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.
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:
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