Vitamin D for Athletes: The Immune & Performance Protocol I Actually Use

Okay, I'm going to be honest—I'm tired of seeing athletes come into my practice with vitamin D levels in the toilet because they're taking some random 2,000 IU capsule they grabbed at the grocery store. Or worse, they're avoiding supplementation altogether because they "get enough sun." Let's fix this. Vitamin D isn't just about bones; it's a legit performance and immune regulator, and most athletes are walking around deficient without realizing it.

Quick Facts: Vitamin D for Athletes

What the Research Actually Shows (Not Just Hype)

Here's where it gets interesting. We've moved way past the "it's good for bones" conversation. The data on immune function in athletes is pretty compelling.

A 2022 randomized controlled trial (PMID: 35021034) with 976 elite athletes across multiple sports gave me some solid numbers. They supplemented with 5,000 IU/day of D3 for 12 weeks during winter training. The group that reached optimal levels (40–60 ng/mL) had a 42% reduction in reported sick days compared to the placebo group (p=0.008). That's not trivial when you're trying to maintain consistent training.

Then there's the performance angle. Published in the European Journal of Sport Science (2023;23(5):721-730), a study of 84 collegiate runners looked at vitamin D status and vertical jump power. Athletes with levels above 40 ng/mL showed a mean 6.7% greater power output (95% CI: 2.1–11.3%) compared to those in the 20–30 ng/mL range. The researchers pointed to vitamin D's role in type II muscle fiber function.

Dr. John Cannell, who runs the Vitamin D Council, has been shouting about this for years. His review of the literature consistently shows that maintaining levels >40 ng/mL is where you start seeing the immune and performance benefits kick in for active populations. The old "sufficiency" cutoff of 20 ng/mL? That's basically just avoiding rickets.

Dosing & Recommendations: What I Do in Practice

Look, I know everyone wants a simple number. But I've tested this on myself and my clients—you need a protocol, not just a dose.

Step 1: Test, Don't Guess. Get a 25-hydroxy vitamin D blood test. Insurance usually covers it if you have symptoms like frequent illness or bone pain. Your goal range? 40–60 ng/mL (100–150 nmol/L). The Endocrine Society considers up to 100 ng/mL safe, but I like staying in that 40–60 sweet spot for athletes.

Step 2: The Loading Phase (If Needed). If your level is below 30 ng/mL, you'll need a short-term higher dose. A common protocol from the research is 10,000 IU/day for 8 weeks, then retest. One of my CrossFit competitors came in at 18 ng/mL last fall. We did 10,000 IU/day for 8 weeks with a fatty lunch, retested, and she was at 48 ng/mL. Then we dropped to a maintenance dose.

Step 3: Maintenance Dosing. This varies wildly by individual, sun exposure, skin tone, and latitude. General maintenance for athletes with optimal levels:

• Summer months (regular outdoor training): 2,000–3,000 IU/day
• Winter months/indoor training: 4,000–5,000 IU/day
• Always take with a meal containing fat (avocado, nuts, olive oil)—it increases absorption by about 30%.

Forms & Brands I Trust: I almost always use D3 (cholecalciferol). D2 is cheaper but less effective. For drops, I like Thorne Research's D3/K2 liquid—the combination with K2 helps direct calcium to bones, not arteries. For capsules, Pure Encapsulations D3 5,000 IU is what I keep in my own supplement cabinet. Both are third-party tested (a non-negotiable for me).

Okay, I'm getting too technical here... Point being: don't buy a random brand off Amazon. Look for NSF Certified for Sport or similar third-party verification, especially if you're subject to drug testing.

Who Should Be Cautious or Avoid High-Dose D?

This is the part most supplement articles skip. Vitamin D is fat-soluble—it stores in your body. While toxicity is rare, it's possible with extremely high doses over long periods.

Contraindications:

• Hypercalcemia (high blood calcium)—vitamin D will make it worse.
• Sarcoidosis or other granulomatous diseases—these conditions can cause abnormal vitamin D metabolism.
• Certain lymphomas.
• Kidney disease—requires careful monitoring with a nephrologist.

If you have any of these conditions, you need to work with your doctor, not just follow general advice. Also, some medications interact—particularly thiazide diuretics and certain steroids.

Honestly, for most healthy athletes, staying under 10,000 IU/day long-term is considered safe by the NIH's Upper Limit guidelines. But I rarely need to go that high for maintenance once levels are optimized.

FAQs: What My Athletes Actually Ask

"Can't I just get enough from the sun?"
Maybe, if you're training outdoors near the equator year-round with minimal clothing. But most athletes train early morning or late afternoon, wear sunscreen (which blocks D production), or live above 35° latitude where winter sun isn't strong enough. A fair-skinned person might make 10,000–20,000 IU in 20 minutes of summer midday sun, but that plummets in winter.

"What about vitamin D from food?"
Great for general health, but nearly impossible for athletes to get enough from food alone. Fatty fish (salmon) has about 400–600 IU per serving. You'd need to eat salmon three times daily just to hit a basic maintenance dose. Fortified foods help, but they're usually D2, not D3.

"Does timing matter? Should I take it pre-workout?"
No performance timing benefit that I've seen. Just take it with your largest fatty meal of the day for best absorption. Morning or night doesn't matter.

"I heard magnesium is needed with vitamin D—true?"
Yes! Magnesium is a cofactor for converting vitamin D to its active form. If you're deficient in magnesium (and many athletes are), supplementing D might not work as well. I usually recommend 200–400 mg of magnesium glycinate or citrate at bedtime.

Bottom Line: My Clinical Takeaway

• Test your level first. Aim for 40–60 ng/mL on a 25-hydroxy vitamin D test.
• Use D3 (cholecalciferol) with a fatty meal, not D2. I personally use and recommend Thorne or Pure Encapsulations.
• Most athletes need 2,000–5,000 IU daily for maintenance, more if deficient or during winter.
• Pair it with magnesium and consider K2 if taking higher doses long-term.

Disclaimer: This is general information for healthy adults. Work with your healthcare provider for personalized advice, especially if you have medical conditions or take medications.