The notion that you could lose weight by cutting out carbohydrates and eating plenty of protein was once tut-tutted by the medical establishment, partly because such diets were based on little more than interesting ideas and speculation. In the past few years, head-to-head trials that pitted high-protein, low-carbohydrate diets against low-fat, high-carbohydrate diets have provided some evidence that a low-carbohydrate diet may help people lose weight more quickly than a low-fat diet, although so far, that evidence is short term.
In two short, head-to-head trials, low-carb approaches worked better than low-fat diets. A more-recent year-long study, published in 2007 in the Journal of the American Medical Association, showed the same thing. In this study, overweight, premenopausal women went on one of four diets: Atkins, Zone, Ornish, or LEARN, a standard low-fat, moderately high-carbohydrate diet. The women in all four groups steadily lost weight for the first six months, with the most rapid weight loss occurring among the Atkins dieters. After that, most of the women started to regain weight. At the end of a year, it looked as though the women in the Atkins group had lost the most weight since the start of the study, about 10 pounds, compared with a loss of almost 6 pounds for the LEARN group, 5 pounds for the Ornish group, and 3½ pounds for the Zone group. Levels of harmful LDL, protective HDL, and other blood lipids were at least as good among women on the Atkins diet as those on the low-fat diet.
If you read the fine print of the study, though, it turns out that few of the women actually stuck with their assigned diets. Those on the Atkins diet were supposed to limit their carbohydrate intake to 50 grams a day, but they took in almost triple that amount. The Ornish dieters were supposed to limit their fat intake to under 10 percent of their daily calories, but they got about 30 percent from fat. There were similar deviations for the Zone and LEARN groups.
What about longer term studies? POUNDS LOST (Preventing Overweight Using Novel Dietary Strategies), a two-year head-to-head trial comparing different weight loss strategies found that low-carb, low-fat, and Mediterranean-style diets worked equally well in the long run, and that there was no speed advantage for one diet over another. What this and other diet comparisons tell us is that sticking with a diet is more important than the diet itself. (Read more about the POUNDS LOST weight loss trial.)
Why, in some studies, do high-protein, low-carb diets seem to work more quickly than low-fat, high-carbohydrate diets, at least in the short run? First, chicken, beef, fish, beans, or other high-protein foods slow the movement of food from the stomach to the intestine. Slower stomach emptying means you feel full for longer and get hungrier later. Second, protein’s gentle, steady effect on blood sugar avoids the quick, steep rise in blood sugar and just as quick hunger-bell-ringing fall that occurs after eating a rapidly digested carbohydrate, like white bread or baked potato. Third, the body uses more energy to digest protein than it does to digest fat or carbohydrate.
No one knows the long-term effects of eating high-protein diets with little or no carbohydrates. Equally worrisome is the inclusion of unhealthy fats in some of these diets. There’s no need to go overboard on protein and eat it to the exclusion of everything else. Avoiding fruits and whole grains means missing out on healthful fiber, vitamins, minerals, and other phytonutrients. It’s also important to pay attention to what accompanies protein. Choosing plant-based high-protein foods that are low in saturated fat will help the heart even as it helps the waistline.