Introduction

Stable weight depends on an even balance between energy intake from food and energy expenditure. Energy expenditure occurs during the day in three ways:

• As energy expended during rest (basal metabolism)
• As energy used to metabolize food (thermogenesis)
• As energy expended during physical activity

Basal metabolism accounts for about two-thirds of expended energy, which is generally used to maintain body temperature and muscle contractions in the heart and intestine.

Thermogenesis accounts for about 10% of expended energy.

When a person's consumes more calories than energy that is used, the body stores the extra calories in fat cells. Fat cells function as energy reservoirs. They enlarge or contract depending on how people use energy. If people do not balance energy input and output by eating right and exercising,  fat can builds up. This can lead to weight gain.

When energy input is equal to energy output, there is no expansion of fat cells (lipocytes) to accommodate excess. It is only when more calories are taken in than used that the extra fat is stored in the lipocytes and the person begins to accumulate fat.

Measurement of Obesity

Obesity is determined by measuring body fat, not just body weight. People might be over the weight limit for normal standards, but if they are very muscular with low body fat, they are not obese. Others might be normal or underweight, but still have excessive body fat. The following measurements and factors are used to determine whether or not a person is overweight to a degree that threatens their health:

• Body mass index (BMI) (a measure of body fat)
• Waist circumference
• Waist-hip ratio
• Anthropometry (skin fold measurement)
• The presence or absence of other disease risk factors (e.g., smoking, high blood pressure, unhealthy cholesterol levels, diabetes, relatives with heart disease)

A person's disease risk factors plus BMI may be the most important components in determining health risks with weight.

The Body Mass Index (BMI). The current standard measurement for obesity is the body mass index (BMI). In general, a BMI of 25 to 29.9 means you are overweight. Obesity is a BMI of 30 and above. Obesity is then classified into three categories:

• Class 1: BMI of 30 to 34.9
• Class II: BMI 35 to 39.9
• Class III: BMI of 40 and greater

These guidelines are very important for people at risk for diabetes, heart disease, or certain cancers. It is also used to determine treatment approaches such as when surgery may be appropriate. The higher the BMI, the greater the risk for significant health problems.

Waist Circumference and Waist-Hip Ratio. The extent of abdominal fat can also be used in assessing risk of disease. Some studies suggest that:

• Women whose waistlines are over 31.5 inches and men whose waists measure over 37 inches should watch their weight.
• A waist size greater than 35 inches in women and 40 inches in men is associated with a higher risk for heart disease, diabetes, and impaired functioning.

Evidence strongly suggests that more body fat around the abdomen and hips (the apple-shape) is a more consistent predictor of heart problems and health risks than BMI.

The distribution of fat can be evaluated by dividing waist size by hip size. For example, a woman with a 30-inch waist and 40-inch hip circumference would have a ratio of 0.75; one with a 41-inch waist and 39-inch hips would have a ratio of 1.05. The lower the ratio the better. The risk of heart disease rises sharply for women with ratios above 0.8 and for men with ratios above 1.0.

Click the icon to see a depiction of the waist-to-hip ratio.

Anthropometry. Anthropometry is the measurement of skin fold thickness in different areas, particularly around the triceps, shoulder blades, and hips. This measurement is useful in determining how much weight is due to muscle or fat.