Cholesterol is mentioned on a daily basis in the media and in ordinary conversations. But what is cholesterol? Why are high levels undesirable? What can you do to lower your cholesterol levels? Cholesterol levels are only one part of the battle against heart disease, the number-one killer of Americans. But cholesterol is a factor that you can address by changing your diet, adding healthy foods, and exercising more. Small changes will add up quickly, and your health will improve within months if you understand and follow American Heart Association and USDA guidelines.