Carbohydrates Offer Some Help In Muscle Protein Synthesis, But Not Enough For The Desired Effect

New study is first to compare net muscle protein balance (protein synthesis minus breakdown) after carbohydrate ingestion with control after exercise

(February 24, 2004) - Bethesda, MD – A visit to the meat counter at any supermarket is proof positive that a good number of Americans are avoiding carbohydrates and consuming high levels of protein and fat, in accordance with the Atkins diet. This carbohydrate-free, fat- and protein- rich diet is for those seeking immediate weight loss, which means most of us. 

But what do others, such as weight lifters and callisthenic enthusiasts, do about carbohydrates?  Their goal is muscle preservation and strengthening, but for years, different theories have been offered about the effectiveness of carbohydrates in maintaining an appropriate muscle protein balance.  A new study may lead to a truce in the debate at the nation’s gymnasiums, and those dedicated to resistance training may finally have an answer as to whether carbohydrates have a positive role in muscle development.

Background

Resistance exercise — also called strength training — increases muscle strength and mass, bone strength, and the body's metabolism. The different methods for resistance training include free weights, weight machines, calisthenics and resistance tubing. When using free weights, dumbbells, and bars stacked with weight plates, you are responsible for both lifting the weight and determining and controlling your body position through the range of motion.

The body’s net muscle protein balance (i.e., the difference between muscle protein synthesis and protein breakdown) generally remains negative in the recovery period after resistance exercise in the absence of nutrient intake, i.e., the muscle’s protein is breaking down complex chemical compounds to simpler ones.  However, it has been demonstrated that infusion or ingestion of amino acids after resistance exercise stimulates muscle protein synthesis. Furthermore, as little as six grams of essential amino acids (EAA) alone effectively stimulates net protein synthesis after a strenuous resistance exercise session.  

The body’s response to the six grams of EAA does not appear to differ when 35 grams of carbohydrates are added.  This reflects the uncertainty of the independent effects of carbohydrates on muscle protein metabolism after resistance exercise.  Additionally, it is unclear how carbohydrate intake causes changes of net protein balance between synthesis and breakdown and how it relates to changes in plasma insulin concentration.

Interpretation of the response of muscle protein to insulin is complicated by the fact that a systemic increase in insulin concentration causes a fall in plasma amino acid concentrations, and this reduced amino acid availability could potentially counteract a direct effect of insulin on synthesis. A past study found that the normal postexercise increase in muscle protein breakdown was slowed by insulin, thus improving net muscle protein balance. However, whereas local infusion of insulin may effectively isolate the effect of insulin per se, the response may differ from when insulin release is stimulated by ingestion of carbohydrates. 

A New Study

Accordingly, a new study set out to investigate the independent effect of carbohydrate intake on muscle protein net balance during recovery from resistance exercise.  The authors of “Effect Of Carbohydrate Intake on Net Muscle Protein Synthesis During Recovery from Resistance Exercise,” are Elisabet Børsheim, Melanie G. Cree, Kevin D. Tipton, Tabatha A. Elliott, Asle Aarsland, and Robert R. Wolfe, all from the Department of Surgery, Metabolism Unit, Shriners Hospitals for Children-Galveston, University of Texas Medical Branch, Galveston, TX.  Their findings appeared in the February 2004 edition of the Journal of Applied Physiology.  The journal is one of 14 peer-reviewed scientific journals published each month by the American Physiological Society (www.APS.org).

Methodology

Sixteen recreationally active and healthy subjects took part in the study. At least one week before an experiment, subjects were familiarized with the exercise protocol, and their one repetition maximum, a maximum weight possible with a leg extension, was determined. The subjects were assigned to one of two groups: carbohydrate group (CHO; n = 8) or placebo group (n = 8).  Subjects were instructed not to exercise for at least 48 hours before an experiment, not to use tobacco or alcohol during the 24 h before an experiment, and not to make any changes in their dietary habits.

The two groups of eight subjects performed a resistance exercise bout (10 sets of eight repetitions of leg presses at 80 percent of one repetition maximum) before they rested in bed for four hours. One group (CHO) received a drink consisting of 100 grams of carbohydrates one hour after exercise; the placebo group received a noncaloric placebo drink. Leg amino acid metabolism was determined by infusion of ²H₅- or ¹³C₆-labeled phenylalanine, sampling from femoral artery and vein, and muscle biopsies from vastus lateralis, the lateral head of quadriceps muscle of anterior (extensor) compartment of thigh.

Results

Key findings of the study included:

·          Plasma glucose concentration was significantly increased in the carbohydrate group until 210 min after intake of drink. 

·          Plasma concentration of insulin reflected the changes in glucose concentration.  The drink intake did not affect arterial insulin concentration in the placebo group, whereas arterial insulin increased by several times after the drink in the CHO group.

·          Arterial phenylalanine (a common amino acid in proteins) concentration did not change after intake of drink in the placebo group but decreased and stabilized in the CHO group.

·          Net muscle protein balance between synthesis and breakdown did not change in the placebo group but improved in the CHO group during the second and third hour after the drink. The improved net balance in the CHO group was due primarily to a progressive decrease in muscle protein breakdown.

Conclusions

This study is the first to compare net muscle protein balance (protein synthesis minus breakdown) after carbohydrate ingestion with control after exercise. The principal finding was that intake of 100 grams of carbohydrates after resistance exercise improved muscle net protein balance.

The findings from this research demonstrate that carbohydrates intake alone can improve net protein balance between synthesis and breakdown. In this work, the gradual improvement in net muscle protein balance after carbohydrate intake was due principally to a progressive reduction in breakdown. However, the improvement was small compared with previous findings after intake of amino acids or amino acids and carbohydrates.

The researchers conclude that intake of carbohydrates alone after resistance exercise will modestly improve the anabolic effect of exercise. However, amino acid intake is necessary for a maximal response, one desired by most participating in resistance exercise programs.

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Source: February 2004 edition of the Journal of Applied Physiology.  The journal is one of 14 peer-reviewed scientific journals published each month by the American Physiological  Society (www.APS.org).