Thursday, July 17, 2008

Among Diabetics, only the Mediterranean diet decreased plasma glucose?

The recent New England Journal of Medicine article comparing long term results with low fat, Mediterranean and low carb diets published yesterday, and made quite a splash in the media. The article is free to the public (Thank you NEJM!) As usual, either the authors or the media, in their fervor to get a "message" out to the public, screwed it up again. The message everyone seems to be taking from this study is that Mediterranean diets are better for diabetics than low fat or low carb diets, the other two diets tested, when that is not what the article says at all!

It doesn't help that the authors state in the abstract that

"Among the 36 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..."

Note that they did NOT say "more favorable than among those assigned to the low-carb diet." Was this a concession to mainstream medicine? Did they feel that they would take too much flak or not get funding for their next study if they went too far in supporting low-carb "fad" diets?

It is beyond me why they did not state in the abstract that those on low carb diets had the lowest glycated hemoglobin levels, since we know that glycated hemoglobin levels are a much better measure of average blood glucose levels and blood glucose control than a few isolated fasting glucose values. This seems to me to be one of the strongest points from a public health perspective, and it needs to be said louder.

Think I am hallucinating? Reading into the data what I want to see? Right there on page 235 (and continuing on page 238) the article says:

"Among the participants with diabetes, the proportion of glycated hemoglobin at 24 months decreased by 0.4±1.3% in the low-fat group, 0.5±1.1% in the Mediterranean-diet group, and 0.9±0.8% in the low-carbohydrate group. The changes were significant (P<0.05) only in the low-carbohydrate group (P = 0.45 for the comparison among groups). I suspect that Mary Vernon and Eric Westman will have similar observations (see their their earlier thoughts on the ACCORD trial and this one (with a hoard of other authors too). (Thank you for pushing the issues with mainstream medicine!)

The authors of the NEJM article calculated the "homeostasis model assessment of insulin resistance" (HOMA-IR) according to the following equation (see Diabetes Care 1997;20: 1087-92) as follows : insulin (U/ml) × fasting glucose (mmol/liter) ÷ 22.5. Their observations about HOMA-IR are interesting, and serve as the basis for their conclusion that the Mediterranean diet worked better for diabetics than the low fat diet. What they again fail to state (though it is also shown in Fig. 4F) is that the HOMA-IR improved most on the low carb diet in non-diabetics. This point is certainly relevant from a public health perspective for diabetes prevention, is it not?

Another curiosity is that the low carb dieters were counseled to choose vegetable sources of fat and protein (ostensibly to reduce trans-fats—Since when is animal fat a significant source of trans-fats?)? I'd like to know what if any effect this urging had on the dieters, and what effect it had on their lipid levels and other parameters measured (if those measures are even relevant…).

Ranting aside, this was a nicely done study, with better control than in many, and for a longer period of time. It should end the speculation and criticism that low carb diets show short term benefit only. It's clear that the weight losses were much better at 5–6 months for both low carb and low fat diets, and that relaxation of dietary stringency most likely resulted in weight re-gains. Certainly, the increase in triglycerides in the low carb group demonstrates an increase in carb consumption (despite the higher fat intake during the more stringent phase) and is likely responsible for the weight re-gains. The Mediterranean diet seemed easier to maintain, judging by the better weight maintenance, but it's not clear why this should be.

The authors were not cagey at least about the more favorable effects on lipids with the low-carb diet, and even with less compliance and some weight re-gain in later phases of the diet, these results were maintained.

I am left with my usual confusion over what constitutes a "Mediterranean diet" though—as described, it seems that the only significant difference between the "low-fat" diet and the "Mediterranean diet" was the substitution of olive oil for some of the meat fat (and a slight increase in the total amount of fat). Does substitution of 5–7 nuts for a few pretzels turn a low fat diet into a Mediterranean diet? Certainly most "low fat" dieters believe they are eating "healthy" diets high in grains and vegetables, but low in "evil" fat. This study represents another nail in the coffin of the low fat dogma, and in that respect as well, I am pleased.

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