Weight Loss and High Protein Diets: Can Soy Help Fight Obesity?

There is an ever-increasing prevalence of obesity throughout the world,1 and a need to formulate interventions for alleviating this problem.2 In the United States, the increases in the prevalence of overweight and obesity are continuing and the potential health benefits of reducing overweight and obesity are of considerable public health importance.3 It is well accepted that losing weight will improve health4 and quality of life.5, 6 This brief summary will provide an overview of what is known about “treating” obesity with high protein diets, and more directly describe what is known about soy protein and why it may be a good choice for weight loss programs.

Can Obesity Actually Be Treated?
Behavioral modifications of diet (reduced energy intake) and exercise (increased energy expenditure) are the primary mechanisms for weight loss programs. However, adherence to these modifications over long periods of time is challenging, especially if participants do not experience “quick” results. Therefore, researchers are seeking ways to promote weight loss by modifying dietary intake patterns. One way to do this is by changing the components or proportions of macronutrients in the diet. Although low fat, high carbohydrate diets were the dominant recommendation for the last several decades, there has been a recent surge of interest in high protein diets as alternatives. For example, one formula for weight loss is to increase protein intake and reduce simple carbohydrate intake.7 Several groups have reported that this program (high protein/low carbohydrate diet) will result in weight loss, and that other health variables will improve as well; however, few long-term protein treatment studies are published.

Clinical Trials
Skov and colleagues7 reported that weight loss in adults after six months was 5.1 kg in a high carbohydrate group (protein 12 percent of total energy) and 8.9 kg in the high protein group (protein 25 percent of total energy) (difference 3.7 kg, P < 0.001), and fat loss was 4.3 kg and 7.6 kg, respectively (difference 3.3 kg, P < 0.0001), whereas no changes were reported in the control group. Another study, a one-year multicenter controlled trial,8 investigated the effects of a low carbohydrate, high protein, high fat diet and a low calorie, high carbohydrate, low fat (conventional) diet. Subjects on the low-carbohydrate diet had lost more weight than subjects on the conventional diet at three months, (-6.8 ± -5.0 [mean ± SD] vs. -2.7 ± 3.7 percent of body weight; P=0.001) and six months (-7.0 ± 6.5 vs. -3.2 ± 5.6 percent of body weight, P=0.02), but the difference at 12 months was not significant (-4.4 ± 6.7 vs. -2.5 ± 6.3 percent of body weight, P=0.26). Another trial9 found that when severely obese subjects are treated with a carbohydrate-restricted diet for six months, they lose more weight than subjects on a calorie- and fat-restricted diet (-5.8 ± 8.6 kg vs. -1.9 ± 4.2 kg; P=0.002).

Protein Types
Soy protein has been studied for its potential health benefits and the reports are promising; therefore, soy may be a good choice for obese patients to consume when trying to lose weight. In a 12-week trial testing the effects of soy-based meal replacement in 100 obese volunteers, Allison and colleagues10 reported that the soybased meal replacement group lost significantly more weight and fat mass than the control group (7.0 kg vs. 2.9 kg; P<0.001 and 4.3 kg vs. 1.4 kg; P=0.003, respectively). The treated group also had a greater change (decrease) in total cholesterol (22.5 vs. 6.8 mg/dl; P=0.013) and LDL cholesterol (21.2 vs. 7.1 mg/dl; P<0.009). In another study, the effects of two distinct patterns of caloric restriction, with both having soy-based meal replacement, were evaluated in a 40-week weight loss trial among 90 overweight/obese adults.11 Both calorically-restricted groups had beneficial weight loss and other health variables, such as serum lipid levels and blood pressure, were improved.11 Similarly, Bosello et al.12 compared 24 obese subjects on a very low calorie diet taking soy protein or casein protein. They found that those subjects taking the soy protein based diet had more favorable improvements in plasma cholesterol and triglycerides despite similar weight loss as the casein group. Several other groups have investigated a low calorie soy-based replacement diet and found that it, in combination with behavioral modification, can be an effective way to treat obese patients.13-17 One study found that when obese diabetic patients were given this soy-based replacement diet for eight weeks, they lost weight and had improved glycemic control.18 Another study on a soy-based replacement supplement involving 30 subjects found reduced body weight after 12 months of treatment to a higher degree than lifestyle education alone.19 A new weight loss drink20 composed of isolated soy protein, milk protein and fructose is being evaluated for its efficacy compared to a standard weight loss program in obese individuals. There are also reports indicating that soy protein based diets compared to casein based diets can reduce body fat and improve cholesterol levels in dietary induced obese rats and genetically obese mice.21, 22

Conclusions
Many concerns and unanswered questions need to be addressed. Will high protein diets have long-term effects on health? Are the beneficial effects seen at six months going to remain after years of high protein diet treatment? Without adequate placebo controlled trials beyond six months of treatment, it is difficult to predict the long-term outcomes of high protein diet treatment. While these few reports do not give a final assessment of the long-term effects of soy-based meal replacement, they do introduce the idea that obese individuals can lose weight and improve other health variables when soy protein is included as part of a reduced-calorie treatment, at least in the short-term.

Mark B. Cope, Ph.D. received his doctorate in nutrition science in 2001 from the University of Alabama at Birmingham. He is currently a postdoctoral fellow in the department of nutrition sciences. Cope is investigating the effects of antipsychotic drugs on weight gain in a mouse model. David B. Allison, Ph.D. received his doctorate from Hofstra University in 1990. In 2001, he joined the faculty of the University of Alabama at Birmingham, where he is currently professor of biostatistics, head of the section on statistical genetics, and director of the NIHfunded Clinical Nutritional Research Center.

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