A study was just published in the New England Journal of Medicine on July 17th comparing the effectiveness and safety of three different weight loss diets. 322 moderately obese subjects were assigned to one of three diets: low-fat, restricted-calorie; Mediterranean, restricted-calorie; or low-carbohydrate, non-restricted calorie.
The rate of adherence to the study diet was 95% at year one and 85% at year two. Among the 272 participants who completed the intervention, the mean weight losses were 3.3 kg for the low-fat group, 4.6 kg for the Mediterranean-diet group, and 5.5 kg for the low-carbohydrate group.
Perhaps more significantly, the relative reduction in the ratio of total cholesterol to HDL was 20% in the low carbohydrate group while only 12% in the low-fat group. Among the 35 subjects with diabetes, changes in fasting plasma glucose and insulin levels were more favorable among those assigned to the Mediterranean diet than among those assigned to the low-fat diet.
Unfortunately, the bias against saturated fat and animal products that is still so prevalent in the mainstream (in spite of the lack of evidence to support it) prevailed in this study. The research team advised those following the low-carb diet to “choose vegetarian sources of fat and protein” and moderate their consumption of saturated fats and meat.
This suggests that the low-carb dieters may have consumed a substantial portion of their calories as fat in the form of omega-6 polyunsaturated fatty acids. Excess intake of omega-6 fatty acids contributes to a host of problems including heart disease, diabetes, and cancer; but even more relevant to this study and its results is the fact that omega-6 fatty acids can cause increased water retention. And as everyone knows, increased water retention equals increased weight.
This certainly causes me to wonder how much more dramatic the results of this study might have been if the low-carb subjects were encouraged to significantly restrict their consumption of omega-6 fats (which cause water retention, and thus weight gain) and replace them with saturated fats (which do not cause water retention). What is remarkable is that in spite of the consumption of omega-6 fats, the low-carb group still lost more weight than both the low-fat and Mediterranean groups. That’s a strong endorsement for the benefits of a low-carb diet for weight loss.
The low-carb and Mediterranean (to a lesser degree) diet also had other benefits beyond promoting weight loss and improving cholesterol measures. The level of high-sensitivity C-reactive protein decreased significantly only in the Mediterranean and ow-carb group, with the low-carb group again showing the greatest decrease (29% vs. 21%). C-reactive protein is a measure of inflammation that has been positively correlated with heart disease in recent studies. Once again, one must wonder if the reduction would have been even greater in the low-carb group had the subjects been told to restrict their intake of omega-6 fats, which are known to promote inflammation.
Another interesting finding is that although caloric intake was only restricted in the low-fat and Mediterranean diet groups, the low-carb group also ended up eating fewer calories during the diet. Many people who follow a low-carb, high protein/high fat diet find that they spontaneously eat less because additional protein, and in particular fat, leads to greater levels of satiety (satisfaction).
One limitation of the study is that it relied on self-reported dietary intake (this is true of almost every dietary study except those performed in tightly controlled conditions, such as an inpatient facility). However, the study was somewhat unique in that it was conducted in a workplace at a research center with an on-site medical clinic. It also had several other strengths. The drop-out rate was exceptionally low for a study of this kind; all participants started simultaneously; the duration was relatively long (2 years); the study group was relatively large; and the monthly measurements of weight remitted a better understanding of the weight-loss trajectory than other studies.