Study: Obesity inhibits muscle growth in strength training

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Obesity decreases muscle growth and recovery after strength training. This is the conclusion of a new study by the University of Illinois.

Obesity and Muscle Growth

Preventing overweight is better than curing it. New research from the University of Illinois has found another reason why it becomes more difficult once overweight is present. Building muscle mass can contribute to combating overweight and its consequences. A greater amount of muscle mass, for example, leads to higher metabolism. More muscle mass also allows more glucose to be absorbed by the muscles, thereby reducing the risk of metabolic complaints such as diabetes. Strength training is therefore an important means to combat overweight. However, research from the University of Illinois shows that an important goal of strength training, muscle growth, is more difficult to achieve when obesity is already present. The same effort thus leads to less result.

Limited Protein Synthesis

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. 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. Nicholas Burd, University of Illinois

For the study, the researchers recruited nine obese individuals and nine individuals with normal weight aged 20 to 23 years. The participants had no experience with regular training but were healthy and not active enough. Prior to the study, measurements were taken such as body composition and glucose tolerance. They also assessed to what extent potential participants were able to perform leg extensions. After this, the ‘labeled’ amino acid phenylalanine was introduced intravenously. By labeling amino acids with a stable isotope, the level of this amino acid in the body can be measured during the study. Perhaps redundant: Muscle growth is determined by the continuous production and breakdown of proteins in the muscles. Protein synthesis utilizes available amino acids for production. During breakdown, amino acids are made available, for example, to be used as fuel. By consuming food with amino acids, you stimulate the production of proteins in the muscles (protein synthesis) by providing amino acids. By eating carbohydrates and fats, you stimulate protein synthesis by providing more fuel. There is then less need to break down proteins in the muscles for amino acids.

Uptake of Amino Acids after Training

By tracking exactly what happens to amino acids in the body, the researchers could see how protein synthesis is influenced when eating and training. They performed a muscle biopsy from one of the participants’ legs. Then they had the participants perform four sets of 10 to 12 repetitions on the leg extension machine with the other leg. Immediately after the training, the participants received 170 grams of lean pork containing 36 grams of protein and 4 grams of fat. Two hours and five hours after the meal, muscle biopsies were taken again, this time from both legs. This allowed them to see the difference in response to food between the trained and untrained leg.

Lower Protein Synthesis in Overweight

As the researchers had expected, there were visible differences between the group of obese participants and those with ‘normal weight’. The obese participants had blood insulin concentrations at the beginning that were about 3.2 times higher. This is an important indication of insulin resistance in the body. Insulin is one of the hormones that stimulates muscles to absorb amino acids when blood sugar rises after a meal. After training and eating the protein-rich meals, differences were also visible. The main difference was seen in the speed of protein synthesis in the muscle fibers.

We show that post-workout muscle building and repair is blunted in young adults with obesity. This is significant because muscle building and repair after exercise has long-term implications for metabolic health and overall physical performance.

The rate of protein synthesis increased in both legs after the participants ate. However, the increased increase in the trained leg was statistically more robust in participants with normal weight.

Don’t Lose Hope

These consequences of overweight mean that you can achieve less result for the same work as someone else. It offers little comfort to hear the advice ‘prevention is better than cure’ when you have already passed that stage. However, that doesn’t mean you should give up hope. This kind of insight can be used as motivation for that difficult beginning in a desired lifestyle change. Knowing that you can expect more results as you persist can prevent you from being discouraged by disappointing initial results. Keep going!

References

1. Joseph W. Beals, Sarah K. Skinner, Colleen F. McKenna, Elizabeth G. Poozhikunnel, Samee A. Farooqi, Stephan van Vliet, Isabel G. Martinez, Alexander V. Ulanov, Zhong Li, Scott A. Paluska, Nicholas A. Burd. Altered anabolic signaling and reduced stimulation of myofibrillar protein synthesis after feeding and resistance exercise in people with obesity. The Journal of Physiology, 2018;
2. https://physoc.onlinelibrary.wiley.com/doi/full/10.1113/JP276210