Vitamin D

Vitamin D is the clearest example of why a single grade for a supplement is sometimes the wrong question. For a deficient person it would be an A. For a replete person it is a D. The population is a mix, so it lands at C.

The whole decision hinges on which one you are, and that is a blood test, not a guess.

What it does, and for whom

Vitamin D is a hormone precursor your skin makes from UVB exposure. Receptors for it appear in skeletal muscle among many other tissues, which is the mechanistic basis for expecting an effect on strength.

Correcting a genuine deficiency does improve muscle function. That is well established, particularly in older adults, where deficiency is associated with weakness and with falls.

The interesting result is what happens above that threshold. When athletes with adequate levels are supplemented, meta-analysis finds no reliable improvement in maximal strength or power. There is no dose-response staircase where more is better. There is a shortfall, and there is not.

Bone health is the other well-supported outcome, and the one with the longest evidence base — relevant to anyone loading a skeleton under a barbell.

Are you actually deficient

Deficiency is common enough that the question is worth asking properly.

The risk factors compound: winter, latitude above roughly 37 degrees, darker skin, working indoors, covering up outdoors, higher body fat — vitamin D is fat-soluble and sequesters in adipose tissue — and being older.

Someone training indoors through a northern winter ticks several of those at once.

The test is serum 25-hydroxyvitamin D. Deficiency is usually drawn below 50 nmol/L (20 ng/mL), with insufficiency in the band above it. Your doctor can order it; it is cheap and it converts this from a guess into a decision.

The dose

For correcting a deficiency, 1,000 to 2,000 IU of D3 daily suits most adults, taken with a meal containing fat because absorption depends on it. Retest after two or three months rather than assuming.

Two things to avoid.

The first is the very large intermittent bolus — 100,000 IU once a month, or annual megadoses. These regimens exist for compliance reasons and some of them have produced worse outcomes than daily dosing, including increased falls in older adults. Daily and modest is the pattern with the better safety record.

The second is open-ended megadosing. Vitamin D is fat-soluble and it accumulates. Toxicity is genuinely rare, but it comes from sustained intakes well above 10,000 IU a day, and it is unpleasant — hypercalcaemia, kidney problems. There is no upside above repletion to justify going near it.

Buy D3, not D2. Cholecalciferol raises and holds serum levels better. Vegan D3 from lichen exists if you need it.

Why we grade it C

Because most people reading this do not know their level, and the supplement only works for half of them.

Take the test. If you are low, this becomes one of the highest-value things in your cabinet for a few pence a day. If you are not, the money is better spent on food.

Side effects and who should skip it

• Vitamin D is fat-soluble and accumulates. Toxicity is rare but real, and comes almost entirely from sustained megadoses above 10,000 IU daily rather than from normal supplementation.
• High-dose intermittent bolus regimens — 100,000 IU monthly and similar — have been associated with worse outcomes including increased falls in older adults. Daily modest dosing is the safer pattern.
• Interacts with several medications including some diuretics, statins and steroids. Worth a conversation with a pharmacist if you take anything long term.