Low-Fat Diet May Not Protect Postmenopausal Women From Diabetes Risk

The Women's Health Initiative Dietary Modification Trial, described in the February 1998 issue of Controlled Clinical Trials, was a multicenter trial of 48,835 postmenopausal women. This randomized controlled trial uses the Women's Health Initiative population to assess whether a low-fat diet intake affects the incidence of diabetes mellitus in postmenopausal women and whether the effect is influenced by weight loss.

"Decreased fat intake with weight loss and increased exercise may reduce the risk of diabetes mellitus in persons with impaired glucose tolerance," write Lesley F. Tinker, PhD, from the Public Health Sciences Division, Fred Hutchinson Cancer Research Center in Seattle, Washington, and colleagues from the Women's Health Initiative Randomized Controlled Dietary Modification Trial. "This study was undertaken to assess the effects of a low-fat dietary pattern on incidence of treated diabetes among generally healthy postmenopausal women."

From 1993 to 2005, a total of 48,835 postmenopausal women aged 50 to 79 years were enrolled at 40 US clinical centers and randomly assigned to a usual-diet comparison group (n = 29,294 [60.0%]) or to an intervention group with a 20% low-fat dietary pattern including increased vegetables, fruits, and grains (n = 19 541 [40.0%]). The outcome measure was self-reported incident diabetes treated with oral agents or insulin.

Incident treated diabetes was reported by 1303 participants following the low-fat diet (7.1%) vs 2039 participants (7.4%) following the usual diet (hazard ratio, 0.96; 95% confidence interval, 0.90 - 1.03; P = .25). The intervention group lost weight, and after 7.5 years, there was a difference of 1.9 kg between the intervention and comparison groups (P < .001). Greater decreases in percentage of energy from total fat were associated with decreased diabetes risk according to subgroup analysis (P for trend = .04), but after adjustment for weight loss, this was not statistically significant.

"A low-fat dietary pattern among generally healthy postmenopausal women showed no evidence of reducing diabetes risk after 8.1 years," the study authors write. "Trends toward reduced incidence were greater with greater decreases in total fat intake and weight loss. Weight loss, rather than macronutrient composition, may be the dominant predictor of reduced risk of diabetes."

In an accompanying editorial, Mark N. Feinglos, MD, CM, and Susan E. Totten, RD, from Duke University Medical Center in Durham, North Carolina, note that current nutritional recommendations for the primary prevention of diabetes mellitus are limited with little that is truly evidence based.

"We do not know whether specific macronutrients put genetically predisposed people at increased risk of developing DM [diabetes mellitus], or whether adding lots of fat or refined carbohydrate to the diet just makes it easier to take in excess calories," Drs. Feinglos and Totten write. "Studies to isolate these effects will be difficult to perform, but, until we have more information, we have to assume that calories trump everything else, and that our number 1 goal for the reduction of new cases of type 2 DM should be to reduce the intake of high-energy, low-benefit foods, particularly in young members of the most vulnerable populations."

Practice Pearls:

• The incidence of diabetes mellitus in postmenopausal women is not reduced by a low-fat diet.
• A trend toward decreased incidence of diabetes mellitus in postmenopausal women following a low-fat diet appears related to weight loss vs the diet (macronutrient) composition.

Arch Intern Med. 2008;168:1485-1486, 1500-1511.

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