The BMI (defined as weight in kg/[height in m]2) provides a measure of relative weight, adjusted for height. This allows comparisons within and between populations. The healthy range for the BMI is between 18.5 and 24.9. Individuals with a BMI between 25 and 29.9 are considered overweight, those with a BMI ≥30 are defined as obese, and a BMI >40 is considered severely (morbidly) obese (Fig. 26.1). These cutoffs are based on studies examining the relationship of BMI to premature death and are similar in men and women. Nearly two thirds of U.S. adults are overweight, and more than one third of those are obese. Children with a BMI-for-age above the 95th percentile are considered obese.
B. Anatomic differences in fat deposition
The anatomic distribution of body fat has a major influence on associated health risks. A waist/hip ratio (WHR) >0.8 for women and >1.0 for men is defined as android, apple-shaped, or upper-body obesity and is associated with more fat deposition in the trunk (Fig. 26.2A). In contrast, a lower WHR reflects a preponderance of fat distributed in the hips and thighs and is called gynoid, pear-shaped, or lower-body obesity. It is defined as a WHR of <0.8 for women and <1.0 for men. The pear shape, more commonly found in women, presents a much lower risk of metabolic disease, and some studies indicate that it may actually be protective. Thus, the clinician can use simple indices of body shape to identify those who may be at higher risk for metabolic diseases associated with obesity.
About 80%–90% of human body fat is stored in subcutaneous (subq) depots in the abdominal (upper body) and the gluteal-femoral (lower body) regions. The remaining 10%–20% is in visceral depots located deep within the abdominal cavity (Fig. 26.2B). Excess fat in visceral and abdominal subq stores increases health risks associated with obesity.
C. Biochemical differences in regional fat depots
The regional types of fat described above are biochemically different. Subq adipocytes from the lower body, particularly in women, are larger, very efficient at fat (triacylglycerol [TAG]) deposition, and tend to mobilize fatty acids (FA) more slowly than subq adipocytes from the upper body. Visceral adipocytes are the most metabolically active. In obese individuals, both abdominal subcutaneous and visceral depots have high rates of lipolysis and contribute to increased availability of free fatty acids (FFA). These metabolic differences may contribute to the higher health risk found in individuals with upper body (abdominal) obesity. [Note: FFA impair insulin signaling and are proinflammatory (see p. 343).] 1.
Endocrine function: White adipose tissue, once thought to be a passive reservoir of TAG, is now known to play an active role in body weight regulatory systems. For example, the adipocyte is an endocrine cell that secretes a number of protein regulators (adipokines), such as the hormones leptin and adiponectin. Leptin regulates appetite as well as metabolism (see p. 352). Adiponectin reduces FFA levels in the blood (by increasing FA oxidation in muscles) and has been associated with improved lipid profiles, increased insulin sensitivity resulting in better glycemic control, and reduced inflammation in patients with diabetes. [Note: Adiponectin levels decrease as body weight increases, whereas leptin levels increase.] 2.