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Am Fam Physician. 2003;67(3):507-508

Clinical Scenario

A 40-year-old overweight woman seeks advice on weight loss.

Clinical Question

Are low-fat diets better than other weight-reducing diets in achieving long-term weight loss?

Evidence-Based Answer

Low-fat diets are no better than low-calorie diets in achieving weight loss in overweight or obese people. In studies, the average weight loss after 18 months on either diet was less than 5 lb.

Cochrane Abstract

Background. Overweight and obesity are global health problems contributing to an ever-increasing disease burden. Calorie restriction can achieve short-term weight loss, but the loss has not been shown to be substantial in the long term. An alternative approach to calorie restriction is to lower the fat content of the diet. However, the long-term effects of fat-restricted diets on weight loss have not been established.

Objectives. To assess the effects of advice about low-fat diets as a means of achieving sustained weight loss, using all available randomized clinical trials. This review1 focused primarily on participants who were overweight or clinically obese and were dieting for the purpose of weight reduction. Because the authors were particularly interested in the ability to sustain weight loss over a longer period of time, they focused on studies of “free living” men and women who were given dietary advice rather than provision of food or money for food.

Search Strategy. The authors searched the Cochrane Library (issue 2, 2001), MEDLINE (up to February 2002), and EMBASE (up to February 2002). They also searched the Science Citation Index (up to January 2001) and bibliographies of identified studies.

Selection Criteria. Trials were included if they fulfilled the following criteria: (1) they were randomized controlled trials of low-fat diets versus other weight-reducing diets; (2) the primary purpose of the study was weight loss; (3) participants were followed for at least six months; and (4) the study participants were adults (18 years or older) who were overweight or obese (body mass index greater than 25 kg per m2) at baseline. Studies that included pregnant women or participants with serious medical conditions were excluded. Two people independently applied the inclusion criteria to the identified studies. Disagreement was resolved by discussion or by intervention of a third party.

Data Collection and Analysis. Data were extracted by three independent reviewers and meta-analysis performed using a random-effects model. Weighted mean differences (WMD) of weight loss were calculated for treatment and control groups at six, 12, and 18 months.

Primary Results. Four studies were included at the six-month follow-up, five studies at the 12-month follow-up, and three studies at the 18-month follow-up. There was no significant difference in weight loss between the two groups at six months (WMD 1.7 kg, 95 percent confidence intervcal [CI], −1.4 to 4.8 kg). The weighted sum of weight loss (WSWL) in the low-fat group was −5.08 kg (95 percent CI, −5.0 to −4.3 kg) and in the control group was −6.5 kg (95 percent CI, −7.3 to −5.7 kg). There was no significant difference in weight loss between the two groups at 12 months (WMD 1.1 kg, 95 percent CI, −1.6 to 3.8 kg). The WSWL in the low-fat group was −2.3 kg (95 percent CI, −3.2 to −1.4 kg) and in the control group was −3.4 kg (95 percent CI, −4.2 to −2.6 kg). There was no significant difference in weight loss between the two groups at 18 months (WMD, 3.7 kg; 95 percent CI, −1.8 to 9.2 kg). The WSWL in the control group was −2.3 kg (95 percent CI, −3.5 to 1.2 kg), and in the low-fat group there was a weight gain of 0.1 kg (95 percent CI, −0.8 to 1 kg). There was significant heterogeneity in the results for weight loss at six months and 12 months.

Except for one study that showed a slight but statistically significant difference in total cholesterol in the low-fat group at one year follow-up, there were no significant differences between the dietary groups for other outcome measures, such as serum lipids, blood pressure, and fasting plasma glucose level. Studies measuring other factors, such as perceived wellness and quality of life, reported conflicting results.

Reviewers' Conclusions. The review suggests that fat-restricted diets are no better than calorie-restricted diets in achieving long-term weight loss in overweight or obese people. Overall, participants lost slightly more weight on the control diets, but this was not significantly different from the weight loss achieved through dietary fat restriction and was so small as to be clinically insignificant.

These summaries have been derived from Cochrane reviews published in the Cochrane Database of Systematic Reviews in the Cochrane Library. Their content has, as far as possible, been checked with the authors of the originalreviews, but the summaries should not be regarded as an official product of the Cochrane Collaboration; minorediting changes have been made to the text (http://www.cochrane.org).

Cochrane Critique

Did the authors address a focused clinical question? Yes.

Were the criteria used to select articles for inclusion appropriate? Yes.

Is it likely that important relevant articles were missed? No.

Was the validity of the individual articles appraised? Yes.

Were the assessments of studies reproducible? Yes.

Were the results similar from study to study? No. Five of the six trials had very similar results (a nonsignificant greater weight loss in patients on low-calorie diets compared with those on low-fat diets). In one study, the greater weight loss on low-calorie diets was significant at each time point. One way in which this trial differed from the others is that the patients on the low-calorie diet reduced both their fat and carbohydrate consumption, instead of restricting only carbohydrates.

Can the results be applied to patient care? Yes.

Do the conclusions make biological and clinical sense? Yes.

Practice Pointers

In the trials reviewed, patients lost an average of 11 to 14 lb after six months on either diet, but by 18 months their net weight loss averaged zero to 5 lb. This review demonstrates the challenge that overweight and obese patients face as they attempt to lose weight.

The National Heart, Lung, and Blood Institute issued evidence-based recommendations to identify, evaluate, and treat overweight and obese adults. According to the recommendations, a variety of methods can be used to nitiate weight loss. However, a long-term weight maintenance program is needed to sustain weight loss. Long-term success after initial weight loss requires a program of dietary therapy, physical activity, and behavior therapy. Drug therapy also might be useful but has not been studied beyond one year. Weight maintenance should begin after the initial six months of weight-loss therapy. Frequent contacts between the patient and practitioner over the long term are more successful.2

Variance, a measure of dispersion, is the degree to which a set of quantities varies. The larger the variance, the further the individual cases are from the average.

Overall, there is no evidence that low-fat diets are any better than low-calorie diets in achieving weight loss in overweight or obese people. This review did not address the addition of pharmacotherapy to fat or calorie restriction, nor did it evaluate trials of restricted carbohydrate (high-protein) diets, which seem to be gaining popularity in the lay population and provoking controversy among some experts.36 There also is little evidence that either diet produces clinically significant weight loss beyond a few months' duration if patients do not follow a weight maintenance program after the initial weight loss. Weight management requires a long-term commitment beyond the initial weight-loss period. As I tell my patients, if there were an easy way to keep weight off, I'd look more like Superman and less like the Michelin Man.

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