You train five days and you're still probably low

Men who train consistently are often still low on vitamin D, and the reason is simple: the gym is indoors. Research comparing athletes has found roughly 80% of indoor athletes with insufficient vitamin D levels, against 48% of outdoor athletes. Training hard does not protect you here. Depending on when and where you train, it can mean less sun exposure than someone who never trains at all but walks outside at lunch.

The short version:

• Indoor athletes show markedly higher rates of low vitamin D than outdoor athletes.
• Early-morning and evening training means daylight hours are spent inside.
• It shows up even in summer, and even in professionals.
• Vitamin D plays a role in muscle function, so this is not a side issue for people who train.*

THE NUMBER THAT SHOULD GET YOUR ATTENTION

A systematic review of elite athletes cites work by Constantini and colleagues showing 80% of indoor athletes with insufficient vitamin D, compared with 48% of outdoor athletes. Same training loads. Same level of dedication. The difference is where the training happens.

And this is not a winter-only problem. A study of professional handball and ice hockey players found a high prevalence of insufficiency in professional indoor athletes, even in summer.

Reviews of the topic list indoor training explicitly as a risk factor for low vitamin D in athletes, alongside living at northern latitudes and limited outdoor time outside of training.

WHY TRAINING HARD DOESN'T HELP

Your body makes vitamin D when skin is exposed to sunlight. That is the main source for most people. Diet contributes relatively little.

Now think about a typical training week for someone with a job and kids. Up early, gym before work or after the kids are down. Commute in a car. Work indoors. The hours when the sun is actually high enough to do anything are the hours you are inside, and the hours you are training are dark or nearly dark.

The discipline that makes someone a consistent trainer, up early, structured, indoors, is the same discipline that removes them from daylight.

"Honestly, no, most of them don't. Between running businesses, working physical jobs that start before sunrise, or training indoors, a lot of my clients go entire days without meaningful sun exposure on bare skin. Even guys who are outside all day landscaping are usually covered up or it's overcast, so it's not the slam dunk people assume. I'd say a lot of my roster is probably running low without even realizing it."

THE PART THAT MATTERS IF YOU LIFT

Vitamin D is not just a bone nutrient. Research links vitamin D status to muscle strength and power, and reviews in this area note that low vitamin D has been associated with an increased rate of muscle injuries in football players, along with observed relationships between vitamin D levels and injury recovery.*

One study of professional indoor athletes found those with sufficient vitamin D levels achieved higher maximal aerobic power than those with insufficient levels. That is an observed association in one cohort rather than proof that supplementing improves performance, and it is worth being precise about that difference.

The honest framing: being low is not good for someone whose training depends on muscle function and recovery, and being low is common in exactly this group.*

"It's a bigger factor than people think. Between early commutes, full days at a desk or on the job, and training sessions that happen before sunrise or after sunset, a lot of guys are basically bookending their daylight hours with darkness. Add in office jobs or indoor gyms, and you've got people going days without direct sun on their skin without even noticing it. That's exactly why I don't leave this to chance, I just build vitamin D into the plan from day one instead of assuming lifestyle will handle it."

DOES AGE MAKE IT WORSE?

This is worth being careful about, because the science is not as settled as most articles claim.

The long-standing view comes from work by MacLaughlin and Holick showing the skin's concentration of the vitamin D precursor 7-dehydrocholesterol declines roughly 50% between age 20 and age 80. That finding is widely repeated.

But a 2023 controlled study comparing healthy younger and older adults found no significant difference in skin precursor concentration, and a similar vitamin D response to UV exposure in both groups. Its conclusion was that this precursor is not the limiting factor people assumed.

So: contested. What is not contested is that older adults consistently show lower vitamin D status, driven substantially by behavior rather than biology alone. Less time outside, more clothing coverage, more time indoors. Which, for a 40-something dad who trains before sunrise and works inside, is the whole point of this article.

"Not really, and that's part of why I bring it up early with everyone. Guys are locked in on protein grams and workout volume, but vitamin D plays a real role in muscle function and recovery, and most guys have never connected those dots. Once I explain it, it clicks pretty fast, since almost everyone's already trying to optimize those exact things anyway."

WHAT TO ACTUALLY DO

1. Test. It is a standard blood test and it is the only way to know. Most people who are low have no idea, because there is no obvious symptom telling you.
2. Count your real daylight. Not time awake. Time actually outside, with skin exposed, while the sun is high. For a lot of people who train indoors, that number is close to zero on a workday.
3. If you supplement, take it with fat. Vitamin D is fat-soluble, so it absorbs better alongside fat. This is why the form matters as much as the dose.
4. Be consistent rather than aggressive. A sensible daily amount taken every day does more than an occasional large dose.

"I care about clean formulation, no unnecessary fillers or junk mixed in, and a company that's transparent about sourcing and third-party testing."

The takeaway is not that training is bad for you. It is that training is not a substitute for sunlight, and the people most confident they are covered are often the ones who are not.

About D3X. D3X makes one thing: a clean liquid vitamin D3 + K2 (2,000 IU D3 / 100 mcg K2) in an olive-oil base, third-party tested and made in the USA. No seed oils, fillers, or sweeteners. Learn more.

*These statements have not been evaluated by Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.