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Chapter: Introduction to Human Nutrition: Energy Metabolism

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Definition of obesity

Obesity has traditionally been defined as an excess accumulation of body energy, in the form of fat or adipose tissue.

Definition of obesity

Obesity has traditionally been defined as an excess accumulation of body energy, in the form of fat or adipose tissue. Thus, obesity is a disease of positive energy balance, which arises as a result of dysregula tion in the energy balance system – a failure of the regulatory systems to make appropriate adjustments between intake and expenditure. It is now becoming clear that the increased health risks of obesity may be conferred by the distribution of body fat. In addition, the influence of altered body fat and/or body fat dis-tribution on health risk may vary across individuals. Thus, obesity is best defined by indices of body fat accumulation, body fat pattern, and alterations in health risk profile.

The body mass index (BMI) is now the most accepted and most widely used crude index of obesity. This index classifies weight relative to height squared. The BMI is therefore calculated as weight in kilo-grams divided by height squared in meters, and expressed in the units of kg/m2. Obesity in adults is defined as a BMI above 30.0 kg/m2, while the normal range for BMI in adults is 18.5–24.9 kg/m2. A BMI in the range of 25–30 kg/m2 is considered overweight. In children, it is more difficult to classify obesity by BMI because height varies with age during growth; thus, age-adjusted BMI percentiles must be used.

 

One of the major disadvantages of using the BMI to classify obesity is that this index does not distin-guish between excess muscle weight and excess fat weight. Thus, although BMI is strongly related to body fatness, at any given BMI in a population, there may be large differences in the range of body fatness. A classic example of misclassification that may arise from the use of the BMI is a heavy football player or body-builder with a large muscle mass who may have a BMI above 30 kg/m2 but is not obese; rather, this man has a high body weight for his height resulting from increased FFM.

 

Since the health risks of obesity are related to body fat distribution, and in particular to excess abdominal fat, other anthropometric indices of body shape are useful in the definition of obesity. Traditionally, the waist-to-hip ratio has been used as a marker of upper versus lower body-fat distribution. More recent studies suggest that waist circumference alone pro-vides the best index of central body-fat pattern and increased risk of obesity-related conditions. The rec-ommended location for the measurement of waist circumference is at the midpoint between the lowest point of the rib cage and the iliac crest. The risk of obesity-related diseases is increased above a waist cir-cumference of 94 cm in men and above 80 cm in women.


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