Exercise puts a significant amount of stress on your body. In response to this stress, your body releases hormones like testosterone and growth hormone, norepinephrine and epinephrine that mobilize fat stores and help you shed body fat. These hormones are important for helping your body adapt to the stress you’re placing on it. With more intense or long-duration exercise cortisol also becomes a factor. Cortisol mobilizes fat stores but it also breaks down muscle tissue, something you don’t want.
The type of hormonal response you get varies with the duration, intensity, and type of exercise you do. For example, endurance exercise causes a different hormonal response than resistance training and more intense resistance training causes a greater release of anabolic hormones that light training. Overall, heavy resistance training is more anabolic response while endurance exercise of long duration is catabolic.
Hormonal Response to Resistance Exercise
A heavy bout of resistance training increases testosterone levels. Testosterone is an anabolic hormone that increases muscle mass by stimulating muscle protein synthesis. At the same time, it helps to counteract the catabolic effects of cortisol on muscle tissue. Some, but not all, research shows testosterone levels also transiently increase in women after resistance training but this testosterone response is blunted in post-menopausal women.
Growth hormone levels also rise and remain higher for up to 30 minutes after resistance training. Growth hormone is anabolic in the sense that it boosts collagen synthesis around the muscle, but research doesn’t support the idea that it increases synthesis of muscle contractile tissue, so it may not have an anabolic effect on muscle growth as many people think. It does seem to have an anti-catabolic effect on muscle, reducing muscle breakdown in response to catabolic hormones like cortisol.
According to some research, it’s more local changes at the level of the muscle fibers that drives muscle growth in response to resistance training, not growth hormone and testosterone. Where growth hormone may play the greatest role is by boosting circulating levels of IGF-1. IGF-1 is a growth factor produced by the liver in response to growth hormone. It acts at the level of the muscle to increase the synthesis of contractile proteins.
Growth hormone has another benefit. It acts directly on fat cells to release stored fat. That can certainly work in your favor if you’re trying to shed body fat. In addition, other hormones including epinephrine and norepinephrine released during exercise enhance fat breakdown.
Hormonal Response to Endurance Training
The hormonal response to endurance exercise varies with the duration and intensity of the exercise. Intense exercise above the lactate threshold increases growth hormone levels while long periods of moderate-intensity endurance exercise lowers growth hormone levels. In addition, long periods of endurance training, greater than an hour, leads to a more sustained rise in cortisol. That’s one reason long periods of moderate intensity cardio breaks down muscle tissue – you get the catabolic effects of cortisol without the anabolic activity of growth hormone and IGF-1. It’s true that too much moderate-intensity cardio can interfere with gains in muscle strength and lean body mass. It’s challenging to build lean body mass and lose body fat at the same time since the way your body adapts to each type of training differs
Long periods of moderate-intensity cardio also lower testosterone and estrogen levels in women. Some women athletes that train in endurance sports stop their menstrual cycles and develop fertility problems due to the impact endurance training has on sex hormones like estrogen. Overall, moderate-intensity endurance exercise has a catabolic effect on muscle tissue and makes it more challenging to build lean body mass.
What Does This Mean?
High-intensity exercise, both heavy resistance training, and high-intensity cardio increase testosterone, growth hormone, and IGF-1. Endurance exercise has the opposite effect. Although both high intensity and moderate intensity exercise elevate cortisol levels, the catabolic effects of cortisol are somewhat offset by the anabolic effects of testosterone, growth hormone and IGF-1.
To maximize the release of anabolic hormones during resistance training, use a challenging resistance and do several sets to expose the muscle to more time under tension. Focus on exercises that target large muscles and activate multiple muscle groups. Squats, deadlifts, bent-over dumbbell rows, push-ups, and pull-ups are good choices.
If your goal is to build muscle strength and lean body mass, keep your cardio sessions shorter and intense, and, of course, make sure you’re consuming enough protein and healthy carbs. Keep these factors in mind when planning your workouts and diet.
References:
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Br J Sports Med 2003;37:100-105.
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