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1 From the Center for Human Nutrition and Departments of Clinical Nutrition and Internal Medicine, University of Texas Southwestern Medical Center, Dallas
See corresponding article on page 38
INTRODUCTION
Obesity is accompanied by many medical complications. Foremost among these is atherosclerotic cardiovascular disease. The term metabolic syndrome is often used to denote the multiple metabolic risk factors that accompany obesity and increase the risk of atherosclerotic cardiovascular disease (1). Risk factors for the metabolic syndrome include atherogenic dyslipidemia, elevated blood pressure, hyperglycemia, a prothrombotic state, and a proinflammatory state. Persons with the metabolic syndrome are also at increased risk of developing type 2 diabetes because most of them have insulin resistance, which is a major risk factor for diabetes. In addition to atherosclerotic cardiovascular disease and diabetes, persons with the metabolic syndrome are more likely to have fatty liver and its complications, nonalcoholic steatohepatitis, and cryptogenic cirrhosis. Finally, persons with the metabolic syndrome may be susceptible to yet another complication, namely, cholesterol gallstones.
OBESITY AND CHOLESTEROL GALLSTONES
A relation between obesity and cholesterol gallstones is well recognized. Obese women are more likely to develop gallstones than are obese men (2). In the past, our laboratory focused on the mechanisms whereby obesity increases the risk of developing gallstones (3, 4). The bile in obese persons is more lithogenic than is that in nonobese persons, ie, the bile of obese persons has a high ratio of cholesterol to solubilizing lipids (bile acids and phospholipids). This high ratio predisposes to crystallization of cholesterol and gallstone formation. The primary reason for lithogenic bile in obese persons is an increase in total body synthesis of cholesterol. The metabolic basis for this increase remains to be elucidated. A likely prime mechanism is overloading of tissues with fatty acids, which provide precursors for cholesterol synthesis. Other adipose tissue–derived substances, which are produced in excess in obesity, may further contribute to overproduction of cholesterol. As a result of this overproduction, cholesterol secretion into bile increases. In many obese persons, the amounts of cholesterol entering the bile exceed the solubilizing capacity of the bile acids and phospholipids. Both obese women and obese men synthesize increased amounts of cholesterol and secrete more cholesterol into bile than do nonobese persons (5). Obese men generally secrete more bile acids and phospholipids into bile than do obese women; consequently, the bile of obese men is less lithogenic, and they have fewer gallstones.
INDICATORS OF OBESITY
The precise relation between obesity and gallstones, however, remains undefined. To understand the relation, an important question must be answered: what is obesity? A simple definition is an excess of total body fat. One proposed simple definition is a body fat content of >25% in men and >33% in women (6). Because the measurement of percentage of body fat requires special methodology, both clinical evaluation and epidemiologic studies use surrogate indicators (7). One such indicator is body mass index (BMI), which is weight (in kg) divided by height (in m) squared. The correlation between BMI and percentage of body fat is positive but by no means perfect. The correlation is higher in women than in men. In men, waist circumference appears to be a better indicator of total body fat than is BMI (7), because waist circumference is strongly determined by fat in the upper body and because men have a tendency to deposit fat in the upper body.
BODY FAT DISTRIBUTION IN OBESE PERSONS
Two commonly identified patterns of obesity are lower body obesity and upper body obesity. In the former, excess fat predominantly accumulates subcutaneously in the gluteofemoral region; in the latter, fat accumulates predominantly in the trunk. The adipose tissue in the trunk occurs in several compartments: truncal subcutaneous, intraperitoneal (visceral), and retroperitoneal (8). When women first become obese, they generally show the pattern of lower body obesity; as obesity increases, excess fat also begins to accumulate in the trunk. Less commonly, women have upper body obesity without excess lower body fat. Most, but not all, obese men show the pattern of upper body obesity from the start. In most cases of upper body obesity, excess fat is located predominantly in the subcutaneous region. In some obese men, however, there can be a disproportionate accumulation of fat intraperitoneally. The latter has been called visceral obesity. This term is often misapplied because persons with predominant upper body subcutaneous obesity are mistakenly said to have visceral obesity (9). The term upper body obesity, which refers to all the fat in the trunk, is preferable. An alternative name for upper body obesity is abdominal obesity (7). The latter name has 2 practical advantages: 1) the presence of excess truncal fat is more noticeable around the waist than around the chest, and 2) simple measurement of waist circumference is a good indicator of upper body fat.
UPPER BODY OBESITY AND METABOLIC DISORDERS
Abdominal obesity is a characteristic feature of the metabolic syndrome (1). Because the presence of abdominal obesity indicates an increase in total body fat in men and in very obese women, some investigators believe that BMI, which is another measure of total body fat, provides the same predictive information for the metabolic complications of obesity that measures of abdominal obesity provide (10). The consensus view, however, is that waist circumference, which is an indicator of abdominal fat, provides predictive power for metabolic complications beyond that provided by BMI (1, 7). Some investigators hypothesize that the visceral component of abdominal obesity is the major determinant of metabolic complications. Others contend that total abdominal fat is more telling than is visceral fat alone (9). Regardless of which view is correct, the literature seems to suggest that upper body fat is more bioactive than is lower body fat. This bioactivity includes increased production of nonesterified fatty acids, inflammatory cytokines, leptin, and prothrombotic factors and decreased production of adiponectin, a putative protective adipokine. Thus, if upper body adipose tissue is the predominant source of these bioactive products, then it may be the major cause of the higher prevalence of metabolic complications in obese persons than in nonobese persons. The report by Tsai et al (11) in this issue of the Journal that abdominal obesity is a stronger predictor of gallstones in men than is BMI is consistent with the concept that upper body adipose tissue is a major cause of metabolic complications. Still, whether the greater predictive power of upper body fat relates to unique metabolic features of this type of adipose tissue or just to the fact that waist circumference in men is a better indictor of total body fat than is BMI is unclear. In any case, the association between abdominal obesity and gallstone formation represents an additional complication that appears to be associated with the metabolic syndrome. It would be interesting to know how well the incidence of gallstone disease correlates with other components of the metabolic syndrome.
REFERENCES
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