Vitamin D: the deficiency that training outdoors does not fix

A 2014 systematic review by Farrokhyar and colleagues pooled 23 studies and 2,313 athletes and found 56 percent had inadequate vitamin D status. These were athletes, including ones who train outside, at latitudes where the sun is not the problem.

That is the finding that should interest a fighter, because it breaks the assumption most people are working from: that this is a winter problem, an indoor problem, somebody else's problem.

Why athletes are worse off than the people watching them

The prevalence numbers are the uncomfortable part, and they need reading carefully. The figure above is often set against a general-population rate of roughly a quarter to two-fifths, which makes athletes look worse off than the people watching them. That comparison holds in some datasets and not in others: later reviews of elite athletes have landed closer to 30 or 40 percent, and the answer moves with how you define inadequacy, which season you sample, and what latitude you sample at.

What survives all that variation is the size of the problem. Whichever number you take, a large minority of athletes are short, and nobody predicted that, because athletes are leaner, more health-conscious and spend more time moving.

And there is no good evidence that athletes get less sun or eat less vitamin D than everyone else. So the usual explanation does not hold, and high rates of deficiency have been documented in athletes who train outdoors at latitudes where sun exposure should be sufficient.

One plausible part of the answer is that heavy training increases utilisation. If you are using more of something and taking in the same amount, status falls.

That mechanism is proposed rather than settled, and I am flagging it as such.

The line in the literature that concerns this sport

Buried in the reviews is a list of groups at elevated risk. Indoor sports. Cold-weather sports. Winter months.

And athletes who follow weight management protocols.

For this website that describes the entire audience. A fighter running a structured camp is, by that description, in a documented higher-risk group before you factor in anything else about his schedule.

Then add the rest of it. Boxing happens indoors. Camps in Ukraine, Kazakhstan and northern Europe run through months when the sun is not producing much of anything at that latitude. And fight week strips food intake down to a narrow list that contains very little of the few foods that carry vitamin D at all.

D3, not D2

Two forms exist. Vitamin D2 is ergocalciferol, D3 is cholecalciferol.

D3 is more potent, acts for longer, and binds more strongly to vitamin D binding protein, which is the mechanism behind its better bioavailability.

Reviews recommend D3 over D2 or over combined therapy for restoring status.

Check the label. If it says D2, or simply says "vitamin D" without specifying, buy something else.

The dose, and the ceiling

Higher doses appear in the research. Some athlete studies have used 5,000 IU per day, and one trial in collegiate basketball players used 10,000 and still did not get every participant into the range it was aiming for.

Those are supervised study protocols with blood testing behind them, not a reason to buy a stronger bottle.

If your result comes back low, the correction dose is a decision for the doctor who ordered the test. Correcting a deficiency and maintaining a normal status are different jobs with different numbers.

What the performance evidence actually shows

Here is where I have to be careful, because this supplement gets sold on a promise the trials do not reliably deliver.

A recent systematic review and meta-analysis of randomised trials in athletes found that supplementation reliably did one thing: it raised blood levels. Nine of the included studies confirmed that.

What those same studies did not establish was a definitive connection between D3 supplementation and muscle strength.

Individual trials point the other way. One found improvements in 10-metre sprint time and vertical jump after supplementation. Another found benefit in athletes who started severely deficient, which is the pattern you would expect: correcting a deficiency helps, topping up somebody who is already sufficient mostly does not.

What the big trials found in everyone else

Three large randomised trials tested vitamin D in the general population, and all three came back flat on the outcomes they were built to test.

• VITAL, in the United States: 25,871 adults took 2,000 IU a day or a placebo for about five years. Vitamin D did not lower the rate of invasive cancer or of major cardiovascular events.
• D-Health, in Australia: 21,315 people aged 60 to 84 took 60,000 IU once a month or a placebo for up to five years. There was no reduction in deaths from any cause.
• ViDA, in New Zealand: 5,110 adults aged 50 to 84 took 100,000 IU a month, after a 200,000 IU starting dose, or a placebo. Cardiovascular disease was not reduced.

