Animals live longer when their calorie intake is restricted to two-thirds of what is considered normal for their species. Animals so restricted are also generally healthier: most disease, including cancer, diabetes, and neurodegenerative illness, are forestalled. This phenomenon was long attributed to a simple slowing of metabolism (cells' production of energy from fuel molecules) and consequent reproduction of its toxic by-products in response to less food. In fact, however, calorie restriction does not slow mammalian metabolism, and in yeast and worms, metabolism is both sped up and altered. Some scientists now theorize that calorie restriction is a biological stressor that, like natural food scarcity, induces a more complex defensive response, which in mammals includes changes in cellular defenses and repair.