Mitochondria are organelles in our cells that are responsible for energy production, and determine our basal metabolism, or the amount of calories we burn at rest.  Exercise and passive heat increased muscle mitochondria proteins and activity to a very similar degree, and the effects of the two combined were additive.  Mitochondrial activity increased 31% with exercise training alone, 35% with heat therapy alone, and 62% with exercise and heat therapy combined (4). Increased metabolism translates to added weight loss when combined with decreased calorie intake.

Another incredible study showed that mice with elevated levels of heat shock proteins did not gain weight, even when consuming a high fat, high calorie diet.  After 10 weeks on this diet, normal mice on the high fat diet had a weight gain of about 30% of their body weight, and developed insulin resistance (pre-diabetes).  Mice with elevated heat shock proteins did not gain weight, and did not become insulin resistant.  They were found to have about 50% more mitochondria in their muscle compared to normal mice, and their basal metabolic rate, the amount of calories they burned at rest, was over twice that of normal mice (5)!