A new study tracked the acute muscle-building response in adults engaged in a weight-training exercise who were fed either high-fat or lean ground pork burgers with the same amount of protein in each. The findings surprised the scientists, adding to the evidence that muscle-protein synthesis in response to weight-training and a post-exercise meal is as complex as the high-protein foods people consume.
The study is reported in the American Journal of Clinical Nutrition.
"What we're finding is that not all high-quality animal protein foods are created equal," said Nicholas Burd, a professor of health and kinesiology at the University of Illinois Urbana-Champaign, who led the research with graduate student Žan Zupančič.
A previous study from Burd's lab found that consuming whole eggs after weight training was better for muscle-protein synthesis than eating only egg whites with equal amounts of protein. Another study from his lab revealed that eating salmon showed a more favorable rate of muscle-building after weight training than a processed mixture containing the same nutrients in the same proportions as the salmon.
These studies suggest that whole foods are better at stimulating post-workout protein synthesis than their processed counterparts, and that the fat content of whole foods may, in some circumstances, improve the rate of muscle-building, Burd said.
In the new study, the researchers used state-of-the-art methods to trace and calculate muscle-protein synthesis in 16 young, physically active adults. The team turned to the U. of I.'s Meat Science Laboratory for formulation of the pork patties.
"That took us a year because it was so hard to get those fat ratios correct," Burd said. All the meat used in the study came from a single pig, and the researchers sent the patties off to another laboratory for analysis. Once the lean-to-fat ratios and other macros were confirmed, the pork burgers were frozen until needed in the feeding part of the study.
Before the weight-training and feeding intervention, all participants received an infusion of isotope-labeled amino acids. This allowed the researchers to track how quickly the labeled amino acids were incorporated into muscle. The team also took blood samples throughout the study to measure amino acid levels in participants' blood.
Before and after the first two hours of the infusion, researchers took muscle biopsies of each participant to get a baseline measure of muscle-protein synthesis.
"And then we took them to the gym," Burd said. "And they were wheeling that infusion pump and everything else with them."
At the gym, the study subjects engaged in an acute bout of leg presses and leg extensions and then returned to the lab for a meal of either a high-fat pork burger, a lean pork burger or a carbohydrate drink. Five hours after the meal, another muscle biopsy was taken to measure protein synthesis in response to the weight-training and feeding intervention.
After a break of a few days, 14 of the 16 participants "crossed over, switching to a different feeding intervention to minimize the impact of individual differences in muscle-building responses," Burd said.
The analysis revealed, as expected, that the amino acid content of the blood was significantly higher in those who ate pork than in those who consumed a carbohydrate drink. But the lean-pork group saw the greatest gains in amino acid levels in the blood. This was true for total and essential amino acids, the team found.
"When you see an increased concentration of amino acids in the blood after you eat, you get a pretty good idea that that is coming from the food that you just ate," Burd said.
Those who consumed the lean pork burger after a bout of weight training also had a greater rate of muscle-protein synthesis than those who ate the high-fat pork burger. This was a surprise to Burd, as "the previous studies using fattier foods, such as whole eggs or salmon, generally showed enhanced post-exercise muscle-protein synthesis compared with lower fat food such as egg whites or nutritional supplements," he said.
Although weight training boosted muscle-protein synthesis in the groups eating pork, the protein in the high-fat burger seemed to have no added benefit in the hours after participants consumed it, while the protein in the lean pork gave muscle-protein synthesis a boost.
"For some reason, the high-fat pork truly blunted the response," Burd said. "In fact, the people who ate the high-fat pork only had slightly better muscle-building potential than those who drank a carbohydrate sports beverage after exercise."
Interpreting the results of this study for people who want to optimize muscle gains from weight-training is tricky, Burd said. It could be that processing the ground pork patties, which involved grinding the meat and adding the fattier meat to the lean, affected the kinetics of digestion.
"There was a little larger rise in the amino acids available from eating lean pork, so it could have been a bigger trigger for muscle-protein synthesis," Burd said. "But that seems to be specific to the ground pork. If you're eating other foods, like eggs or salmon, the whole foods appear to be better despite not eliciting a large rise in blood amino acids."
Burd stresses that exercise is the strongest stimulus for muscle-protein synthesis.
"Most of the muscle response is to weight-training, and we use nutrition to try to squeeze out the remaining potential," he said. "When it comes to eating after weight-training, what we're finding is that some foods, particularly whole, unprocessed foods seem to be a better stimulus."
Burd also is a professor of nutritional sciences and is affiliated with the Carl R. Woese Institute for Genomic Biology at the U. of I.
The National Pork Board's Pork Checkoff program supported this research. The funder had no involvement in study design, data collection or analysis.
