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Part 2 of a series on diet and weight loss
By Dónal P. O’Mathúna
Part 1 of this series last month examined the effectiveness of low-carbohydrate (Atkins-type) diets for weight loss. This Part 2 will focus on the effectiveness of the more traditional low-fat diets. These diets have come under criticism from some who claim that fat consumption in the range of 18-40% has little impact on body fat.1 They note that while obesity was becoming an epidemic, Americans decreased the proportion of fat in their diets from 40% to 32%.2 However, while the percent fat in the diet has decreased, the actual per capita consumption of fat and simple carbohydrates (like sugar, syrup, and processed flour and rice) has increased every year.3 This review will examine recent evidence on whether reducing fat in people’s diet is an effective way to lose weight.
Interest in low-fat diets arose following the revelation of little success with initial efforts to curb the prevalence of obesity.4 Protocols that only restricted calories or added appetite suppressant drugs were found to have beneficial short-term effects, but not long-term benefits. Meanwhile, the dietary fat content was being studied for its effect on heart disease. A number of randomized controlled trials (RCTs) reported an almost accidental finding that subjects with heart disease randomized to low-fat and ultra-low-fat diets lost weight—even without intending to. Some cardiac trials followed subjects on low-fat diets for up to four years and found small, but sustained, weight loss.4 The next step was to directly examine whether low-fat diets were effective in helping overweight people lose weight.
Mechanism of Action
Fat is a logical target for weight-loss strategies. Fat contains more than twice as many calories per gram as protein or carbohydrate (9 kcal/g vs. 4 kcal/g, respectively).3 Therefore, replacing fat with protein or carbohydrate leads to fewer calories without requiring a reduction in the amount of food eaten. Thus, reducing fat may be an effective way to reduce total calorie intake. Another factor that may contribute to total calorie reduction is that fat is relatively less satiating compared to protein or carbohydrate.5 Replacing fat with other food groups may therefore leave people more satiated, thus reducing how much they eat and leaving them less inclined to snack between meals. Also, a large body of short-term studies has shown that fat tends to improve the taste of foods, which increases the tendency for people to over-consume them.1 One of the limitations with low-fat diets then is a tendency for the foods to be less tasty, which makes long-term adherence problematic.
A meta-analysis of 16 controlled trials of low-fat diets was published in 2000.1 Included trials lasted at least two months and did not restrict total calorie intake. Those on the low-fat diets reduced the percent energy from fat by between 3% and 24%, compared to the control group. The change in body weight in different studies ranged from losing 11.4 kg to gaining 0.8 kg. The amount of weight lost correlated best with pretreatment weight (P < 0.05). Those entering studies within the normal weight range (60-72 kg) showed no weight change if on a low-fat diet, in contrast to those in a control group who showed slight weight gain. A dose-response relationship was found between the percentage dietary fat intake and weight loss (P < 0.005). This meta-analysis was not without its limitations, however.5 Only two of the reviewed trials had weight loss as a primary outcome measure (the others focused on blood lipids), four of the trials involved subjects who were not overweight, and less than half of the trials lasted more than six months.
A more recent systematic review examined RCTs conducted specifically for the purpose of examining weight loss in overweight or obese subjects without serious disease.5 All trials lasted at least six months and compared a low-fat diet with another diet in which all subjects were given dietary advice. Six studies met the review’s inclusion criteria, with the low-fat diets recommending either 20-30 g/d fat or a diet containing 20% fat. The control diets included low-carbohydrate, restricted-calorie, and moderate-fat diets. A meta-analysis was conducted on the results of the 594 participants in the six trials, 92% of whom were women. The results found no significant differences between the low-fat diets and the other diets for changes in weight, with overall weight loss in all trials at 12-18 months follow-up being 2-4 kg. A limitation with the studies included in this review was the relatively high drop-out rates, up to 40% in one study.
The Health Technology Assessment Programme makes evidence-based clinical reports available to the United Kingdom’s National Health Service. A 2004 report systematically reviewed RCTs of weight-loss strategies that lasted longer than 12 months.6 The best strategy (with participants losing on average 5.31 kg after 12 months) were low-fat diets. The next most effective strategies were various weight-loss drugs (leading to weight loss of 2-3 kg after 24 months). The report concluded there was insufficient evidence on the long-term effectiveness of other diets. Adding a planned exercise program to a low-fat diet resulted in loss of an additional 1.95 kg at 12 months. Adding behavioral therapy to a low-fat diet resulted in even greater weight loss: an additional 7.67 kg. However, the report noted that the latter finding was based on studies with very small numbers of subjects and should therefore be viewed tentatively. Two studies combining a low-fat diet with both exercise and behavioral therapy found no additional benefit.
