Which Supplements Do You Actually Need?

Introduction

Walk into any pharmacy and the shelves make an implicit promise: that these bottles are good for everyone, and more is better.

Neither is true.

Supplements are treatments for documented deficiency, not daily insurance for people who already have enough. That single principle would change what most people buy — and it is the principle this article is built on.

The evidence for it is not controversial. Correcting a deficiency produces measurable benefit. Adding a nutrient to someone who already has enough produces, at best, nothing — and occasionally harm. The difference between those two situations is a blood test, and almost nobody takes one before buying.

This article covers three things the supplement conversation usually skips: which deficiencies are actually common, why single nutrients underperform — the cofactor problem, with a major trial published this year that demonstrates it — and why an iron deficiency often is not about iron at all.

Who Is Actually Deficient

Start with the real numbers, because “widespread deficiency” is used to sell things and it deserves to be checked.

Zinc. Wessells and Brown estimated global prevalence from national food supply data and stunting rates, and found the burden substantial and heavily concentrated in populations with limited access to animal foods and high phytate intake [1]. This is a real global problem — and it is not evenly distributed. Deficiency in a well-fed adult in Europe is not the same proposition as deficiency in a low-income setting.

Vitamin D. Genuinely widespread, and the mechanism is understood: it is synthesised in skin exposed to UVB, and modern life is largely indoors [2]. Latitude, skin pigmentation, age, clothing and season all shift it.

Vitamin C. Frank deficiency — scurvy — is uncommon where fresh produce is available, but low status persists in smokers, in people on low incomes, in older adults, and in anyone whose diet contains very little fruit or vegetable matter [3].

The point of these numbers is not that everyone should supplement. It is that deficiency is a real, testable, treatable state — and it is a different thing from the vague depletion the marketing implies.

The Principle: Test First

Vitamin D is the clearest case, because the thresholds are defined.

The Endocrine Society’s clinical practice guideline sets them out [4]:

Those numbers do real work. Someone at 12 ng/mL has a correctable medical problem with documented consequences. Someone at 45 ng/mL does not, and a daily capsule will not move them anywhere useful.

Correcting deficiency works. Martineau’s individual-participant-data meta-analysis found vitamin D supplementation protected against acute respiratory infection — with the benefit concentrated in those who were most deficient at baseline [5]. That subgroup finding is the whole argument in one result: the people who benefited were the people who were short.

Not correcting anything does not work. We covered this in detail in vitamin D and B12 deficiency, where 18,353 adults took vitamin D3 for a median of 5.3 years and their risk of depression did not change.

Same nutrient. Opposite results. The variable is whether the person needed it.

Zinc follows the same logic. Prasad’s work established both the clinical picture of deficiency and its effects on immune cell function [6][7] — and it is a picture of deficiency, not of ordinary people topping up. Our coverage of zinc alongside magnesium and selenium is in the war on supplements.

Vitamin C: The One That Food Actually Handles

Vitamin C is the clearest example of a nutrient the food supply solves, when the food supply is any good at all.

It is abundant in citrus, peppers, broccoli, kale, strawberries, tomatoes and potatoes. A person eating vegetables and fruit in ordinary quantity reaches sufficiency without thinking about it. Our citrus article covers the food side.

And the trial evidence on supplementation is unusually clear. Hemilä and Chalker’s Cochrane review of vitamin C and the common cold found that routine supplementation does not reduce how often adults in the general population catch colds [8]. Regular supplementation modestly shortened duration — by about 8% in adults. Taking it once symptoms had started did not consistently help.

There is one exception worth knowing: in people under sustained heavy physical stress — marathon runners, soldiers on exercise, subarctic troops — supplementation roughly halved cold incidence [8]. Which is a precise finding about a specific population, and it is not a general recommendation.

Padayatty’s review of vitamin C’s antioxidant role reaches a similar conclusion from the other direction: the biochemistry is genuine, and it does not translate into benefit for people who already have enough [9].

If you eat fruit and vegetables, you almost certainly do not need a vitamin C supplement. That is a boring conclusion and it is what the evidence says.

The Cofactor Problem — and the Trial That Proves It

Here is the deeper argument, and it is the one that separates a serious approach to supplementation from a shelf of bottles.

The body does not use nutrients in isolation. It uses them in networks. Take one out of its context, deliver it at a dose nothing in nature provides, and it does not necessarily behave the way it does in food.

Vitamin D is the case in point. Its job is to raise calcium absorption. But absorbed calcium has to go somewhere, and where it goes depends partly on vitamin K2, which activates matrix Gla protein — the protein that inhibits calcium deposition in arterial walls. Schurgers and colleagues showed that undercarboxylated matrix Gla protein functions as a marker of vascular calcification [10], which is the mechanistic basis for the concern that vitamin D without adequate K2 could direct calcium to the wrong place.

For years that remained a hypothesis. This year it was tested properly.

