The vitamin D deficiency most men don't know about

Vitamin D acts more like a hormone than a vitamin. Receptors sit on cells throughout the body — including immune cells and energy-regulating tissues.

The deficiency

Most adults in northern climates produce essentially no vitamin D from sunlight between October and April. Food sources are limited (fatty fish is the only meaningful one). Most modern diets don't include enough.

The fix

Get tested. The 25-hydroxyvitamin D blood test costs $15–30. Most men in northern climates need 2,000–3,000 IU/day. Take with food containing fat — D is fat-soluble.