What they actually do
Almost every reaction that releases energy from food needs a vitamin or mineral to help it along. B vitamins work inside energy metabolism. Iron carries oxygen. Magnesium takes part in hundreds of enzyme reactions, including those that handle the body’s energy currency, ATP. Zinc is needed for immune function and building protein. None of these supply calories, but all of them control what the calories can be used for.
The practical result is that a shortfall rarely shows up as one specific, obvious symptom. It shows up as things working less well — fatigue, poor recovery, weaker immunity — which is exactly the kind of thing most easily blamed on something else.
Fat-soluble and water-soluble vitamins
Vitamins A, D, E and K dissolve in fat, are absorbed alongside the fat in food, and are stored in the body. That means you need them less regularly, but also that an excess builds up rather than being flushed out. The B vitamins and vitamin C dissolve in water, are stored far less, and need topping up more consistently.
Vitamin D is the one most often taken as a supplement, and the one whose trial evidence is most instructive. The VITAL trial randomised nearly 26,000 adults to vitamin D or a placebo and found no significant reduction in cancer or cardiovascular events overall. Correcting a genuine deficiency and adding more for people who already have enough are different questions, and trials like this answer the second one.
- Fat-soluble
- A, D, E, K — absorbed with fat, stored in the body.
- Water-soluble
- B-complex and C — minimally stored, needed regularly.
Minerals and electrolytes
Electrolytes — sodium, potassium, magnesium, calcium and chloride — carry an electrical charge in the body’s fluids and govern hydration, the firing of nerves and the contraction of muscle. They are lost in sweat, which is why they matter especially around long or hot-weather training, and why replacing fluid without replacing electrolytes can leave the real problem in place.
Iron, zinc, selenium and iodine are trace minerals where both too little and too much carry consequences, and where the gap between the two can be narrower than it is for the water-soluble vitamins.
The key micronutrients, one by one
A varied whole-food diet covers the range, but a handful of micronutrients account for most of the shortfalls. It is worth knowing what each one does and where it is found — the food sources below are foods you can look up in the food database.
- Vitamin D
- Works with calcium in bone and has a hand in immune function. Made in skin from sunlight and hard to get from food alone — oily fish and eggs are among the few sources. Its largest trial, VITAL, found no overall reduction in cancer or heart events from supplementing it.
- Magnesium
- A cofactor in hundreds of enzyme reactions, including the ones that handle ATP, the body’s energy currency. Concentrated in leafy greens, pumpkin seeds, legumes, nuts and whole grains.
- Vitamin K
- Needed for blood clotting and for directing calcium in bone. Leafy greens supply the K1 form; fermented foods such as natto carry the K2 form.
- Vitamin C
- Water-soluble, barely stored, and needed for collagen and as an antioxidant. Peppers, citrus, berries and brassicas are dense sources.
- B vitamins
- A family that works inside energy metabolism, turning food into usable energy and building red blood cells and DNA. Spread across whole grains, legumes, eggs, meat and leafy greens; B12 comes essentially only from animal foods.
- Zinc
- Needed for immune function, wound healing and building protein. Richest in shellfish and meat, and — less absorbably — in legumes, seeds and whole grains.
- Iron
- Carries oxygen in the blood. The heme iron in meat and fish is absorbed far more readily than the non-heme iron in legumes and leafy greens; pairing plant iron with vitamin C helps.
- Selenium
- A trace mineral built into the body’s antioxidant enzymes, where both too little and too much carry consequences. Concentrated in seafood and eggs, and very densely in Brazil nuts.
- Omega-3 fatty acids
- Long-chain fats (EPA and DHA) that build cell membranes and sit at the centre of inflammation signalling. Oily fish — salmon, sardines, mackerel — are the direct source; the plant form in flax and walnuts converts to them only partly. Pooled trials of supplementation report a modest cardiovascular signal.
Where they come from
Whole foods deliver micronutrients packaged in a way that affects how much is absorbed — the iron in meat is absorbed more readily than the iron in plants, and the fat-soluble vitamins need fat present to be taken up at all. A varied diet built on vegetables, fruit, whole grains, legumes, dairy or its alternatives, and good protein covers the range for most people, without any single food having to do the heavy lifting.
The food database lists the micronutrients each food is a meaningful source of, so the question "where do I get this" has a concrete answer rather than a supplement aisle.
A note on supplements
Supplements can correct a shortfall; they do not manufacture extra health on top. Tracking emergency-department visits across 63 US hospitals, researchers estimated thousands of visits a year caused by harm from dietary supplements, concentrated in weight-loss and energy products. That is a small risk spread across enormous use — but it is not zero.
No recommendation on what to take — only what the trials measured.
