Here's something Dr. Kevin Sprouse — team physician for EF Education First Pro Cycling and one of the most respected sports medicine doctors in endurance athletics — told us when we asked about daily supplements for serious athletes: he's seen very few elite athletes with sufficient vitamin D levels. Not low. Not borderline. Straight-up insufficient. Thorne Vitamin D is the go-to D3 supplement recommended by Dr. Sprouse as the foundation of any athlete's daily supplement protocol — and it's why it's been part of The Feed's athlete stack from the beginning. The very habits that make you an athlete — early morning training, sunscreen on long rides, indoor workouts — are exactly what cuts off your primary vitamin D source. And the RDA wasn't set for someone logging 15-hour training weeks. Vitamin D functions more like a hormone than a standard vitamin, involved in muscle function, immune response, calcium absorption, and recovery. When your levels are low, your ceiling drops: slower recovery, more frequent illness, weaker performance when it matters most. D3, Not D2 — The Form Matters Thorne uses D3 — the same form your skin produces from sunlight, and the form research shows is more potent and better absorbed. NSF Certified for Sport — Clean for Tested Athletes Every batch of Thorne D-5,000 is independently tested to verify label accuracy and screened for nearly 300 banned substances. No surprises. No guessing. Two Doses, One Decision D-5,000 (5,000 IU) is the standard daily maintenance dose for most athletes. D-10,000 (10,000 IU) is for athletes who've tested low or need to correct a confirmed deficiency. Start with bloodwork — it tells you exactly which dose you need. For athletes, adequate vitamin D supports lean muscle mass, immune consistency, calcium absorption for high-impact bones, and cardiovascular function. Thorne's formula is pure D3 in a clean capsule — no lactose, no preservatives, nothing you don't need. A small amount of vitamin E is included to stabilize the D3 for full potency. How to Use Thorne Vitamin D Daily dose: Take 1 capsule daily with a meal containing fat — vitamin D is fat-soluble, so taking it with food meaningfully improves absorption. Choosing your dose: D-5,000 IU is the standard starting point for most athletes. D-10,000 IU is better suited for correcting a confirmed deficiency or for athletes in colder climates with limited winter sun exposure. When in doubt, get bloodwork first. Pro tip: Take it consistently in the morning with breakfast. Pair it with an omega-3 for enhanced absorption and synergistic anti-inflammatory benefits — exactly the combo Dr. Sprouse recommends as part of his Athlete Daily Protocol.
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