Definitions

“Obesity is a chronic, relapsing, neurochemical disease produced by the interaction of environment and host” . In the late 1970s and 1980s, health researchers began to report an association between body fat and cardiovascular disease comorbidities, including hypertension, hyperlipidemia, and diabetes. Obesity was defined as an accumulation of excess body fat, with thresholds associated with cardiovascular disease greater than 22% body fat (%BF) for men and greater than 35% for women.

Adipose tissue is a specialized connective tissue that is comprised of lipid-filled cells (adipocytes) contained within a collagen framework. The adipocyte is 90% triglycerides, with small amounts of diglycerides, monoglycerides, cholesterol, phospholipids, free fatty acids, protein, and water. Its primary function is as a reservoir for stored energy, but it also serves as a mechanical cushion and insulator for heat conservation. Adipose tissue is the least heavy of the stored energy substrates within the human body, with a molecular density of 0.901 g/mL, although with 9 kcal/g of tissue, it has a much higher energy density than either carbohydrate (4 kcal/g) or protein (4 kcal/g). Storing energy as fat in the human body therefore provides much better mobility because of its high energy and low molecular density .

Fat storage (lipogenesis) occurs from the processing of primary foodstuffs during parasympathetic-mediated postprandial conditions. Carbohydrates, proteins, and fats are processed in the digestive tract and immediately used for reparative and metabolic functions or are stored as future fuel sources. Only small amounts of carbohydrates are stored as glycogen in muscle (150 g) and liver (90 g), and even smaller amounts are used for immediate metabolism and repair. Excess carbohydrates are converted to fat and stored efficiently as adipose tissue. Similarly, ingested protein is digested into amino acids, and those that are not immediately incorporated into structural proteins, hormones, or enzymes are converted into adipose tissue, with the excess nitrogen excreted in the urine and feces. Dietary fat and cholesterol is readily digested and packaged into water-soluble chylomicrons, which can be transported to the liver and adipocytes for processing and storage. Regardless of foodstuff composition, a diet high in caloric density will ultimately result in adipose accumulation when energy needs are sufficiently met. As the energy system is stressed in times of relative famine, carbohydrates and proteins are actually catabolized sooner and at higher rates than adipose tissue. Subsequently, attempts to diet for rapid weight loss often result in early loss of water, protein, and carbohydrates with relatively small amounts of adipose reduction. Because the protein stores used come primarily from skeletal muscle, resting metabolism is subsequently reduced after rapid weight loss, resulting in a lower rate of energy expenditure at rest and during exercise that ultimately favors accumulation of more energy storage (ie, adipose tissue).

As rates of obesity increased within the United States, the World Health Organization (WHO) began to take note and, in the mid-1990s, began to look for an easier method to identify persons at risk than the labor-intensive body composition assessment tools used by early researchers. Impressive data had been collected in the United States by the National Health and Nutrition Examination Surveys (NHANES I-III) in three different cycles, using height and weight to correlate obesity with morbidity and mortality. In 1998, the WHO introduced the use of body mass index (BMI) as a screening tool to stratify persons for risk of obesity-related disorders. BMI is determined by dividing weight (kg) by height (m) squared, and has been used by American insurance companies for decades to predict risk for morbidity and mortality. According to the WHO definitions, BMI less than 18 kg/m ² is considered underweight, 18 to 24.9 kg/m ² is normal weight, 25 to 29.9 kg/m ² is overweight, and greater than 30 kg/m ² is obese. In recent years, additional classifications for moderate (30–34.9 kg/m ² ), severe (35–39.9 kg/m ² ), and very severe (>40 kg/m ² ) obesity have also been added, reflecting exponentially increased risk for hypertension, hyperlipidemia, heart disease, and diabetes. BMI is easily calculated in most populations with minimal time, expense, and effort, and therefore provides a simple and efficient way to collect epidemiologic data and stratify risk. Unfortunately, BMI lacks the sensitivity to differentiate body fat from fat-free lean mass (FFM). Therefore, a large, muscular athlete who has only 4%BF but a BMI of 31 kg/m ² would be inappropriately classified as obese according to the WHO definition. Conversely, a person who has tetraplegia might have normal weight relative to height, but have 38%BF despite a BMI of 25 kg/m ² . This relationship will be discussed in more detail later.