The study – published in JAMA – reports that when overeating, calories alone contribute to increases in body fat. In contrast, protein contributes to the changes in energy expenditure and lean body mass, but does not play a role in the increased in body fat.
“The key finding of this study is that calories are more important than protein while consuming excess amounts of energy with respect to increases in body fat,” explained the research team, led by Dr George Bray of the Pennington Biomedical Research Center, USA.
In an accompanying editorial, Dr Zhaoping Li and his colleagues from the University of California, Los Angeles, stated that the results of this study "suggest that overeating low protein diets may increase fat deposition leading to loss of lean body mass despite lesser increases in body weight.”
They argued that policy makers need to understand the role of the Western diet in promoting overweight and obesity.
“Because this diet increases the risks of over-nutrition through fat deposition beyond that detected by body mass index, the method used to assess the current obesity epidemic and the magnitude of the obesity epidemic may have been underestimated,” they said.
Obesity has become a major international public health concern. The authors noted that over 60% of adults in the USA are categorised as overweight, whilst more than 30% are obese.
Bray and his team explained that although many people are overweight or obese, there is a significant number of people who despite overeating have normal weight and do not become overweight or obese.
“These differences may reflect differences in the way individuals handle the food they eat each day both during weight gain and weight loss,” argued the researchers, noting that “the concept that when people overeat, the amount of weight gain is highly individual, has intrigued medical science for a century."
Previous research has suggested such a phenomenon may be due to macronutrient composition and responses to overfeeding – with various human studies supporting the view that when people overeat a diet that contains either high or low protein, they are less “metabolically efficient” than diets of average protein intake.
The new study aimed to determine whether levels of dietary protein differentially affected body composition, weight gain, or energy expenditure “under tightly controlled conditions in a randomized controlled trial.”
The team randomised 25 healthy people, living in a controlled setting, to consume diets or varying protein levels. Participants were randomised to receive diets containing 5% energy from protein (low protein), 15% (normal protein), or 25% (high protein) – which they were overfed for 8 weeks.
Bray and his colleagues reported that all participants in the study gained weight – with no differences by sex.
They found that the rate of weight gain in the low protein diet group was significantly less than in the other two groups.
The researchers explained that their data is consistent with previous findings, and implies “that a diet providing only 5% of energy from protein was metabolically different with a higher energy cost of weight gain compared with diets that contained 15% and 25% of energy from protein.”
However, the team noted that calories alone, contributed to the increases in body fat witnessed, whilst protein contributed to the changes in energy expenditure and lean body mass, “but not to the increase in body fat.”
You watch a video of Dr Bray explaining his findings by clicking here.
Volume 307, Number 1, Pages 47-55, doi: 10.1001/jama.2011.1918
“Effect of Dietary Protein Content on Weight Gain, Energy Expenditure, and Body Composition During Overeating: A Randomized Controlled Trial”
Authors: George A. Bray, Steven R. Smith, Lilian de Jonge, Hui Xie, Jennifer Rood, Corby K. Martin, et al