Cholesterol: The most common type of steroid in the body. Cholesterol has a reputation for being associated with an increased risk for heart and blood vessel disease. However, cholesterol is essential to the formation of bile acids, vitamin D, progesterone, estrogens (estradiol, estrone, estriol), androgens (androsterone, testosterone), mineralocorticoid hormones (aldosterone, corticosterone), and glucocorticoid hormones (cortisol). Cholesterol is also necessary to the normal permeability and function of the membranes that surround cells. A diet high in saturated fats tends to increase blood cholesterol levels, whereas a diet high in unsaturated fats tends to lower blood cholesterol levels. Although some cholesterol is obtained from the diet, most cholesterol is made in the liver and other tissues. The treatment of elevated cholesterol involves not only diet but also weight loss, regular exercise, and medications. After the age of 20, cholesterol testing is recommended every 5 years.