- Reduced rate of glycogen depletion: When glycogen levels are low, muscles use protein and amino acids to produce glucose.(29) Protein and amino acids are the building blocks of muscle.(29) With shortages of glycogen, muscle starts using vital protein and amino acids for energy purposes.(29) This leads to muscle damage and overtraining (it has been shown that muscle damage limits and interferes with glycogen storage and synthesis). (29) Glycogen is the storage form of glucose + carbohydrates. About 80% of total carbohydrate is stored in skeletal muscle (about 14% is stored in the liver and 6% in the blood in the form of glucose).(29) Glycogen is important but humans have a limited capacity to store it.(29) Muscle glycogen is crucial for ATP re-synthesis during exercise.(29) Studies show that exercising in hot environments reduces muscle glycogen use by 40 to 50% and show reduced rates of glycogen depletion due to improved muscle perfusion.(7,8). Additional studies show that heat acclimation leads to sparing of muscle glycogen associated with enhanced ability to perform highly intense exercise following prolonged exertion in the heat.(7)
- Increased release of human growth hormone* (HGH): HGH is a vital hormone that affects the muscle loss and atrophy that typically occurs with aging.(12,13) The higher your levels of HGH, the healthier and stronger you will be. For most people, at about the age of 30 a stage called “somatopause” is reached. When this point is reached, HGH levels begin to drop off dramatically. This decline in HGH levels contributes to the aging process, so the maintenance of high HGH levels is increasingly important as we age.(43) A study has shown that exercise in a warm environment induced significant elevations in HGH concentrations (exercise in induced elevations of plasma HGH levels with increments exceeding 20 ng/ml in 29 degree C. water and 30 ng/ml in 36 degree C. water).(68)
*Note: See appendix for additional information.
Studies have documented that hyperthermic conditioning can significantly induce the release of human growth hormone (HGH). (68,11,12,13) One study showed a doubling of HGH levels with only two 20-minute heat sessions at 176 degrees F.(11,12) A second study showed that HGH levels can be increased fivefold with only two 15-minute heat-conditioning sessions,(11,12) and a third study showed that two one hour heat sessions each day at 176 degrees F. for one week increased HGH levels by sixteen times on the third day.(13) When hyperthermia and exercise are combined, the synergistic effect causes even greater increases in HGH.(93)
- Increased protein synthesis: Stimulation of the uptake of amino acids into muscle cells increases protein synthesis. Exercise in heat has been shown to contribute to improved protein synthesis.(11,14,15)
- Inhibited cellular protein degradation (and enzymes responsible for same): Hyperthermic conditioning and exercise in heat contribute to improved regulation of protein metabolism.(11,14,15,18)
- Reduced blood lactate levels: Reduced lactate levels result from incomplete glucose burning because the cardiovascular system cannot furnish enough oxygen to break down pyruvic acid). Pyruvic acid is converted to lactic acid. (29) Increased levels of lactate in muscles causes fatigue during exercise. Reduced lactate production can increase the capacity for prolonged physical activity (it is believed that this is because of the increased blood flow to the muscles). (29) Exercise performed in a hot environment has been shown to reduce blood lactate levels.(16)
- Increased concentrations of heat shock proteins (HSPs): HSPs and variations in the HSP70 gene can reduce protein degradation and promote muscle growth. HSPs also provide longevity and anti-aging benefits. (36,37,38,39) A growing body of literature supports the role of heat shock proteins in heat adaptation which allows organisms to perform work in high-temperature environments.(24)
- Increased prolactin release: Prolactin is a hormone produced in the pituitary gland. Named originally after its function to promote milk production (lactation) in mammals, it has since been shown to have more than 300 functions (reproductive, metabolic, fluid regulation, regulation of the immune system and behavior). Prolactin is an indirect marker of central fatigue.(49) A study comparing the prolactin responses of subjects reaching exhaustion via cycling to subjects heated to the same core temperature passively found that with both forms of heating the prolactin response was the same. The conclusion is that core temperature is the key stimulus for prolactin release.(20)
H. Reduced insulin resistance. Hyperthermic conditioning has been shown to protect skeletal muscle from high-fat diet–induced insulin resistance and provide strong evidence that HSP induction in skeletal muscle could be a potential therapeutic treatment for obesity-induced insulin resistance.(50)