A growing body of evidence strongly suggests that vitamin D in high doses not only helps keep bones strong but also reduces the risk of colon, ovarian, and breast cancers, and diseases such as diabetes and multiple sclerosis. And many of us don't get enough because of a lack of exposure to sunlight (the sun triggers D's production in the skin) or diets that omit good sources (fatty fish such as salmon, mackerel, and tuna, and fortified milk and cereal).

As long as you are in good health, a brief liquid fast or cleanse is fine. But don't expect wonders—other than a sense of personal accomplishment, perhaps: Any physiologist will tell you that properly functioning lungs, liver, kidneys, and intestines do a fantastic job of keeping your body free of impurities without the help of fasting.