Why do weight-loss results of high protein diets vary? Not all protein is created equal, according to a new, rigorously controlled Agricultural Research Service trial. Specifically,
whey protein (from milk) looks like a winner in terms of weight loss & body fat-to-muscle ratio, even when calories are not reduced.
With nearly 68% of American adults either overweight or obese, finding ways to help people lose excess pounds is a national priority. Though the most obvious advice is to eat less and exercise more, this long-standing recommendation has proven unsuccessful for the lion's share of the population struggling to maintain a healthy body weight.
An alternative approach is to help people focus on consuming specific foods that promote weight loss. Some studies have suggested that high-protein foods might function in this capacity, although the data are conflicting.
Scientists speculate that there are several reasons for these inconsistencies, including the type of protein used (animal vs. plant).
A Six-Month Comparison
To better understand the relation between protein intake and weight loss, researchers with the Agricultural Research Service - the principal scientific research agency of the U.S. Department of Agriculture (USDA) - recently conducted a rigorously-controlled human intervention trial comparing milk proteins (whey) vs. soy proteins in overweight and obese adults.
Their paper, published in the Aug 2011 issue of The Journal of Nutrition, reports “Whey protein but not soy protein supplementation alters body weight and composition in free-living overweight and obese adults.”
Pounds and Fat Lost Even Without Calorie Reductions
Subjects (n = 73) were relatively healthy, other than having a body mass index (BMI) between 28 and 38 kg/m2. As a point of reference, the U.S. Centers for Disease Control and Prevention (CDC) considers BMIs between 25 and 29.9 kg/m2 as signifying overweight; BMI values above this level are indicative of obesity.
For 23 weeks, participants consumed either:
• Whey (milk) protein (about 104 g protein),
• Soy protein (about 104 g protein),
• Or carbohydrate supplements.
All supplements provided 400 kcal/day. Overall dietary intake was assessed every 10 days, and sensations of satiety and hunger were monitored daily. Prior to the start of the intervention and then monthly thereafter, body weight and composition were measured; fasting blood samples were also collected.
There was no effect of dietary treatment on overall caloric intake. Similarly, dietary treatment did not influence hunger, desire to eat, or the feeling of stomach fullness.
Nonetheless, by the end of the study:
• Body weight of the group consuming the whey protein was significantly (2%) lower than that of the group consuming the carbohydrate treatment.
• Moreover, body fat mass was 2.3 kg (about 5 lb) lower in the whey protein group compared to the carbohydrate group.
These beneficial effects were not seen in subjects consuming the soy protein supplement.
Whey Protein Reduced ‘Hunger Hormone’
Consumption of the whey protein (but not soy protein) also resulted in lower circulating concentrations of ghrelin, a stomach-derived hormone thought to stimulate hunger.
Both groups consuming protein supplements exhibited lower fasting insulin concentrations when compared to that assigned to the carbohydrate treatment.
The researchers concluded that different sources of dietary protein may differentially facilitate weight loss and affect body composition.
They also suggested that dietary recommendations related to weight loss, especially those that emphasize the role of protein, consider the source of the protein in helping meet weight-loss goals.
Source: Journal of Nutrition media alert, Jul 14, 2011.