A high protein intake is associated with reduced weight gain and improved body composition, and clinical trials investigating high protein diets have found them effective at aiding weight loss. The ability of protein to decrease body weight is likely a combination of a satiety effect, an increased gut transit time and a decrease in the glycaemic response to a mixed meal. However, protein can be consumed in a number of different forms, and evidence suggests that some proteins are more beneficial for weight loss than others. For example, whey protein is unique in that it is able to stimulate insulin secretion and has a beneficial effect on immunity. Whey protein also contains high amounts of branched chain amino acids that can aid muscle growth and reduce lean mass losses during calorie restriction. Little research has directly compared the benefits of different proteins.
Because of the lack of direct comparison studies, researchers1 compared the weight loss effects of whey with soy protein, when added to the habitual diets of free-living obese and overweight individuals. Subjects were randomly assigned to receive 56g/d whey protein, 56g/d soy protein or an isoenergetic amount of carbohydrate for 23 weeks, in addition to their normal diet. Body weight was assessed monthly and dietary intake was recorded on a 24-hour diet recall every 10 days. By the end of the study, the body weight of the carbohydrate and soy protein groups was not significantly different. However, those subjects consuming the whey protein had a body weight 1.8kg lower and a fat mass 2.3kg lower than the carbohydrate group. The carbohydrate group showed a trend to increased fat mass and increased body weight. The waist circumference was also smaller in the whey protein group, compared to the other groups.
These results suggest that whey protein is a superior source of protein for weight loss when compared to soy protein. In addition, this research raises questions about the benefits of soy protein in a weight loss diet. Interestingly the plasma ghrelin levels of those subjects consuming the whey protein was lower compared to the other groups, suggesting that whey protein was effective at reducing the appetite of the subjects. Appetite suppression therefore could be one of the mechanisms by which whey protein is able to cause weight loss. In this study the subjects were not calorie restricted and could eat their normal diet ad libitum. This provides further evidence that calorie restrictive diets are not required to cause weight loss. Increased production of thyroid hormones are one possible mechanism by which protein may cause weight loss, but in this study, soy protein caused a grater increase compared to whey.
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