But the psychological or spiritual effect can't be discounted, says Dillard. "People love the idea of cleansing, of purification rituals, going to the Ganges, to the spa. It has powerful psychological, religious, spiritual meaning. That has its own positive effect on health. But we need to separate that from saying this is science or good medicine."
Some still consider fasting -- in any form -- to be "out there." "When I review diets that are not based on science, the question I ask myself is: Would I feed them to my family? In this case, the answer is a clear no," says Susan Roberts, PhD, chief of the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging and a professor of nutrition at Tufts University in Boston.