Vitamin D.

Vitamin D may prevent cancer (Scientific journal abstract here.)

A daily dose of vitamin D could cut the risk of cancers of the breast, colon and ovary by up to a half, a 40-year review of research has found. The evidence for the protective effect of the "sunshine vitamin" is so overwhelming that urgent action must be taken by public health authorities to boost blood levels, say cancer specialists. . . .Heart disease, lung disease, cancer, diabetes, high blood pressure, schizophrenia and multiple sclerosis are among the conditions in which it is believed to play a vital role. The vitamin is also essential for bone health and protects against rickets in children and osteoporosis in the elderly.

Reduced exposure to sunlight has depleted natural Vitamin D supplies in many people.