The Latest Studies on Protein ("Low-Carb") Diets, and How They Affect Weight Loss and Muscle Mass

Tuesday, January 31, 2012, 12:00 am

Less than a month ago, George Bray and colleagues from the Pennington Biomedical Research Center made headlines with a research study showing that subjects on a low-protein, high-fat weight-gain diet (eating nearly 1000 extra calories per day for 8 weeks) gained only half as much weight (7 pounds vs. 13-14) as those on a higher-protein, lower-fat diet (the carb content of both diets was the same). But the low-pro group also lost muscle, while the high-pro group gained muscle. Both groups lost the same amount of fat.
So the study, published in the Journal of the American Medical Association, seemed to convey a bit of a mixed message. The low-pro diets didn't pack on as many pounds, but they also didn't add muscle mass, which would increase metabolism, which would tend to bring one's weight back in line over time. Hope you followed that.
This week Bray has returned with the opposite study, sort of, in the American Journal of Clinical Nutrition. Instead of adding 1000 calories a day, he took away 750 calories a day. So this is a classic weight-loss study--the kind many people are interested in. Since the weight-gain study produced different results from different diets, you might expect the same-thing from the weight-loss study. Especially since you've read so much about low-carb (high-protein) weight-loss diets in recent years. Here's a summary of the new study, called POUNDS LOST.
Why was this new study conducted? We live in an increasingly overweight and obese world, and it's important to help people find diets that will help them lose weight and keep the weight off. Some other studies have suggested that varying amounts of "macronutrients" (fat, protein, carbohydrates) in the diet could produce different outcomes. In particular, it has been suggested that low-fat diets might help people lose body fat, and that high-protein diets (often termed "low-carb") might assist efforts to lose weight while maintaining muscle. Some studies have also hinted that low-carb diets might help people lose more of the deep "visceral" belly fat that's believed to be a particularly pernicious aspect of weight gain.
How was this new study conducted? The 424 male and female subjects had an average age in their low 50s. The men had an average weight of 226 pounds, and the women, 185. Both men and women had BMIs in the low 30s, had no diabetes or heart disease, were not on meds that affect body weight, and were not exercisers. They were given complete physicals to determine muscle mass, fat mass, and visceral-fat. Then they were divided into 4 dietary groups, and asked to eat 750 calories a day less than their maintenance calorie intake. The diet groups ranged from 20% fat-15% protein-65% carbohydrates to 40% fat-25% protein-35% carbs. (About 40 percent of all subjects were men, higher than in most other weight-loss studies.) Results were reported after six months, and two years.
What were the results? 1) After six months, all subjects on all diets lost about the same amount of weight, roughly 14 pounds. There were no differences in results from the four different diet groups. Eighteen months later, subjects had gained back about 5.6 lbs, again with no differences among diet groups.
2) The subjects lost about nine pounds of fat and five pounds of muscle in six months, with no differences among diet groups.
3) About five of the nine pounds of fat loss came from the belly region. Most of this was subcutaneous fat, but about 37% of the belly-loss came from visceral fat.
4) Men lost more visceral fat than women, apparently because they had more to begin with. (Men tend to gain body fat you-know-where, and women tend to put it in that other place.)
What should we conclude? The study clearly gives support to the "it's the calories, stupid" crowd over those who argue that changes in fats, proteins, and carbs can make a significant difference. The authors admit that it was tough to get subjects to stick to their diets, and they couldn't push them as far as they wanted in terms of the extreme ends of macronutrient consumption (i.e., super-high proteins, or super-high carbs, etc). They also contend that such diets would almost surely fall outside one of the macronutrient targets generally regarded as healthy. They believe their results fall in line with most, but not all, similar studies. That is, "The macronutrient composition of the diet does not play a significant role in changes in body composition." Therefore, pick any healthy, low-calorie diet that works for you. The authors like the DASH diet.
Important note, not from the study authors: All the weight-losers on all the diets lost about one-third of their pounds from muscle. Being in a high-protein group offered no protection from this muscle loss. However, with exercise, the subjects might have decreased or even eliminated the loss of muscle mass, which is important for function and for maintaining basal metabolic rate (daily calorie burn), which could help keep the pounds off.