The recommended level for vitamin D maintenance is 2,000 vitamin D IU per day. The good news is that this supplement is relatively inexpensive and can be purchased for less than two dollars per month (Now Foods Vitamins is one option).
Consider supplementing unless you a health condition that predisposes you to one of the following conditions:
- Hypercalcemia (high calcium in the blood)
- Granulomatous disease
- Lyme disease
- Kidney disease or are taking the diuretic known as hydrochlorothiazide (HCTZ), a "water pill" used to treat high blood pressure and fluid retention caused by a range of conditions, including heart disease.
The average healthy body synthesizes 10,000 to 15,000 IU of vitamin D3 in the skin within minutes of sun exposure without ill effect. We have a natural system of checks and balances within the body that shut off vitamin D buildup once we get enough. The half-life of 25 (OH) D is 15 days. This means that sun exposure once every 3 to 4 days is adequate.
Safe sun exposure recommendations suggest to expose only the arms, legs and back for 5 to 30 minutes at close to solar noon twice weekly without sunscreen. Never stay in the sun unprotected long enough to get burned.
Since most cyclists are in the sun for much longer periods of time than this, the following list may be one of the causes for a vitamin D deficiency:
- Train indoors or outdoors in a clouded or polluted environment.
- Live at latitudes greater than 35 degrees north between the middle of October and the middle of February.
- Wear sunscreen or protective clothing regularly.
- Have dark or very light pigmented skin.
- Have excess body fat.
- Have a malabsorption disorder.
- Are genetically predisposed.