Two more recent studies reviewed in Part 1 of this series compared low-fat diets to low-carbohydrate (e.g., Atkins) diets. These found that weight-loss after six months was greater on the low-carbohydrate diet, but then equivalent at 12 months. A subsequent study found similar results after a 10-week trial, with participants losing 6.8 kg on a low-carbohydrate diet and 7.0 kg on a low-fat diet (no significant difference).7
Concerns have been raised that low-fat diets may adversely affect blood lipids. Increases in total triglycerides and reductions in HDL-cholesterol serum levels have been found in some studies of low-fat diets. However, these findings appeared in trials where carbohydrate replaced fat in isoeneregetic studies in which weight loss was prevented.1 When studies permitted ad libitum consumption of carbohydrates (like all those reviewed here), some weight loss occurred and all lipid levels changed in a more heart-healthy direction.
Low-fat diets can be problematic if simple carbohydrates replace the fat. Instead, complex carbohydrates should be added, in particular those of vegetable, fruit, and whole-grain origin, that carry the additional benefit of being rich in vitamins, minerals, and trace elements.1 Vegetables, in particular oat bran, legumes, and barley, also provide additional fiber, especially soluble fiber, which helps increase satiety.
Low-fat diets have been found to effectively lead to weight loss. In the short term, weight loss may be greater with low-carbohydrate diets. However, at 12 months and later, low-fat diets are at least as effective as other diets. In tackling obesity and weight gain, long-term results are extremely important, but have been hampered by a lack of data. For the last 10 years, the National Weight Control Registry has tracked people (mostly women) who have successfully lost weight, at least initially.8 Results from a study of 2,700 people were presented at the North American Association for the Study of Obesity meeting in November 2004. The type of diet used made no difference in how people lost weight, but those who subsequently increased their fat intake regained the most weight. Low-fat diets may therefore be an important step towards developing long-term healthy eating patterns.
Low-fat diets are as effective as other diets in achieving weight loss when participants are monitored for at least a year. The additional carbohydrate should be made up primarily of vegetables and fruits. Exercise is also crucial, and probably not only to increase energy expenditure. Researchers have hypothesized that weight loss leads to a reduction in fat oxidation and resting energy expenditure.5 Therefore, unless exercise is increased, resting metabolic status will make weight gain almost inevitable. Evidence also supports the importance of group therapy and support. Overall, then, patients should be advised that weight loss still depends on developing a balanced, healthy diet, increasing exercise, and finding support from those around them.
Dr. O’Mathúna is a lecturer in Health Care Ethics at the School of Nursing, Dublin City University, Ireland.
1. Astrup A, et al. The role of dietary fat in body fatness: Evidence from a preliminary meta-analysis of ad libitum low-fat dietary intervention studies. Br J Nutr 2000;83(Suppl 1):S25-S32.
2. Atkins RC, et al. Low-carb, low-fat diet gurus face off. JAMA 2003;289:1767-1768, 1773.
3. Ornish D. Was Dr. Atkins right? J Am Diet Assoc 2004;104: 537-542.
4. Pirozzo S, et al. Advice on low-fat diets for obesity. The Cochrane Database Syst Rev 2002;(2):CD003640.
5. Pirozzo S. Should we recommend low-fat diets for obesity? Obes Rev 2003;4:83-90.
6. Avenell A, et al. Systematic review of the long-term effects and economic consequences of treatments for obesity and implications for health improvement. Health Technol Assess 2004;8:1-182.
7. Meckling KA, et al. Comparison of a low-fat diet to a low-carbohydrate diet on weight loss, body composition, and risk factors for diabetes and cardiovascular disease in free-living, overweight men and women. J Clin Endocrinol Metab 2004; 89:2717-2723.
8. Marchione M. Study: Low-fat diets better for keeping weight off. AP on-line (Nov. 16, 2004). Available at: www.highbeam.com. Accessed Dec. 2, 2004.