Imagine a treatment that could build bones, strengthen the immune system and lower the risks of illnesses like diabetes, heart and kidney disease, high blood pressure and cancer. Some research suggests that such a wonder treatment already exists. It’s vitamin D, a nutrient that the body makes from sunlight and that is also found in fish and fortified milk. Doctors are increasingly testing their patients’ vitamin D levels and prescribing daily supplements to raise them. But don’t start gobbling down vitamin D supplements just yet. The excitement about their health potential is still far ahead of the science. Although numerous studies have been promising, there are scant data from randomized clinical trials. Little is known about what the ideal level of vitamin D really is, whether raising it can improve health, and what potential side effects are caused by high doses. And since most of the data on vitamin D comes from observational research, it may be that high doses of the nutrient don’t really make people healthier, but that healthy people simply do the sorts of things that happen to raise vitamin D.