Overnutrition manifested by obesity has emerged as a major health problem in affluent countries. In spite of increased interest in fitness, obesity is on the increase in the United States. This is particularly so among children and adolescents. Although obesity is associated with many risk factors for diseases, the mechanisms whereby it enhances disease risk are not fully understood. Such an understanding is needed to develop strategies for management of these conditions. In this report we suggest that overnutrition produces clinical diseases only in individuals who already possess a metabolic weakness or "defect" in a given system. In the absence of such underlying defects, overnutrition, or obesity, is well tolerated. One of the most common consequences of obesity is dyslipidemia, that is, elevations of very low-density lipoprotein (VLDL) triglycerides and low-density lipoprotein (LDL) cholesterol and low concentrations of high-density lipoprotein (HDL) cholesterol. The major effect of overnutrition on lipoprotein metabolism is to stimulate the production of VLDL. For patients who have an underlying defect in lypolysis of VLDL triglycerides, hypertriglyceridemia will develop in the obese state. For those who have defective clearance of LDL, obesity will accentuate hypercholesterolemia. Both of these effects can be explained by overproduction of VLDL, due to obesity, combined with a genetic defect in clearance of VLDL or LDL. The mechanism whereby obesity causes a lowering of HDL cholesterol is uncertain, although it could enhance removal of HDL by an excess of adipose tissue. Another disease associated with obesity is cholesterol gallstones. The presence of obesity more than doubles the risk for gallstones. Two underlying factors increase the danger for gallstones: a deficiency of hepatic secretion of bile acids and a tendency for formation of cholesterol crystals in bile. Overnutrition promotes the synthesis of whole-body cholesterol, and the only route for excretion of this excess cholesterol is through the biliary tree.(ABSTRACT TRUNCATED AT 400 WORDS)