Train To Gain: Rat Rates

Studies evaluating the effects of endurance exercise on testosterone levels have been equivocal. Some studies show a definite drop in testosterone following aerobic exercise, while others find either an increase or no change. The type of exercise also influences testosterone release. That’s apparent in a study that compared the effects of continuous and intermittent exercise in male rats.1

The rats were divided into two groups, with one group doing intermittent swimming exercise and the other continuous swimming. The study lasted six weeks and measured testosterone responses to the exercise sessions as well as other hormones associated with testosterone release in the body that are synthesized and secreted in the brain, including luteinizing hormone (LH) and gonadotropin-releasing hormone (GRH).

The study found that the rats’ testosterone levels increased after intermittent exercise but were lower following continuous exercise. The continuous-training group also showed lower resting testosterone levels than a group of sedentary rats at the three-week point, but hormone levels had risen by the six-week point.

Past studies have found that the decreased testosterone levels following continuous exercise, such as typical aerobic sessions, may occur because of decreased blood flow to the testes, the site of testosterone synthesis; decreased activity of enzymes in the testes required for testosterone synthesis; or decreased activity of cyclic AMP, an ATP product also needed for testosterone synthesis.

In the rat study reported here, the researchers noted increased LH following intermittent exercise but not continuous exercise. That’s significant because LH, a pituitary gland hormone, dictates the production of testosterone in the Leydig cells of the testes. The higher LH flow after intermittent exercise explains why that type of exercise leads to higher testosterone levels.

The question is, Does endurance exercise have the same effect on human testosterone levels as it has on rats? [See ‘Test Run’ in the April Eat to Grow section for a study of older male runners and testosterone.] IM 1 Hu, Y., et al. (1999). Serum testosterone responses to continuous and intermittent exercise training in male rats. Int J Sports Medicine. 20:12-16.