A recent headline-grabbing study seemed to settle the issue of low-carb versus low-fat diets definitively, reporting that a low-carb diet was not only more effective for weight loss, but also better for cardiovascular risk factors. In the yearlong study of 148 obese volunteers, those assigned to a low-carbohydrate regimen lost nearly eight pounds more and showed greater improvements in triglycerides, HDL cholesterol and a standard risk score for heart disease than a low-fat group. Both groups saw similar decreases in unhealthy LDL cholesterol. Researchers said the cardiovascular findings were notable because of concerns that low-carb diets might boost unhealthy cholesterol levels as dieters substitute fats for carbohydrates.
But Susan B. Roberts, PhD, Tufts professor of nutrition and founder of the online iDiet weight loss program <www.myidiet.com>, cautions, This study had quite a few flaws. In particular, they measured body composition with the bioelectrical impedance method, which is hopelessly inaccurate as a scientific tool. Plus, the dieters did not stick with the recommendations, and the low-carb group, in reality, became moderate-carb consumers.
With these qualifications, however, Roberts says, the message that moderately low-carb diets work better than low-fat diets for weight control fits with what we already know about carbohydrate and fat metabolism by the body.
COMPARING INTERVENTIONS: The study, published in Annals of Internal Medicine, instructed half of participants to limit digestible carbohydrates (total carbohydrates minus fiber) to less than 40 grams a day. Since baseline carb intake was 240 grams a day, this represented a sharp 75% decrease. The other participants were told to consume less than 30% of calories from fats and 55% from carbohydrates. Critics of the study have pointed out that since the low-fat group averaged just over 35% of calories from fat before the study began, this wasnt as significant a dietary intervention.
Fiber intake was similar between the two groups, at 15-16 grams a day, as was physical activity. Researchers described average calorie consumption also as similar between the groups, at 1,448 for the low-carb participants and 1,527 for the low-fat group. But critics noted that over the first few months of the study, when adherence was best, the low-carb dieters ate about 200 fewer calories daily. And over the yearlong study, the 7.7-pound difference between the groups weight loss-11.7 pounds on the low-carb diet versus 3.9 pounds on the low-fat diet-could be entirely explained by the small but significant 79 calories a day in average consumption, almost 29,000 total calories.
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SMART CHOICES: Given these issues with the study, what should people concerned about weight and cardiovascular disease take away from the headlines? First, as readers of this newsletter know, its true that science has long ago moved away from supporting low-fat diets. Its a mistake to lump all fats together in terms of nutrition, and the fad that led to substituting refined carbohydrates for total fats actually contributed to the current obesity epidemic. (See the November Special Supplement for more on smart fat choices.)
But its also true that not all carbohydrates are the same. Carbs include everything from candy to barley, snack chips to broccoli. Making smart choices about carbohydrates, emphasizing plant foods and whole grains while avoiding added sugars and processed foods, remains a better approach than wholesale avoidance of all carbs.
Roberts says the new study wont lead her to change recommendations about the balance of carbohydrates, fat and protein in her iDiet program, because she already supports a moderately (but not very) low-carb approach. If youre looking to lose weight, she says, strategies on hunger and cravings-replacing unhealthy habits with healthy ones-are the key, and diets that are very low-carb or very low-fat make that harder.
You may have also heard about another recent study, published in JAMA , that combined 48 prior trials of popular diets, such as Atkins and the Zone, and concluded that low-carb and low-fat diets were equally effective. It recommended people use any diet that they can stick with, says Tufts Roberts. But she discounts those results, noting, It includes all kinds of reports that should never be bundled together in a summary of this kind, including on people undergoing bariatric surgery and those eating food provide by the scientist for weeks on end. I think we cant conclude anything here.”