's word for anything.

• Every benefit statement in this guide is written to the exact wording of the GB Nutrition and Health Claims Register, and applies only where a nutrient is present at 15 percent or more of its nutrient reference value.
• Intake figures are UK figures, drawn from NHS advice and UK reference nutrient intakes, not converted from American numbers.
• Studies are linked to their stable PubMed Central or PubMed record so you can read the paper itself, and I have deliberately included the limitations of each one rather than only the headline finding.
• Where the evidence is weak, contested or based on menus rather than measured blood levels, this guide says so.
• This guide is reviewed and updated as new evidence appears, and the date at the top reflects the last review rather than the original publication.
• Written by Dr. Miron, Founder of Pure Vitamins UK. Where the honest answer is a blood test or a GP appointment rather than a product, that is what it says.

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Dr. Miron, Founder of Pure Vitamins UK

Food supplements are not a substitute for a varied and balanced diet and a healthy lifestyle. This article is for information only and is not medical advice. If you have a health condition, take medication, or are pregnant or breastfeeding, speak to your GP or pharmacist first.