The DANCODE trial, published in Circulation, randomised 400 patients with severe coronary calcification — a calcium score of 400 Agatston units or more — to vitamin K2 at 720 μg daily plus vitamin D3 at 25 μg, or placebo, for 24 months, with cardiac CT at baseline, 12 and 24 months [11].

The combination significantly slowed calcification progression.

Two things to hold together here. This is a genuine positive trial for a nutrient combination, in a real clinical population, with a hard imaging endpoint — and that is rare enough to be worth stating plainly.

And it slowed progression. The calcium score in the treated group still rose, by 196 points. Nothing reversed. Anyone selling K2 as a way to clean out your arteries is describing a trial that has not been run. One further limit: the trial tested the combination, so it cannot say how much of the effect belongs to K2 alone — and a calcium score is a marker, not a heart attack; whether the slower progression translates into fewer events is a question this trial was not designed to answer.

But as evidence that nutrients work in combination rather than alone, it is about as good as this field gets.

Iron: When the Deficiency Is Not About the Nutrient

Iron deficiency is the most common nutritional deficiency in the world, and it is also the one most often treated by adding iron to a person who cannot absorb it.

Sometimes intake genuinely is the problem — in menstruating women with heavy losses, in growing children, in populations whose diets are low in bioavailable iron. That is real and this article is not dismissing it.

But a great deal of iron deficiency is an absorption or handling problem, and treating it as an intake problem is why so many people take iron for months without their numbers moving. Abbaspour’s review covers the mechanisms [12]:

And the gut microbiota is part of this too. Yilmaz and Li reviewed the relationship between gut bacteria and iron and found it running both ways: the microbiota influences iron availability, and iron availability shapes the microbiota [13]. Broader work on gut dysbiosis and vitamin-dependent immune regulation points the same direction — nutrient status and gut ecology are not separable questions [14].

The practical consequence: a ferritin that will not rise on supplements is not a reason for a higher dose. It is a reason to ask why. We set out how iron handling shapes outcomes in hepatitis B and C: how immunity, iron and diet decide what happens next.

When Supplementation Is Genuinely Indicated

Nothing here argues against supplements. It argues against taking them blind. These are the situations where the case is strong:

• ✓ A documented deficiency on a blood test
• ✓ Vitamin B12 for anyone eating no animal foods at all — not optional, and not a patchwork
• ✓ Folic acid before and in early pregnancy
• ✓ Vitamin D where sun exposure is genuinely limited — latitude, indoor work, covered skin, older age, care-home residence
• ✓ Malabsorption — coeliac disease, bariatric surgery, inflammatory bowel disease
• ✓ Nutrients depleted by long-term medication — B12 with metformin is the common one
• ✓ Iron where deficiency is confirmed and the cause has been looked for

Outside those, the honest default is food — and the money is better spent on it.

Collagen is the clearest case of the whole problem. It is sold on the assumption that swallowing a protein delivers that protein — but the body dismantles collagen into amino acids like any other, and whether it rebuilds them into collagen depends on copper, zinc and vitamin C being present. We take it apart in you cannot eat collagen to get collagen.

The rest of what supports immunity is not on a shelf at all. Sleep, movement, stress and the medication list do more than any capsule, and we covered that in what actually weakens your immune system.

Frequently Asked Questions

Should I take vitamin D every day?

Only if you need it. The Endocrine Society defines deficiency as below 20 ng/mL and sufficiency as 30–100 [4]. Correcting a genuine deficiency works — the protective effect against respiratory infection was concentrated in those most deficient at baseline [5]. In people who already have enough, large trials have found no benefit. Get the blood test first.

Does vitamin C prevent colds?

Not in the general population. The Cochrane review found routine supplementation does not reduce how often ordinary adults catch colds, though it modestly shortens duration [8]. The exception is people under extreme sustained physical stress, where it did reduce incidence. If you eat fruit and vegetables, a supplement adds nothing.

Do I need vitamin K2 with vitamin D?

The case is now stronger than it was. Vitamin D raises calcium absorption, and K2 activates the protein that keeps calcium out of arterial walls [10]. In 2026 the DANCODE trial randomised 400 patients with severe coronary calcification to K2 plus D3 or placebo for two years and found significantly slower calcification progression [11]. Worth discussing with your doctor — and note it slowed progression rather than reversing it, in a population that already had severe disease.

Why isn’t my iron going up on supplements?

Often because the deficiency was never really about intake. Chronic inflammation raises hepcidin and blocks iron handling; H. pylori, gastritis and coeliac disease impair absorption; ongoing blood loss outpaces replacement [12]. A ferritin that won’t rise is a reason to investigate the cause, not to increase the dose.

Is it dangerous to take supplements I don’t need?

It can be. Fat-soluble vitamins accumulate, unlike water-soluble ones. Excess vitamin D can cause hypercalcaemia. Excess zinc interferes with copper absorption. And a supplement can mask a problem worth diagnosing — treating low iron without finding the bleeding source is the classic example.

