Obesity means having too much body fat. It is different from being overweight, which means weighing too much. The weight may come from muscle, bone, fat and/or body water. Both terms mean that a person's weight is greater than what's considered healthy for his or her height.
Obesity occurs over time when you eat more calories than you use. The balance between calories-in and calories-out differs for each person. Factors that might tip the balance include your genetic makeup, overeating, eating high-fat foods and not being physically active.
Being obese increases your risk of diabetes, heart disease, stroke, arthritis and some cancers. If you are obese, losing even 5 to 10 percent of your weight can delay or prevent some of these diseases.
National Institute of Diabetes and Digestive and Kidney Diseases