Effect of Obesity on Body Introduction Obesity is a medical condition in which abnormal or excessive fat accumulation takes place throughout the body which may show a negative effect on health. A person is considered obese if they have a high body mass index (BMI). BMI measures your healthy weight in relation to your height. For adults, a BMI of: 5 to 24.9 – healthy weight 25 to 29.9 – overweight 30 to 39.9 – obese 40 or above –