Can I get everything from food?

Mostly, with real exceptions. B12 is bacterial in origin and unsupplemented vegans genuinely need it. Vitamin D depends on sunlight more than diet. Folate needs supplementing around pregnancy. Beyond those, a varied diet covers it, and food delivers nutrients with the cofactors that make them work.

Which test should I ask for?

For vitamin D, 25-hydroxyvitamin D — the storage form [4]. For iron, ferritin plus a full blood count, and ideally an inflammatory marker, since inflammation raises ferritin and can hide a deficiency. For B12, a serum B12 level. Ask your doctor which are appropriate for you rather than ordering a panel of everything.

Conclusion

The supplement industry sells a general idea: that modern life has depleted everyone, and that a bottle restores what was lost.

The evidence supports something narrower and more useful. Deficiency is real, common in identifiable groups, testable, and worth correcting. Sufficiency is also real, and adding more to it does nothing.

The distinction between those two states costs one blood test, and it is the single most valuable thing you can do before spending money on supplements.

Three findings in this article point the same way. Vitamin D worked for the people who were most deficient [5], and did nothing for a large population that mostly wasn’t. Vitamin C did not prevent colds in ordinary adults, but did in people under extreme physical stress [8]. And K2 with D3 slowed arterial calcification in patients who already had severe disease [11].

Every one of those is a targeted result in a defined group. None is a general recommendation, and none of them was found by giving a nutrient to everybody.

Test first. Supplement second. Fix the cause where there is one. And spend the rest on food, which delivers nutrients in the combinations the body evolved to use.

Nature provides. Physiology responds. Precision heals.

Scientific Sources and References

1. Wessells, K. R., & Brown, K. H. (2012). “Estimating the global prevalence of zinc deficiency: results based on zinc availability in national food supplies and the prevalence of stunting.” PLoS One, 7(11), e50568. PMID 23209782
2. Holick, M. F. (2007). “Vitamin D deficiency.” New England Journal of Medicine, 357(3), 266–281. PMID 17634462
3. Rowe, S., & Carr, A. C. (2020). “Global vitamin C status and prevalence of deficiency: a cause for concern?” Nutrients, 12(7), 2008. PMID 32640674
4. Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., et al. (2011). “Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline.” Journal of Clinical Endocrinology & Metabolism, 96(7), 1911–1930. PMID 21646368
5. Martineau, A. R., Jolliffe, D. A., Hooper, R. L., et al. (2017). “Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data.” BMJ, 356, i6583. PMID 28202713
6. Prasad, A. S. (2003). “Zinc deficiency.” BMJ, 326(7386), 409–410. PMID 12595353
7. Prasad, A. S. (2008). “Zinc in human health: effect of zinc on immune cells.” Molecular Medicine, 14(5–6), 353–357. PMID 18385818
8. Hemilä, H., & Chalker, E. (2013). “Vitamin C for preventing and treating the common cold.” Cochrane Database of Systematic Reviews, (1), CD000980. PMID 23440782
9. Padayatty, S. J., Katz, A., Wang, Y., et al. (2003). “Vitamin C as an antioxidant: evaluation of its role in disease prevention.” Journal of the American College of Nutrition, 22(1), 18–35. PMID 12569111
10. Schurgers, L. J., Teunissen, K. J., Knapen, M. H., et al. (2005). “Novel conformation-specific antibodies against matrix gamma-carboxyglutamic acid (Gla) protein: undercarboxylated matrix Gla protein as marker for vascular calcification.” Arteriosclerosis, Thrombosis, and Vascular Biology, 25(8), 1629–1633. PMID 15961706
11. Hasific, S., Øvrehus, K. A., Mejldal, A., et al. (2026). “Vitamin K2 and vitamin D3 supplementation in patients with severe coronary artery calcification: the DANCODE trial.” Circulation, published online 28 August 2026. PMID 42661461
12. Abbaspour, N., Hurrell, R., & Kelishadi, R. (2014). “Review on iron and its importance for human health.” Journal of Research in Medical Sciences, 19(2), 164–174. PMID 24778671
13. Yilmaz, B., & Li, H. (2018). “Gut microbiota and iron: the crucial actors in health and disease.” Pharmaceuticals, 11(4), 98. PMID 30301142
14. Stefanelli, N., et al. (2026). “Gut dysbiosis and vitamin-dependent immune regulation in degenerative musculoskeletal and bone diseases.” Frontiers in Immunology, 17. PMID 42597253

Last reviewed: September 2026. Vivid Remedy publishes independently researched, evidence-based health content cross-referenced against current peer-reviewed scientific literature. This content does not constitute medical advice. Do not start, stop or change supplementation without consulting a qualified healthcare professional, particularly if you take prescribed medication, are pregnant, or have kidney or liver disease.