It’s perhaps one of the most contentious issues in nutrition: should individuals who want to control their weight eat a diet low in fat, or low in carbohydrate? The ‘conventional’ view is that fat is ‘calorific’ and fattening, so low-fat is the way to go. However, some (e.g. Gary Taubes and the late Robert Atkins) have argued that it’s carbohydrate that, essentially, makes us fat, so we should be reigning in our consumption of such foods. One way to settle this argument is to study the effects of these diets on weight loss, and this week saw the publication of a study in which a low-fat was pitted against a low-carb diet over a period of two years. This study also assessed the effects of a so-called ‘Mediterranean diet’ too.
Participants eating the low-fat and Mediterranean diets were asked to restrict calories (1500 and 1800 calories per day for women and men respectively). Individuals on the low-carb diet could eat as much as they liked. For full details of the 3 diets in this trial, see the free full text article.
322 individuals were enrolled in the study, of whom 272 completed it. Of those who completed the study, the average weight losses were:
Low-fat diet group ” 3.3 kg
Mediterranean diet group ” 4.6 kg
Low-carb diet group ” 5.5 kg
The participants of the study also had certain blood parameters checked, including blood fat levels. In medicine, the ratio of total cholesterol (generally taken to be a bad thing) to so-called HDL (generally taken to be a good thing) cholesterol is believed to be a marker for the risk of cardiovascular disease: the higher the ratio, the greater the risk. Individuals on the low-carb diet saw a reduction in this ratio which exceeded that achieved on the low fat diet. Compared to the low-fat group, the low-carb group also saw a statistically significant drop in their levels of unhealthy blood fats known as ‘triglycerides’.
In short, compared to the low-fat group, the low-carb group lost more weight and saw improved changes in their blood fat levels.
At the start of the study, 36 participants had been diagnosed with diabetes. These (as well as other) participants had their fasting blood sugar levels checked as part of the study. Compared to the diabetics eating a low-fat diet, those eating a Mediterranean diet experienced a statistically significant reduction in their blood glucose levels, but the low-carb group did not. I find this result somewhat surprising, seeing as there is quite a lot of evidence now which shows carbohydrate restriction can be effective for helping to control blood sugar levels in diabetes.
However, it is perhaps worth pointing out that at the start of the study, more than three-quarters of the diabetics in the low-carb group were on medication for their diabetes, compared to 50 and 47 per cent of diabetics in the low-fat and Mediterranean diet groups respectively. In other words, it is possible that, overall, the diabetics in the low-carb group had more advanced disease which may not have been so amenable to a nutritional approach.
What this study does do, I think, is add to the body of evidence which suggests that lower carb/carb-controlled eating has distinct merit, and generally has the capacity to out-perform low-fat diets in the weight loss stakes (and without any conscious restriction of food intake, either). The results of this study suggest that such a diet may be superior in terms of cardiovascular disease risk too.
Shai I, et al. Weight Loss with a Low-Carbohydrate, Mediterranean, or Low-Fat Diet. NEJM 2008;359:229-241