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food source · 4
Vitamin D dissolves in oil and travels with fat. Salmon stores its oil in the flesh, so the fish itself carries a useful amount of vitamin D: about 250–650 IU per 100 g of farmed Atlantic salmon.
Mechanism · Vitamin D travels with the oil
Vitamin D dissolves in oil, so it sits wherever the fat is. Salmon stores its oil in muscle, so the vitamin D is in the fillet you eat; cod stores its oil in the liver, so its flesh has almost none. What you eat is only raw material, which the liver and then the kidney each have to process before it becomes the active form.
Numbers · A rare food source in winter
Farmed Atlantic salmon has about 250–650 IU of vitamin D per 100 g, and wild salmon usually has more. In a high-latitude winter the skin makes almost no vitamin D, and few natural foods supply a useful amount; fatty fish are one of them.
Oily fish is one of the few natural food sources of vitamin D. Sardines eaten with their bones put calcium and the vitamin D that helps it into the blood in the same bite.
Mechanism · Vitamin D opens the calcium route
Vitamin D from food is processed first in the liver and then in the kidney before it becomes active. The active form makes the lining cells of the duodenum build more calcium channels, calcium-binding protein and calcium pumps, opening the route that carries calcium into the blood. Without it, most of the bone calcium you chew never gets in.
In practice · Calcium and vitamin D in one bite
Most foods supply one end of this chain: natural milk mainly supplies calcium, and sunlight supplies vitamin D. A can of bone-in sardines puts both into the same bite. Canned sardines also have about 1.4–1.5 g of EPA and DHA per 100 g, which is their other side as an omega-3 source.
←Eggs
Egg is one of the few natural food sources of vitamin D. It is concentrated in the yolk, though the amount is far below that in fatty fish.
Mechanism · The vitamin D is in the yolk
Vitamin D dissolves in fat, and almost all of an egg's fat is in the yolk, so its vitamin D is there too, and egg whites alone supply none. It is still one of the few natural sources, just not as concentrated as fatty fish.
In practice · Yolk fat is a carrier too
The fat in the yolk also helps fat-soluble nutrients such as lutein get absorbed: the nutrient and its carrier come in the same bite, which is where a whole yolk differs from a single-nutrient supplement.
Ergosterol in the cell membranes of mushrooms turns into vitamin D2 under ultraviolet light. Mushrooms grown in the dark have little vitamin D; only those that have been in the sun or treated with UV light count as a vitamin D source.
Mechanism · Mushrooms make vitamin D in sunlight
Ultraviolet light striking ergosterol in fungal cell membranes turns it into vitamin D2, the same kind of light-driven reaction by which sunlight turns 7-dehydrocholesterol in human skin into vitamin D3. The D2 you eat is still raw material that the liver and then the kidney have to process.
In practice · Look for UV treatment on the label
Mushrooms grown in the dark have little vitamin D, so unless the label says they were UV-treated or sun-dried, do not count them as a vitamin D food. Mushrooms exposed to sun or UV light can raise blood vitamin D levels, which is especially useful for people who eat no animal foods, though D2 does this less well than D3, which is why supplements usually use D3.
synergy · 3
Vitamin D helps the gut absorb more calcium; K2 activates osteocalcin and matrix Gla protein, the first of which helps lay calcium into bone while the second helps keep it from settling in artery walls. One brings calcium in, the other steers where it goes. The evidence for K2's half comes mainly from mechanism studies and observational research.
For older adults who train, vitamin D is more like the material that lets training work: low vitamin D impairs muscle function and is linked with falls, so a deficiency should be corrected, but vitamin D on its own cannot replace training. In Fiatarone 1994, older people who took a nutritional supplement without training did not improve on the main outcomes.
Vitamin D is a helper for eczema, not a switch: Nielsen 2024's pooled analysis saw a small drop in severity scores, while an earlier Cochrane review judged the evidence insufficient for established eczema. Topping up a deficiency makes sense, but expecting vitamin D alone to control the chain of skin-barrier failure and allergic (Th2) inflammation asks more than the evidence can carry.
cofactor · 7
Vitamin D has to be processed once in the liver and once in the kidney before it becomes active, and the enzymes for both steps need magnesium. When magnesium is short, supplemented vitamin D may not be converted well; this point comes mainly from mechanism studies and reviews.
The final step in activating vitamin D is carried out by an enzyme called CYP27B1 in the kidney. As kidney function falls, this step stalls, active vitamin D drops, and calcium and phosphate control goes out of balance; this is one of the starting points of chronic kidney disease–mineral and bone disorder (CKD-MBD).
