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And as Emily Harris just mentioned, some services offered at spas aren't backed up by a lot of hard science. Fasting is one of the more controversial treatments.

NPR's Patty Neighmond looks at what happens to the body when it's deprived of food.

PATTY NEIGHMOND: Naomi Neufeld is an endocrinologist at UCLA. Neufeld says most adults need about 2,000 calories a day. But it doesn't hurt, she says, to fast, to stop eating for short periods of time, say 24 hours once a week or so, as long as you drink water.

Dr. NAOMI NEUFELD (Endocrinologist, UCLA): You re-tune your body. You suppress insulin secretion. You reduce the taste for sugar, so sugar becomes something that you're less fond of taking.

NEIGHMOND: Eventually, the body burns up stored sugars and so less insulin is needed to help the body digest food. That gives the pancreas a rest. On juice diets recommended by some spas, you may lose weight, but your digestive system doesn't get a rest.

Mark Mattson is a scientist with the National Institute on Aging. He says that when we convert food into energy, our bodies create a lot of by-products we could do without, including free radicals.

Dr. MARK MATTSON (Scientist, National Institute on Aging): And these free radicals will attack proteins, they'll attack DNA and the nucleus of cells, and they'll also attack the membranes of cells and damage all of those different molecules in the cells.

NEIGHMOND: And even if you don't fast, regularly limiting the calories you consume may be beneficial. Mattson says that in studies where rats and mice were fed every other day, there was a reduction in disease. He says those findings hold promise for humans who may partake in partial fasting.

Dr. MATTSON: It can improve glucose regulation and protect against diabetes. It can have beneficial effects in the - on the heart and lowering blood pressure. And in our lab, at least in the animal studies, has - have very beneficial effects on the brain protecting in our animal models that are relevant to Alzheimer's and Parkinson's and stroke.

NEIGHMOND: Even extend life span. And it turns out that eating less sends a message to the cells to conserve and use energy more efficiently.

Dr. MATTSON: When they're exposed to a mild stress, they're sort of expecting that maybe this is going to happen again, maybe next time the animal that -which I'm living in is going to have to go longer without food. And so I'd better be able to deal with that when it comes on.

NEIGHMOND: Similar to the way muscles get built up when they're stressed by exercise. But these are benefits from partial fasting. It's far less clear what can be gained by complete water-only fasting. Proponents claim it lowers blood pressure and reduces cancer risk, but scientists have yet to prove that.

Dr. Naomi Neufeld says long-term fasting, going for days with just water, can be harmful, that's because after the first few days of liquid only, the body uses up all its stored glucose to make energy. And then it turns to other sources, including fat and muscle.

Dr. NEUFELD: The main tissue that's the target in long-term fasting is muscle, because muscle has readily available amino acids that can be converted to glucose right away. And those can happen within minutes, so that your brain is never deprived of glucose.

NEIGHMOND: And when muscles break down, they release proteins which are partly composed of nitrogen. Too much nitrogen can be toxic to the kidneys and liver, that's when starvation is officially underway. And that's why today, in most religious fasts, people don't go without food more than 12 to 24 hours.

Patty Neighmond, NPR News.

NORRIS: There are more stories from our International Medicine series at npr.org/health.

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