Yet Another “Which Diet Plan Is Best” For Weight Loss” Study: Anything New?

This week’s New England Journal of Medicine (Volume 359, No. 3, July 17,2008) contained an article entitled “Weight loss with low-carbohydrate, Mediterranean, or low-fat diet,” written by Iris Shai and colleagues. Although the study was conducted in Israel, it was a large international effort with co-investigators from all around the globe. The study results even made the New York Times (“Healthy diets shown to have benefit despite modest weight losses,” written by Tara Parker-Pope, Thursday, July 17, 2008, page C3).

The Study Design

The purpose of the study was to evaluate the effectiveness of several different diet plans long-term with an effort to minimize “drop-outs.” The investigators claimed (correctly) that most previous studies had been hampered by their short-term nature and/or large drop-out rates. The plan was to recruit slightly obese subjects from a single work-site and monitor them for 2 years. The investigators recruited 322 subjects willing to be randomly assigned to 1 of 3 dietary regimens: a low-fat, restricted calorie plan (group 1) ; a Mediterranean, restricted calorie plan (group 2) ; and a low-carbohydrate, non-restricted calorie plan, based on the Adkins diet (group 3). The mean age of study subjects was 52 years and the mean BMI was 31 (I’m sure you remember from previous entries that the BMI is calculated by dividing the weight in kilograms by the height in meters X height in meters; normal is <24.9, overweight is 25-29.9, and obese is 30 or more). Eight-six percent of subjects were male. The study subjects had intensive education regarding the diet plans and frequent follow-up.

The Results

First, adherence to the regimens (I think that meant that the study subjects showed up for their follow-up visits?) was pretty good, about 95% after 1 year and 84.6% after 2 years (272 out of 322 subjects). For each study group the maximum weigh loss was between 1-6 months into the study. At the end of the study, the mean weight loss in subjects who completed the study was 3.3 kg, 4.6 kg, and 5.5 kg in groups 1-3, respectively. The mean BMI fell by 1 in group 1 and 1.5 in groups 2 and 3. There was a difference in results comparing males and females; the 45 females in the study lost 0.1 kg in group 1, 6.2 kg in group 2, and 2.4 kg in group 3.

The investigators found no particular safety problems with any of the diet plans. The subjects with diabetes mellitus fared best with the Mediterranean diet plan in terms of improved fating plasma glucose values (the improvements in hemoglobin A1cs were about the same in the 3 groups, about 0.5%). Lipid profiles showed modest improvements with all 3 diet plans but group 3 (the low-fat diet group) generally showed the greatest improvements.

The investigators concluded that based on their results, Mediterranean and low-carbohydrate diets may be effective alternatives to “classic” low fat diets. They also concluded that a workplace medical care model for weight reduction such as in their study might be a useful way to achieve beneficial results and that even modest weight reductions can improve risk factors for undesirable health outcomes. The investigators did not really focus on the benefits of one diet plan vs. another (I do wonder what the first draft of the paper submitted to the journal looked like, but we’ll never know).

What do the results mean?

I found it somewhat difficult to interpret the results. First, the investigators should have presented the male and female data separately given the fact that 86% of the subjects were male but also given the results in the 45 female participants; the females lost quite a bit more weight on the Mediterranean diet (average of 6.6 kg) than on either of the other 2 plans. In fact, the females lost only a trivial amount on the other 2 plans. I have no idea what the data mean except that we should think twice before trying to generalize the study results to males and females alike.

The investigators are to be commended for having a pretty low drop-out rate but even 15% is of concern. In addition, the data showed that the drop-out rate differed among the 3 groups- it was highest in the low-carbohydrate group (22%) and lowest in the low-fat group (9.6%) with the differences statistically significant.

Finally, even two years of follow-up is far too brief a period of time to come to any firm conclusions about long-term effects of the of the 3 diet plans with respect to efficacy and safety.

What do the results really mean?

If one takes a hypothetical study subject with a BMI of 31, it might be a male about 175 cm tall (almost 69 inches tall) who weighs 95 kg (209 lbs). To attain a weight at the upper limit of the “normal” BMI range (24.9) he would need to lose about 18 kg, far more than was achieved in the study conducted by Shai and colleagues. Clearly, even heroic efforts to achieve weight loss by dietary manipulation as undertaken by the investigators still fall far short of what is desirable. But, still, every little bit helps. I do wonder what the results would have been in study subjects with much higher BMIs?

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