The Science Behind 9 Big Diets

From low-fat to paleo, here's what the latest research says about diets' effect on weight and health.

1. Low-carb

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Some research has found that high-fat, high-protein, low-carb diets help people burn more calories than low-fat variants. However, a large study found that such diets increased women's risk of heart disease. The evidence for low-carb is decidedly mixed.

2. Low-fat

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This old standby is not looking that awesome these days. The same study that praised the low-glycemic-index diet found that people on low-fat diets burned the fewest calories of any sampled dieters, and also experienced an increase in potentially unhealthy insulin resistance. Even Dean Ornish, a low-fat pioneer, is now advocating a more mixed approach that's low in processed foods and refined carbs as well as fat.

3. Low Glycemic Index

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This diet revolves around minimally-processed foods whose carbohydrates digest slowly (like whole barley or bulgur, as opposed to white bread). A 2012 study found that it might have more beneficial effects on metabolism than a low-fat diet, without some of the risks of low-carb. According to the study author, it also might be easier to stick to, because "unlike low-fat and very-low carbohydrate diets, a low-glycemic-index diet doesn't eliminate entire classes of food."

4. Dessert With Breakfast

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In one study, people who ate a big breakfast with a "dessert," like a doughnut or piece of cake, lost weight and kept it off better than those who ate a more conventional low-carb diet. The researchers think the dessert reduced cravings for sweets later in the day. A caveat: the dessert-with-breakfast diet was still very low-calorie, at just 1,400 a day for women.

5. Sugar-free

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Cutting out sugar is trendy right now — Tom Hanks and Alec Baldwin are supposedly doing it, and at least one crusading doctor is touting a reduction in sugar as a cure for obesity and diabetes. One 2012 study found that drinking sugary drinks might strengthen people's genetic risk of obesity. However, other experts have been skeptical that cutting out sugary drinks will really make that much difference, and research on sugar-free or low-sugar diets isn't yet as advanced as research into other approaches.

6. Mediterranean

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The Mediterranean diet, high in fish and veggies and low in meat, has been getting good press for years. A 2011 Swedish study found that people who followed it lived longer than more carnivorous types. A form of the diet high in olives and olive oil also appears to be good for bone health. One caveat: this diet may be harder for low-income people to follow, since foods like fish can be expensive.

7. Gluten-free

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This diet is great for people with celiac disease, who have a variety of bad reactions to gluten. For others, there may not be any benefit. Several experts caution that going gluten-free doesn't necessarily lead to weight loss.

8. Paleo

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An effort to approximate what humans ate in paleolithic times, this diet centers on meat and vegetables and bans most grains and sweets. Most experts say there's not yet enough data to determine whether it affects weight or health. One biologist ">points out that human bodies are actually capable of adapting to their circumstances, so that eating the way we did thousands of years ago may not make any sense.

9. No Diet

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Lots of research (and anecdotal evidence) has shown that it's hard to keep weight off — according to one study, just one in six Americans who have ever been overweight is able to maintain significant weight loss. And Dean Ornish is now advocating that everyone eat with an eye to health — not necessarily weight loss.