The nutritional foundation for bone is 1000–1200 mg of calcium a day (food first, supplements only to fill the gap) plus enough vitamin D. Vitamin D governs whether calcium is absorbed and laid down in bone; a shortfall should be corrected, but it is not something high doses can buy.
Vitamin D decides whether the calcium you eat can be absorbed. On the NIH Office of Dietary Supplements tiers, a 25(OH)D below 30 nmol/L (12 ng/mL) is deficiency, 30–50 nmol/L may be inadequate, and 50 nmol/L (20 ng/mL) or more is enough for most people. When vitamin D is deficient, calcium absorption drops clearly, and taking more calcium keeps little of it.
Skeletal muscle carries vitamin D receptors. People who are seriously short of vitamin D, especially older adults, have weaker muscles and fall more often, mainly as an observed association; whether vitamin D supplements make training build more muscle has no consistent answer. Correct a deficiency, but there is no need to push levels higher.
Immune cells in the lungs use an antimicrobial peptide called LL-37 against pathogens, and vitamin D raises its expression; this is one mechanism linking vitamin D to airway defense. A meta-analysis of individual participant data found that vitamin D supplements had some effect on asthma attacks.
Vitamin D's effect on sarcopenia shows up mainly in people who are deficient: once they are corrected, strength may improve and falls may decrease. For people who already have enough, taking more does not add strength, which matches the conclusion for bone.
regulates · 10
Active vitamin D (calcitriol) makes gut cells build the channels and carrier proteins that actively absorb calcium. When vitamin D is low, this active route stays barely open, and much less of the calcium you eat gets absorbed.
Phosphate and calcium in the blood are held steady by several hormones working together: vitamin D makes the gut absorb more calcium and phosphate, while parathyroid hormone and FGF23, a hormone made by bone, make the kidneys excrete more phosphate. They keep each other in check as one system that regulates mineral balance.
Active vitamin D acts through receptors on immune cells such as macrophages to regulate the production of antimicrobial peptides. Martineau 2017 pooled 25 randomized trials: vitamin D supplements lowered the odds of acute respiratory infection by about 12% overall, with the benefit concentrated in people dosed daily or weekly rather than with large boluses, and largest in those whose baseline 25(OH)D was below 25 nmol/L.
Vitamin D is fat-soluble and has to dissolve in fat before the gut can absorb it, so taking it with a meal that contains fat gets more of it absorbed than taking it on an empty stomach. Mulligan 2010 had 17 patients whose vitamin D treatment was not working switch to taking it with the largest meal of the day, and after 2-3 months their blood 25(OH)D rose by 56.7% on average; this was a before-and-after comparison without a control group, so the direction is credible but the size should not be copied as is.
A severe shortage of vitamin D or calcium causes rickets in children: the growth plate cartilage cannot mineralize, the legs bow, the wrists and ankles thicken, and growth slows. Without a shortage, extra calcium adds bone mass, not height.
←After birth and breastfeeding
Breast milk carries little vitamin D, far less than a baby needs, so the American Academy of Pediatrics recommends that breastfed babies, fully or partly breastfed, get a vitamin D supplement every day from the first few days of life, at a dose set by the pediatrician.
In a fatigue work-up, a 25(OH)D below 50 nmol/L (20 ng/mL) is worth checking once and correcting; but once you reach sufficiency, taking more does not bring more energy, and that is the part people often get backward.
The skin's raw material for vitamin D declines with age: in skin samples removed during surgery, skin from people in their late 70s and early 80s made less than half as much vitamin D precursor under the same ultraviolet light as skin from children and young people. Older people also go out less and show less skin, so they run short more easily.
A Korean multicenter randomized trial (Jeong 2020) gave vitamin D and calcium only to people with BPPV whose repositioning had worked and whose blood vitamin D was low (25(OH)D below 20 ng/mL, that is, 50 nmol/L), and recurrence over a year fell from 46.7% to 37.8%. The trial did not test whether people who are not short of vitamin D benefit; repositioning is still the treatment itself, and the result does not carry over to falls or fractures.
Keep two things apart: filling a gap back to normal is worthwhile, and people who were deficient may get slightly fewer respiratory infections once corrected; topping up someone who already has enough adds nothing. Immunity is not better the higher it goes; a supplement can bring you back to normal, not push you beyond it.
contrast · 1
The VITAL trial used a two-by-two factorial design to test vitamin D and fish oil at the same time: participants took one, both, or placebo for both. On the vitamin D side, neither of the two main endpoints, cancer and major cardiovascular events, showed an effect (Manson 2019). So healthy people are not advised to take both as a matter of course; correct a deficiency when one is found.