The transition from healthy, active young adulthood (left), with healthy amounts and function of adipose tissue/muscle, through a sedentary lifestyle and weight gain shift to middle age (middle) to more extreme old age (right) is depicted. With a sedentary lifestyle and plentiful food, adults accumulate excess visceral fat, develop adipocyte hypertrophy in subcutaneous fat, and lose muscle mass and strength. Increased adipocyte size is associated with excess release of free fatty acids (FFAs), which have been shown to cause insulin resistance and other metabolic abnormalities; excess visceral fat causes excess FFA delivery to the liver. With extreme old age comes reduced anabolic hormones, and these reductions combined with direct effects of aging and further declines in activity result in more muscle atrophy and greater adipose tissue dysfunction. There are primary aging mechanisms at both the cellular and the organismal level. SQ AT, subcutaneous adipose tissue.