Vitamin D and Bone Health: What Triathletes Need to Know

Endurance athletes — runners, cyclists and swimmers alike — tend to have lower bone density than athletes from strength or ball sports, and in some studies, lower than inactive people altogether. Vitamin D is one of the more fixable pieces of that picture, and it’s a particular issue for anyone training through a UK winter.

Why Vitamin D Matters for Bone Health

Vitamin D helps your body absorb calcium and regulates bone turnover. When levels run low, bone turnover increases and bones become more prone to stress injuries — a meaningful concern for triathletes given the impact loading from running, especially during high-volume run blocks or a shift onto harder surfaces. Research on stress fractures has flagged a vitamin D level below roughly 75.8 nmol/L as a risk factor, with a widely-used target of getting levels above 50 nmol/L if you’re deficient or insufficient. One well-known supplementation trial in female recruits found an 800 IU/day vitamin D dose over eight weeks cut stress fracture incidence by 20% compared with placebo.

The UK Sunlight Problem

Your body makes most of its vitamin D from sunlight on skin, and in the UK, the sun isn’t strong enough to do that between roughly October and March. NHS guidance for the general public recommends considering a daily supplement of 10 micrograms (400 IU) of vitamin D during autumn and winter, and year-round for anyone who gets limited sun exposure — which describes plenty of triathletes doing early-morning pool sessions and evening turbo rides in the dark.

Who’s Most at Risk

Anyone training indoors through winter with minimal outdoor daylight exposure.
Athletes with darker skin, who need more sun exposure to produce the same amount of vitamin D.
Anyone already dealing with low energy availability or RED-S — the two issues compound each other’s effect on bone health.
Runners and triathletes with a history of recurrent stress fractures.
Women, who carry a documented higher baseline stress fracture risk than men.

When to Get Tested

If you’ve had a stress fracture, feel persistently fatigued through winter, or simply want to know where you stand, your GP can arrange a blood test for serum vitamin D. Over-the-counter supplements at the standard 10mcg/400 IU dose are considered safe for most adults without testing first, but if you’re considering a much higher dose, talk to your GP or a sports dietitian rather than guessing — very high vitamin D intake over long periods can cause problems of its own.