Vitamin D Supplements for Athletes: How Much Do You Really Need?
As a sports dietitian, I usually take a "food first" approach to meeting micronutrient needs, but vitamin D is one of the exceptions. Few foods naturally contain significant amounts of it, and our ability to make this nutrient from sunlight varies quite a bit. That's why I often recommend vitamin D supplements for athletes, especially heading into the colder months.
So, how much vitamin D do you actually need? Why is it so important? When should you get your levels tested, and when does supplementation make sense?
In episode 147 of The Eat for Endurance Podcast, I’m joined by Kelly Pritchett, PhD, RD, CSSD, to answer these questions and more. Kelly is a sports dietitian and Professor of Nutrition and Exercise Science at Central Washington University. She has conducted research on vitamin D in athletes, and I was excited to bring her back on the show to cover this important and timely topic.
Kelly and I discuss:
Why vitamin D matters for bone health, muscle function, immunity, and athletic recovery
The best food sources of vitamin D, and why these foods alone won't cut it
How sun exposure, skin tone, sunscreen, clothing, season, and geography impact your ability to produce vitamin D
Common causes of low vitamin D and who's most at risk
What vitamin D blood test results mean and why recommendations for optimal levels differ
When to get your vitamin D tested and how time of year affects your results
When you should consider supplementation, and how dosage can vary
How vitamin D works with calcium, magnesium, and vitamin K, especially for bone stress injuries
What the research tells us about vitamin D in athletes
Tune in to the full episode below, or keep reading for the key takeaways.
Why Vitamin D Matters (Especially for Athletes)
Vitamin D is a fat-soluble vitamin that is best known for its role in bone health, as it promotes the absorption of calcium in the gut and performs other critical functions that allow you to build and maintain strong, healthy bones across your lifespan. Vitamin D also supports the muscles, brain, immunity, cell growth, and more.
When it comes to athletes specifically, Kelly explained that research on vitamin D and performance is mixed. Where correcting a deficiency seems to help most is indirectly: low vitamin D has been linked to more frequent upper respiratory infections, longer illnesses, and more missed training days, as well as a higher risk of stress fractures.
It's important to remember that vitamin D is only one small piece of the puzzle when it comes to bone and immune health. Adequate energy and macronutrient intake, consuming enough of other key micronutrients (e.g. calcium and magnesium for bones), and other lifestyle behaviors (e.g. strength training, hand-washing) all play important roles too.
Why Getting Enough Vitamin D Can Be Challenging
Vitamin D is a little different from other micronutrients in that you can actually make it yourself and consume it in some foods. And yet, many people struggle to maintain healthy levels because it's not as straightforward as "just eat these foods" or "go sit in the sun."
Can you get enough vitamin D from food?
The short answer: most likely not. But that doesn't mean you shouldn't still make an effort to eat foods that are rich in vitamin D. While there are relatively few foods that naturally contain substantial amounts, plenty of foods are fortified with this nutrient.
Some food sources of vitamin D include:
Fatty fish, such as salmon, tuna, mackerel, and sardines
Fortified milk and some plant-based milk alternatives
Fortified orange juice
Fortified breakfast cereals
Egg yolks (contain smaller amounts)
UV-exposed mushrooms
Cod liver oil is another source, although it's not exactly a food most people are eager to add to their daily routine.
Kelly shared data from previous studies involving athletes, where estimated dietary vitamin D intake ranged from just 100 to 300 IU per day, compared with the adult RDA of 600 IU per day.
This doesn't mean every athlete needs a supplement. But it does illustrate why food alone can be challenging, especially if you don't regularly eat vitamin D-rich or fortified foods.
For vegan and vegetarian athletes, this can require a little more effort since many of the naturally rich sources of vitamin D are animal-derived. Fortified foods can help fill the gap, but it depends on what you're actually eating.
How much does sun exposure matter?
Sun exposure matters a lot, because sunlight is a major source of vitamin D. Your skin makes vitamin D3 (cholecalciferol) in response to UVB rays. Your liver then converts it to 25-hydroxyvitamin D (calcidiol), which is what's measured on a blood test, and your kidneys convert that into the active form your body uses.
But don't be fooled: just because you spend time outside doesn't automatically mean you're making enough.
Many factors like season, latitude, time of day, skin pigmentation, sunscreen, clothing, and age all influence how much vitamin D your skin produces.
Here are some specific examples that may lead to reduced vitamin D production:
Exercising in the early morning or evening
Exercising fully covered in clothing
Blocking UVB rays with sunscreen
Limited time outside (e.g. exercise indoors)
Living at latitudes > 37° (e.g. Boston, SF, NYC, Chicago, Seattle etc) from late fall through early spring
Age > 70 (reduced ability to make it in the skin)
Kelly noted there's a lot of individual variation. After one summer, her graduate student's levels were much higher than her own, even without supplementing.
