Door Closing on Low-fat Diets for Long-term Weight Management

The best way to keep a steady body weight over the long term remains elusive, but a new meta-analysis suggests that a low-fat diet is not the answer for people who want to lose pounds. In fact, when the intensity of strategies is comparable, weight loss over a year or more of follow-up tends to be greater with low-carbohydrate, higher-fat diets.

“Previous trials comparing low-fat diet interventions with participants’ usual diets or minimal-intensity control groups have misled perceptions of the efficacy of reductions in fat intake as a strategy for long-term weight loss,” Deirdre Tobias, ScD, of Brigham and Women’s Hospital (Boston, MA), and colleagues say in their paper published online October 29, 2015, in The Lancet Diabetes and Endocrinology.

“Health and nutrition guidelines should cease recommending low-fat diets for weight loss in view of the clear absence of long-term efficacy when compared with other similar-intensity dietary interventions,” they argue. “Additional research is needed to identify optimum intervention strategies for long-term weight loss and weight maintenance, including the need to look beyond variations in macronutrient composition.”

Commenting on the meta-analysis, David Katz, MD, MPH, director of the Yale-Griffin Prevention Research Center (Derby, CT), agreed that the focus of future studies should shift from macronutrients to foods, adding that an emphasis should be placed on the effects of traditional dietary patterns—such as the Mediterranean diet and traditional Asian diets—on indicators of health as well as weight.

“The people who manage to maintain a healthy weight for a lifetime are not cutting fat or cutting carbohydrate.… [W]hat they do is eat wholesome foods in sensible combinations, and that facilitates both weight control and good health outcomes,” Katz told TCTMD.

Low-fat Only Wins Against Usual Diets

The researchers pooled data from 53 RCTs with a total of 68,128 adult participants. Among the trials, 35 were weight-loss trials, 13 had no intended intervention on weight, and 5 were weight-maintenance trials. All had at least 1 year of follow-up.

Across all participants and trials types combined, mean weight loss was 2.71 kg through a median follow-up of 1 year. That figure was 3.75 kg in trials designed to reduce weight.

In weight-loss trials, low-carbohydrate interventions were more effective than low-fat diets, with a weighted mean difference (WMD) of 1.15 kg (95% CI 0.52-1.79 kg). Weight loss was similar with low-fat and other higher-fat interventions (WMD 0.36 kg; 95% CI -0.66 to 1.37 kg).

Compared with usual diets, however, low-fat interventions were associated a 5.41-kg greater (95% CI 3.54-7.29 kg) reduction in body weight.

The findings were consistent in analyses of trials with no weight loss goal or weight-maintenance trials, none of which included low-carbohydrate comparisons. In the maintenance trials, low-fat interventions provided similar weight loss when compared with higher-fat interventions and greater weight loss when compared with usual diets.

A deeper look at the weight-loss trials showed that “trials achieving greater differences in dietary fat intake and serum triglyceride concentrations resulted in greater weight loss in participants on the higher-fat interventions than in those on the low-fat diet,” the researchers note.

Weight Loss Poor Regardless of Intervention

In an editorial accompanying the study, Kevin Hall, PhD, of the National Institute of Diabetes and Digestive and Kidney Diseases (Bethesda, MD), says it is premature to declare low-carbohydrate diets superior to low-fat diets, highlighting the “clinically meaningless” difference in weight loss between the 2 groups and the “unimpressive” average weight loss across trials and interventions.

He identifies poor adherence as a likely reason for the latter finding. “Outpatient weight-loss studies ubiquitously achieve a maximum weight loss after about 6 to 8 months, followed by weight regain,” he writes, adding that the meta-analysis “only included diet studies lasting at least 1 year, so any reported differences in weight loss were probably due to diet differences that had long since dissipated.”

Hall calls for further research “to determine the factors that affect diet adherence and thereby help maintain weight loss over the long term. What seems to be clear is that long-term diet adherence is abysmal, irrespective of whether low-fat or other diets, such as low-carbohydrate diets, are prescribed.”

Wrong Question Being Asked

Katz said that the question of low-fat vs low-carbohydrate diets is “crummy” and that “you can’t get a good answer to a bad question.”

He mentioned several issues, including the wide range of diets considered low-fat—with 10% to 30% of calories from fat fitting the definition across studies—and the differential effects on caloric intake from restricting fats or carbohydrates. Because carbohydrates make up a much larger part of the typical diet than do fats, restricting carbohydrates instead of fats will inevitably result in a deeper cut to caloric intake, Katz said, pointing out that the meta-analysis did not adjust for overall energy intake.

“The focus on macronutrients is an obsolete concept,” he said. “We have encumbered ourselves with this historical question pitting one macronutrient class against another, but I think we’ve reached a point in our understanding about the relationship between food choice, macronutrient distribution, weight, and health to acknowledge that it’s a bad question and it’s time to move on.”

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Sources: 
1. Tobias DK, Manson JE, Ludwig DS, et al. Effect of low-fat diet interventions versus other diet interventions on long-term weight change in adults: a systematic review and meta-analysis. Lancet Diabetes Endocrinol. 2015;Epub ahead of print.
2. Hall KD. Prescribing low-fat diets: useless for long-term weight loss [editorial]? Lancet Diabetes Endocrinol. 2015;Epub ahead of print.

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