"We show that post-workout muscle building and repair is blunted in young adults with obesity," said Nicholas Burd from the University of Illinois and lead researcher on the study.
The research, published in the Journal of Physiology, puts some serious doubts on trends such as ‘dirty bulking’, with implications going beyond aesthetics and highlighting the need to maintain a healthy weight in order properly repair muscle tissue.
"Several previous studies, including some from our lab, have shown reductions in muscle protein synthesis after food ingestion in obese adults compared with normal-weight adults," said Burd. "Our new study goes further, showing there is an obesity-related impairment in building new muscle proteins in the fed state after a weightlifting session."
To reach their conclusions, the scientists took data from 18 young adults, all of whom were inactive. Half of the study participants were deemed obese, with the other half at a health weight.
Amino acid levels and muscle biopsies were taken from the legs of all participants who were then subjected to 10-12 reps of an unspecified resistance exercise on the opposing leg.
"By using participants' own nonexercised legs as controls in the experiment, we were able to directly compare how weightlifting affects muscles' ability to build new proteins versus a nonexercised state," Burd said.
Following the training, the participants all ate the same meal, 170g of lean ground pork, before further muscle biopsies were taken at 120 and 300 minutes.
"The obese adults had plasma insulin concentrations that were approximately 3.2 times higher at baseline, which highlights some level of whole-body insulin resistance," he said. "Given that obesity is associated with increased muscle mass, but of poorer metabolic quality, the amount of work they could perform per unit of lean muscle mass also was lower than that of their normal-weight peers."
Scientists also noticed differences in the protein uptake between the two trail groups following their protein-rich meal, with obesity inhibiting the rates of myofibrillar protein synthesis, responsible for generating force in the muscles and for an increase in size as a response to exercise.
Obesity is a real issue among Australian men, with a 2017 report from the Australian Institute of Health and Welfare estimating that 71 per cent of Aussie men being overweight, costing the country $8.6 billion dollars.