The common thread is that most people in these trials were not deficient to begin with. Topping up somebody who already has enough does not do much, which is the same pattern the athlete trials show for strength and power.

The 2024 Endocrine Society guideline drew the practical conclusion. Healthy adults under 75 should take the recommended daily allowance and not more, unless sun exposure or diet is inadequate. Empiric supplementation above that is suggested only for children and adolescents, adults over 75, pregnancy and high-risk prediabetes. Routine blood testing is not suggested for any of those groups. Where treatment is needed, daily lower doses are preferred over large intermittent ones.

Why a fighter is a different question

The guideline is written for people with no reason to be short. A fighter in camp often has several: training indoors, camps through winter at northern latitudes, and weeks of restricted eating, which is exactly the inadequate sun exposure or dietary intake the guideline makes its exception for. That is why this page still recommends D3 for fighters in those conditions, and why it no longer recommends it to everyone else.

The K2 question, and a mark against my own guide

My Fighter's Supplement Guide lists vitamin D3 paired with K2, and says K2 directs calcium to bones rather than arteries.

The mechanism behind that is real. Vitamin K activates proteins involved in getting calcium into bone and keeping it out of soft tissue. What is thinner than the confidence of that sentence is the clinical evidence that adding K2 is necessary alongside D3 at the doses above, in healthy young athletes, in the timeframe of a fight career.

So I am softening my own position rather than defending it. A combined D3 and K2 product is a reasonable choice and I still use them. Presenting K2 as critical overstated what is established.

If a product you like is D3 alone, that is not a problem.

Testing, and why there are no numbers on this page

For healthy people, routine vitamin D testing is not recommended. The 2024 Endocrine Society guideline found no benefit from screening, including in people with darker skin. A test makes sense when a doctor has a reason to order one, and it is the doctor's decision to make.

What you will not find here is a target figure. Reference ranges differ between laboratories and guidelines, the interpretation depends on the person in front of the doctor, and a number printed on a website invites a fighter to self-diagnose and self-dose against it. That is a bad trade for both of us.

Take the result to somebody qualified to read it.

Sun, and why it is not a plan

Sensible sun exposure does work, within limits that are narrower than people assume. The guidance that appears in the sports science literature is specific: middle of the day, around 15 minutes, most days of the week, at moderate latitudes, with a reasonable share of skin uncovered, during summer.

Read those conditions against a fight camp. Training happens indoors. Sessions are morning and late afternoon. Camps run through winter as often as summer, and travel puts you at latitudes you did not choose.

Skin tone changes the equation again, and so does anything covering you.

Sun is a contributor. It is not a protocol, and treating it as one is how athletes at good latitudes still turn up deficient.

What goes in the bag

• D3, from a product listed on a certification register, taken daily with a meal that contains fat.
• Up to 4,000 IU a day in an indoor or winter camp on restricted intake, and the recommended daily allowance when sun and food are covering it.
• Adjusted when the season or the camp changes, instead of the same dose on autopilot all year.
• Logged with its lot number like everything else, for the reasons in the certification article.

Where this leaves a fighter

Most of what I write about supplements comes down to the same instruction: take the sensible dose, keep the record, stop expecting miracles.

Vitamin D fits that instruction better than its reputation suggests. For most people the answer is food, sun and the recommended daily allowance.

For a fighter who spends the winter indoors making weight, the conditions change, and a daily D3 in the range above covers what sun and food are not supplying. When the season or the camp changes, the dose can change with it.

The other fat-soluble one worth getting right, and the one with a combat-sports argument behind it, is omega-3.

Common questions

Do most people need to take vitamin D?

No. Large trials in the general population found little benefit from extra vitamin D in people who were not deficient, and the 2024 Endocrine Society guideline advises healthy adults under 75 to stay at the recommended daily allowance unless sun or diet is inadequate.

Should I get my vitamin D level tested?

Not routinely. The 2024 guideline suggests against routine testing in healthy people. Whether you need a test is a decision for a doctor who has a reason to order one.