Low-Fat Diets Not More Effective Than Other Diets

Pam Harrison

November 02, 2015

BOSTON, MA — Low-fat diets are not more effective than other diets of similar intensity in helping people lose weight over the long term, and nutrition guidelines need to stop recommending low-fat diets as an effective weight-loss intervention, a meta-analysis shows.

"For decades now, we've been told that to lose weight or to keep from gaining weight we need to not eat fat, yet look where we are today, we're all overweight and obese," lead author Deirdre Tobias, MD, from Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, told Medscape Medical News.

"So our hunch was that to simply reduce fat if we want to not be fat was not going to be an effective strategy for long-term weight loss, and in fact we found that all diets did fairly poorly in terms of helping people lose weight. I think focusing on nutrients like fat instead of foods is causing a lot of confusion and allowing people to make poor choices."

The study was published online October 30, 2015 in Lancet: Diabetes & Endocrinology.

Fifty-three randomized controlled trials of both weight-loss and non–weight-loss studies as well as weight-maintenance trials were included in the meta-analysis. In all, the 53 studies involved 68,128 adults.

Low-fat diets included in the analysis ranged from very low-fat diets containing ≤10% of calories from fat to more moderate goals of diets containing ≤30% of calories from fat.

Comparator diets of higher fat intake were diverse and ranged from having participants maintain their usual diet to following a low-carbohydrate and other moderate- to high-fat diets.

The intensity of the dietary interventions also varied, from the use of simple pamphlets or instructions given at baseline to multicomponent programs involving counseling, regular meetings with dieticians, food diaries, and cooking lessons.

Caloric restriction was a component of many weight loss interventions but not all.

"Most of the trials were undertaken in North America (n=37) and were 1 year in duration (n=27)," Dr Tobias and colleagues write.

In all participants combined, study subjects reported a mean weight loss of 2.71 kg on any diet after a median of 1 year of follow-up. The mean weight loss in trials designed for weight loss was 3.75 kg.

"We noted no difference between low-fat and higher-fat dietary interventions when all weight-loss trials were combined, although there was significant between-study heterogeneity," investigators write.

Nineteen weight-loss trials compared low-fat interventions with higher-fat diets, and low-fat diets did not lead to significant differences in weight loss over the long term compared with higher-fat interventions at a weighted mean difference of 0.36 kg.

Eighteen weight-loss trials compared low-carbohydrate diets with low-fat diets, and in these comparisons, low-carbohydrate diets led to significantly greater long-term weight loss than did low-fat diets, at a weighted mean difference of 1.15 kg in favor of low-carbohydrate diets.

In fact, low-fat diets led to a greater decrease in weight, at a difference of 5.41 kg, only when they were compared with a control arm of subjects' following their usual diet.

Indeed, 18 clinical weight-loss trials showed that higher-fat diets led to significantly greater weight loss than low-fat diets when groups differed by more than 5% of calories coming from fat, at a weighted mean difference of 1.04 kg.

Findings from non–weight-loss trials and weight-maintenance trials also showed that low-fat diets had a similar effect on weight loss as did higher-fat diets and that low-fat diets again led to greater weight loss only when compared with subjects' usual diet.

"Results from this comprehensive meta-analysis of [randomized clinical trials] with at least 1 year of follow-up suggest that low-fat dietary interventions do not lead to greater weight loss than do low-carbohydrate and other higher-fat dietary interventions of a similar intensity," Dr Tobias and colleagues state.

"In fact, in the setting of weight-loss trials, higher-fat, low-carbohydrate dietary interventions led to a slight but significantly greater long-term weight loss than did low-fat interventions."

Asked how people might more successfully lose weight and keep it off, Dr Tobias felt that people should simply be told what foods they should and should not be eating and not leave them alone to do the math.

"I think once we get the message out about which foods are good for people and which foods they need to avoid, people can pick and choose, and at the end of the day, they will stick to a diet because they'll be choosing things they like and effects will be more lasting," she said.

Adherence a Long-term Issue

Editorialist  Kevin Hall, MD, from the National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland, is convinced that not being able to stick to a diet for much longer than 6 to 8 months is probably the reason why long-term weight loss on any diet is so disappointing.

"Irrespective of the diet prescription, the overall average weight loss in trials testing interventions designed to reduce body weight was unimpressive (3.75 kg)," Dr Hall observes in his accompanying editorial.

The question then becomes: why is long-term weight loss so poor, regardless of the type of diet prescribed?

"We know that there are certain biological changes that take place when people try to lose weight," Dr Hall told Medscape Medical News.

However, as Dr Hall and others have shown, changes in metabolic rate when dieting only explain a very small part of the typical trajectory that people experience during a diet, which is that they tend to lose weight for up to 6 to 8 months into their program, after which they plateau and then on average slowly regain the weight.

"I don't think we know why this happens," Dr Hall observed.

"But it may be as simple as people have a hard time sticking to a change in their dietary habits, and they return back to their old habits even though they may not be fully aware they are doing that."

On the other hand, Dr Hall emphasized that in any meta-analysis, weight loss reported at the end of a trial is a group average, and some individuals are able to lose weight and keep it off regardless of the diet prescribed.

"This means that some people prescribed even a low-fat diet did lose weight in these clinical trials, while others gained weight, so I wouldn't suggest physicians stop advising people to lose weight, because then you won't get those few people who actually do well on a weight-loss program," Dr Hall said.

"The real trick is trying to figure out whether there is a special diet that would work for an individual patient, and right now, we don't have a good answer to these kinds of questions."

The study was funded by the National Institutes of Health and the American Diabetes Association. Dr Tobias has no relevant financial relationships; disclosures for the coauthors are listed in the paper. Dr Hall has no relevant financial relationships.

Lancet Diabetes Endocrinol 2015. Published online October 30, 2015. Abstract, Editorial

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