The trouble with L-arginine

(Edmonton) An amino acid supplement that is popular with young athletes looking to boost performance shows no lasting enhancements for fit young men at rest, says Scott Forbes, a doctoral student in the U of A's Faculty of Physical Education and Recreation.

L-arginine is a naturally occurring amino acid available over the counter at health stores. It’s often prescribed for older adults with cardiovascular disease, hypertension or endothelial dysfunction for its effects as a vasodilator. This is the first study to look at its impacts on young, vigorous men.

Forbes, a doctoral student in exercise physiology, says there are two reasons for L-arginine’s popularity. “First, L-arginine is a precursor for nitric oxide that is known to improve blood flow, which in turn may aid the delivery of important nutrients to working muscles and assist with metabolic waste product removal. Secondly, L-arginine has been shown to increase growth hormone levels in the blood.”

The benefits of growth hormone are diverse, including increasing the use of fat as a fuel as well as insulin and insulin-growth factor-1 (IGF-1) levels. However, most of the research conducted on L-arginine has been in a clinical setting, and the benefits for physically active individuals are not as established. In some cases, they are conflicting.

“One of the reasons for this is that the amount an individual has to consume has not been clearly established and this information is rarely provided by the manufacturers of such products,” explains Forbes.

For Forbes it was a theory worth testing. He wanted to test two different L-arginine doses on healthy, athletic men, the group most likely to purchase this readily available supplement.

“L-arginine is interesting for a few reasons,” says Forbes. “It can increase growth hormone response, and so can increase muscle mass. Also, it has an impact on insulin, which is another anabolic hormone. A recent hot topic has been about nitric oxide as a vasodilator. The theory is that if you can vasodilate your arteries, you can potentially enhance blood flow to the muscles and enhance nutrient delivery and waste product removal.”

For this study, Forbes examined the effects of a low and high dose of oral L-arginine on blood L-arginine, markers of nitric oxide, growth hormone, insulin and insulin-like growth factor-1. The double-blind, randomized, placebo-controlled study recruited 14 healthy, young, physically active males who didn’t use nutritional supplements. They were pre-screened, completing a one-day food record which was analyzed for carbohydrates, protein and fat consumption and caloric intake, then required to follow a modified diet to regulate intake of food and water prior to being dosed with L-arginine.

After a 10-hour overnight fast and no breakfast, they were given a dose of L-arginine—either .075 g per kilogram of body mass or .15 g per kg of body mass for the high dose—or a placebo, says Forbes.

Blood samples were drawn, with the athlete at rest, every half hour for three hours after the L-arginine or placebo dose. The reason, explains Forbes, is that “previous studies show that two hours after consumption, L-arginine tends to reach baseline again.”

Forbes found that the two different doses did significantly elevate L-arginine concentrations in the blood at rest, and both a low dose and a high dose were equally effective in doing so. But neither dose promoted a significant increase in nitric oxide, growth hormone, insulin or insulin-like growth factor-1.

Forbes has now embarked on two studies to see the effects of L-arginine on the fit, young body during exercise: one is working with strength-trained athletes and the other with aerobically trained athletes—cyclists in this case.

“This time we’re looking at the effects of supplements under two extremes: aerobic and strength exercise.

“There’s a lot of money in nutritional supplements,” he adds. “The industry might not be too happy when they see the results at rest, but who knows, it may be different with exercise.”

Forbes has completed both of the exercise studies and hopes to publish the results in the near future. His paper, “The acute effects of a low and high dose of oral L-arginine supplementation in young, active males at rest,” was published in Applied Physiology, Nutrition and Metabolism, 2011, 36(3): 405-411.