Kelly also emphasized being mindful of skin cancer risk, so this is a balancing act. A 15-minute walk between 10 a.m. and 2 p.m. with your sleeves (or pants) rolled up can help, and people with darker skin tones may need closer to 30 minutes. If you're going out for longer, apply sunscreen once you're outside.
The practical takeaway is that your vitamin D status depends on more than how sunny it is where you live. Your individual circumstances, diet, and seasonal habits all play a role.
What Are Signs of Vitamin D Deficiency?
This is where things get a little tricky. While there are clear signs in children (i.e. rickets), adults cannot reliably tell that they're vitamin D deficient based on how they feel.
For adult athletes, a history of bone stress injuries may be a reason to discuss vitamin D status with a healthcare professional. Things like bone pain, fatigue, frequent illness, and muscle discomfort may have other explanations, so I wouldn't automatically blame vitamin D.
Additionally, anyone with malabsorption issues (e.g. inflammatory bowel disease) is at risk of deficiency, as they may not be adequately absorbing any vitamin D they consume.
If you're concerned about your vitamin D status, a blood test is much more useful than playing a guessing game. It can also help determine whether supplementation is appropriate and, if so, how much you need to take.
Vitamin D Supplements for Athletes: Testing & Dosage
What is an "optimal" vitamin D level?
Different organizations and research studies use different thresholds for deficiency, insufficiency, and sufficiency. As Kelly pointed out, the National Academy of Medicine and the Endocrine Society have used different cutoffs, which is one reason you may find conflicting information online about what your vitamin D level “should” be.
In our conversation, Kelly discussed a range of approximately 75–100 nmol/L, equivalent to 30–40 ng/mL, as a reasonable target range for athletes. She also cautioned against assuming that higher is always better.
For context, vitamin D blood results are commonly reported in nanograms per milliliter (ng/mL) in the United States, while research may use nanomoles per liter (nmol/L). Here's what you'll likely see on your reports:
Deficiency: < 20 ng/mL
Insufficiency: 20-29 ng/mL
Sufficiency: ≥ 30 ng/mL
Remember that your results need to be interpreted as part of the bigger picture, and your treatment plan should be individualized.
When should you get your vitamin D tested?
Kelly and I recommend testing your vitamin D regularly, at least once a year and ideally twice. It's important to keep in mind that the time of year can affect what your results show.
Kelly explained that if you're only testing once a year, late winter or early spring can be useful because levels may be lower after months of reduced UVB exposure. Another option is late summer or early fall, which can give you a sense of your seasonal high.
If you have a history of bone stress injuries, other risk factors for deficiency (e.g. vegan diet) or you're considering taking a higher-dose vitamin D supplement, it's worth talking with your healthcare professional about getting tested.
How much vitamin D do you need daily?
There isn't one vitamin D dosage that's right for every athlete. It depends on your current level, diet, sun exposure, medical history, and whether you're correcting a deficiency or maintaining adequate levels. For reference, the RDA for most adults ages 19–70 is 600 IU per day (800 IU after age 70) from all sources combined.
Kelly explained that athletes heading into winter with adequate levels may need 1,000–2,000 IU of vitamin D3 daily to maintain them, while correcting a deficiency requires a higher dose for a set period. In one of her studies with Paralympic athletes, doses were based on starting levels:
Deficient: 50,000 IU per week for 8 weeks (about 7,000 IU per day), then a maintenance dose
Insufficient: 35,000 IU per week for 4 weeks (about 5,000 IU per day), then a maintenance dose
Sufficient: 15,000 IU per week (about 2,000 IU per day) for maintenance
By the end, 91% of athletes were sufficient, up from just 25% at the start. These doses were supervised, so work with your healthcare provider rather than replicating them on your own.
When it comes to vitamin D supplements for athletes, keep these in mind:
Choose D3: This form is more effective at raising your levels.
More isn't better: Excessive intake can cause toxicity.
Time of day doesn't seem to matter: Just take it with a meal or snack that contains fat to support absorption.
Retest: Levels can improve within 2–4 weeks, and 3 months is a common checkpoint.
As for vitamin D and K combination supplements, these nutrients work together in bone health, but that doesn't mean everyone needs both in supplement form. Kelly recommends starting with food sources of vitamin K, like leafy greens, fermented foods, meat, and eggs.
And if you're dealing with a bone stress injury, look beyond vitamin D. Calcium, magnesium, energy intake, and carbohydrates all matter, too.
As Kelly put it:
“We can look at all these things holistically versus just take vitamin D and it's going to solve everything, because that's not the case.”
Vitamin D supplementation can be useful when you need it, but the right dose depends on the individual, and taking more isn't automatically better. Testing and individualized guidance can give you a much better answer than guessing based on something you read online.
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