Web MD -- All protein may not be created equal when it comes to weight loss.
Whey, or milk, protein may offer people who want to slim down a slight edge over soy, a new study shows.
Researchers at the U.S. Department of Agriculture's Human Nutrition Research Center randomly assigned 90 overweight and obese middle-aged adults to one of three groups. The first group was asked to add protein drinks made with whey to their normal diets, the second group drank protein drinks made with soy protein, and the third group drank carbohydrate drinks.
Study participants weren't told which group they had been assigned to. All the drinks, which were drunk twice daily, at breakfast and dinner, had the same number of calories: 200. They also all had had 52 grams per packet, for a daily total of 104 grams of added protein or carbs.
They also had people keep records of the other foods they were eating. The researchers made sure participants were drinking the shakes during random urine tests for levels of a chemical tracer they had added to the powders.
Seventy-three people completed the study, which is published in the Journal of Nutrition.
When the study started, there were no significant differences between groups, researchers report.
Men weighed an average of 218 pounds, while women weighed an average of about 190 pounds.
Throughout the study, all the groups ate roughly the same number of average daily calories, about 2,200.
After six months, people drinking the carbohydrate shakes had gained a little bit of weight, about 2 pounds, which appeared to be mainly added fat, compared to where they started.
People drinking the soy shakes had stayed about the same weight as where they started.
But people drinking the whey protein had lost a little bit of weight and body fat, about 2 pounds. Additionally, while the other groups saw little change in the size of their waists, the whey protein group lost about an inch around the middle.
The study was partially funded by the dairy industry.
Researchers say a couple of things may help to explain the weight and fat loss seen with whey protein.
People in the whey protein group had significantly lower blood levels of the hormone ghrelin than people eating the soy protein or carbohydrate.
"It's a hormone that helps regulate food intake," says David J. Baer, PhD, research physiologist at the USDA's agriculture research service in Beltsville, Md. "So the higher concentration, the more hungry somebody feels. The lower concentration, the fuller somebody feels."
And though researchers really can't explain why this happened or what it means, they found that people drinking the whey protein had cut back on their carbohydrate intake by the end of the study, even though they weren't eating fewer total calories and didn't know what kind of supplement they were getting.
Though people drinking soy protein saw little change in their weight or body composition during the study, they had higher levels of thyroid hormones compared to those drinking whey. Thyroid hormones control metabolism and higher levels may indicate a metabolic boost, though more research is needed to fully explain what that may mean for weight loss.
For people who are hoping to replicate the results at home, researchers advise picking a whey product that is also low in calories and fat.
"A lot of the whey products on the market also have a lot of calories in them," Baer says, "Consumers just need to read those labels."