Harvard researchers wanted to find out which diet was most successful at keeping the weight off. So 21 people were asked go on three different eating plans to see which worked best.
Each subject tried the low-carbohydrate diet, similar to the Atkins Diet; the low-fat diet, what the American Heart Association recommends; and the low-glycemic index diet, which attempts to keep blood sugar levels stable.
"Glycemic index is actually sort of hard for patients to understand. But if you tell them, 'By the way, we just want you to eat foods that haven't been processed,' it's easier for people to understand," said internal medicine expert Dr. John De Beixedon. "So I usually call that a 'unprocessed' or 'low-processed' or 'whole food' or 'natural food' type of diet."
The result: low-carb and low glycemic diets were better than low fat.
"If you're looking at the way that foods are utilized by the human body, if you go on a low-fat diet, then your body says 'Hey wait, I'm starving,'" said Dr. De Beixedon. "And when it says 'Hey wait, I'm starving,' it actually tries to hold on to fat."
Although eating a low-carb diet burned the most calories, it increased markers of stress and inflammation, which is not healthy.
The bottom line: Study authors recommend eating the low-glycemic way.
"And that is the way that the body has been designed," said Dr. De Beixedon. "It's actually been designed for low-glycemic foods."
Dr. De Beixedon says whole foods like fruits, vegetables, lean proteins and whole grains are what humans were meant to eat.
But whole grain does not mean bread, no matter what the label says.
"Really, honestly, even though you hear when they say 'whole grain wheat' -- any bread has been processed," said Dr. De Beixedon.
While some purists argue you can lose weight on any kind of food if you restrict your calories, not all calories make the cut.
Experts say while the research challenges the notion "all calories are created equal," it's not definitive.
At end of the day, if it's just about weight loss, common sense still wins out: Eat fewer calories than you burn by exercising.