Benefits

Physical exercise is important for maintaining physical fitness and can contribute positively to maintaining a healthy weight, building and maintaining healthy bone density, muscle strength, and joint mobility, promoting physiological well-being, reducing surgical risks, and strengthening the immune system.

Exercise also reduces levels of cortisol. Cortisol is a stress hormone that builds fat in the abdominal region, making weight loss difficult.^{[citation needed]} Cortisol causes many health problems, both physical and mental.^[8]

Frequent and regular aerobic exercise has been shown to help prevent or treat serious and life-threatening chronic conditions such as high blood pressure, obesity, heart disease, Type 2 diabetes,insomnia, and depression.^[9] Endurance exercise before meals lowers blood glucose more than the same exercise after meals.^[10]

There is some evidence that vigorous exercise (90-95% of VO₂ Max) is more beneficial than moderate exercise (40 to 70% of VO₂ Max).^[11] Some studies have shown that vigorous exercise executed by healthy individuals can increase opioid peptides (a.k.a. endorphins, naturally occurring opioids that in conjunction with other neurotransmitters are responsible for exercise-induced euphoria and have been shown to be addictive), increase testosterone and growth hormone,^[12] effects that are not as fully realized with moderate exercise. More recent research^[13][14] indicates that anandamide may play a greater role than endorphins in "runner's high".

Both aerobic and anaerobic exercise also work to increase the mechanical efficiency of the heart by increasing cardiac volume (aerobic exercise), or myocardial thickness (strength training). Such changesare generally beneficial and healthy if they occur in response to exercise.

Not everyone benefits equally from exercise. There is tremendous variation in individual response to training: where most people will see a moderate increase in endurance from aerobic exercise, some individuals will as much as double their oxygen uptake, while others can never augment endurance.^[15][16] Similarly, only a minority of people will show significant muscle growth after prolonged weight training, while a larger fraction experience improvements in strength.^[17] This genetic variation in improvement from training is one of the key physiological differences between elite athletes and the larger population.^[18][19] Studies have shown that exercising in middle age leads to better physical ability later in life.^[20]