(BMI): a method used by scientists and researchers to define degrees of over and underweight. In combination with waist circumference, it is a reliable guide to how much fat you carry. BMI is derived from extensive data held by life-insurance companies and is calculated by dividing a person's weight by the square of his or her height (kilograms/metres2). The risk of certain obesity-related illnesses increases as BMI goes up, so the higher your BMI the more at risk you are. Underweight adults have a BMI less than 18.5; acceptable BMIs range from 18.5 to 24.9. A level of 25 – 29.9 represents overweight and any figure above 30 characterises obesity and puts the person at a higher risk of conditions such as diabetes, heart disease, hypertension, stroke and arthritis. BMI overestimates fat in athletes or very muscular people and may underestimate it in older persons. The Index may be used (with some modification) to assess children and adolescents. (See OBESITY, BARIATRIC SURGERY.)