April 7, 2009 -- Too much "good" protein makes bad fats worse, new research suggests.
A high-fat diet may lead to insulin resistance, a major step on the path to type 2 diabetes. But cutting back on fat may not help those who continue to eat too much protein, find Christopher Newgard, PhD, director of the Sarah Stedman Nutrition and Metabolism Center at Duke University, and colleagues.
"There's not only fat in that hamburger but plenty of protein," Newgard tells WebMD. "We are overconsuming calories composed of all the different macronutrients, and together they have harmful effects."
When they began their studies, Newgard and colleagues weren't trying to give protein a bad name. They were just trying to find out how the metabolism of obese people differs from that of lean people.
To do this, they collected vast amounts of information -- including high-tech lab tests on blood and urine samples -- from 74 healthy obese people and 67 healthy lean people.
Unexpectedly, obese people had a distinct metabolic "signature" related to a particular subtype of amino acids called BCAA (branched-chain amino acids). About 20% of the protein in the typical American diet is made up of BCAAs.
Lean people's bodies tend to make new proteins out of BCAAs. In obese people, Newgard and colleagues suggest, this process gets overloaded. Instead of making new protein, the BCAAs are diverted into a deviant pathway that leads to insulin resistance.
Can too much protein really be bad? Yes -- at least in lab rats. Newgard's team fed rats all the high-fat food they wanted. Two other groups of rats got less food: either standard chow or chow enriched with fats and BCAAs.
The rats on the BCAA/fat diet didn't eat as much food or gain as much weight as the rats on the high-fat diet -- but they became just as insulin resistant.
"Under circumstances of overconsumption, not only does excess fat and carbohydrate have injurious effects, but also the protein component of the diet can lead to some of the co-morbidities of obesity," Newgard says.
Human studies will be needed to confirm the rat findings. But Ronald B. Goldberg, MD, director of the lipid disorders clinic at the University of Miami, says the findings could have major implications.
"What they show is that the combination of high fat and protein might be what's important in developing insulin resistance," Goldberg tells WebMD. "The truth is that in Western diets we do eat a high-protein, high-fat diet. The stress previously has not been on the high-protein component."
The Newgard study appears in the April 8 issue of the journal Cell Metabolism.