Definition and Classification
Obesity, now a worldwide epidemic, has recently been declared one of the major concerns for global health by the World Health Organization (WHO) (see Chapter 74). Obesity is associated with, or a precursor state to, numerous serious diseases, including type 2 diabetes mellitus, cardiovascular disease, stroke, and certain carcinomas (see Chapter 84). In order to identify the prevalence as well as diagnose the disease in clinical settings, it is necessary to define the condition by reliable measurements. Such measurements should not only include total body fat mass, but also distribution of body fat, because central localization of excess depot fat is associated with much higher risks for serious complications than more even, or peripheral, distribution of depot fat masses (see Chapter 11).
Obesity is the condition with elevated fat masses in the body. A problem is how this should be defined. Depot fat is stored in essence only in adipose tissue as triglycerides. The function of adipose tissue is to supply the body with energy when needed. In the postprandial condition, fat is drawn from adipose tissue in the form of mobilized free fatty acids. This is then replenished by energy intake. In the absence of external sources of energy, adipose tissue is the major supplier of energy for bodily functions. A normal man or woman has about 10–15 kg of fat in adipose tissue, which is sufficient for survival during starvation for about a month. This is clearly an unnecessarily large reserve depot in current urbanized society, where food is immediately available when needed. One may then suggest that in terms of needs of reserve energy supply, the currently “normal” amount of fat in adipose tissue is not needed.
There is, however, another, more important aspect of this issue. When adipose tissue