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What is obesity and how is it measured?

Obesity is a term used to describe body weight that is significantly greater than what is considered healthy. If you are obese, you have a higher amount of body fat compared to lean muscle mass. Importantly, obesity is a diagnosis, not a judgment of character.
One of the simplest and most accepted ways to measure obesity involves calculation of the body mass index (BMI). BMI is a determination of body fat based on height and weight. Adults with a BMI of 25 to 29.9 are considered overweight while those with a BMI of 30 or more are considered obese. Obesity can be further classified as Class I, II or III depending upon the degree of increase in BMI over 30. Class III obesity is sometimes referred to as extreme obesity. Morbid obesity is when an obesity-associated complication (see below) exists in an individual with any class of obesity. BMI does have some limitations - it may overestimate body fat in those with a muscular build and it may underestimate body fat in those who have lost muscle mass.

Weight Classification


Below 18.5
Normal weight

18.5 - 24.9

25.0 - 29.9
Class I
30.0 - 34.9

Class II

Class III
40.0 and above

You can calculate your own BMI from your height and weight using the National Heart Lung and Blood Institute (NHLBI) calculator at www.nhlbisupport.com/bmi/bmicalc.htm or you can look it up on the NHLBI BMI table at www.nhlbi.nih.gov/guidelines/obesity/bmi_tbl.htm
The places where you store your body fat may affect your health. Some people store fat primarily in their hips and buttocks while others store fat primarily in their abdomen. If most of your fat is in your abdomen rather than in your hips, you're at higher risk for heart disease, cancer and diabetes. Waist circumference can be used as a measure of abdominal fat. A waist circumference of more than 35 inches around in women or more than 40 inches around in men puts them at higher risk of weight-related health problems.
To measure your waist circumference, stand straight, loop a tape measure around your bare abdomen just above your hip bones, bringing the tape measure together at the belly button. Take the measurement when the tape is snug but not pinching and just after you breathe out.

What causes obesity?

Obesity occurs when you eat more calories than you use doing activities - your body stores the difference. A calorie is a unit of energy in food and drink. Your body needs energy to be healthy and active but if you take in more energy than your body uses, over time you will gain weight. Be aware that eating a diet consisting of low-fat or low-carbohydrate/sugar foods without accounting for the total number of calories consumed can result in weight gain! In addition to eating, activity level and exercise habits, a person's weight is a result of many factors including the environment and culture in which they live, their family history and genetics, and how their body uses the energy consumed ('metabolism').Other factors such as certain medical conditions, pregnancy, emotional factors, smoking cessation, aging and lack of sleep may also contribute. Importantly, while hypothyroidism (low thyroid function) is often blamed as a cause of obesity, its role has largely been exaggerated. Certainly, obesity should not be attributed to thyroid disease if adequate thyroid replacement treatment is in place. Finally, some medications may also cause weight gain and increase body fat, although the propensity of these medications to increase body weight differs considerably. Examples of some medications that may cause
3weight gain are as follows:

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