Influence of obesity on immune function.

Obesity has been linked with a wide variety of health problems including hypertension, dyslipidemia, diabetes mellitus, certain forms of cancer, cardiovascular disease, and gallstones (1). Clinical and epidemiologic data also indicate that the incidence and severity of specific types of infectious illnesses are higher in obese persons compared with lean persons (2-5). For example, obesity has been identified as a risk factor for infection and poor wound healing after surgical procedures (3,4), and overweight patients with burns have been shown to be at greater risk for infection and bacteremia than their nonobese counterparts (5). Obesity has also been associated with a poor antibody response to hepatitis B plasma vaccine (6,7). Finally, several animal studies have suggested that obesity is linked to impaired immunity (2,8-10).

Although these data provide indirect evidence of reduced immunocompetence among obese human beings, only a small number of studies have compared immune function in obese and nonobese persons, and these have included a limited range of immune measures (2,11-14). In a study by Tanaka et al (11), impaired T- and B-cell function was reported in 34 obese men and women who were compared with 35 nonobese control subjects. Obese children and adolescents have been reported to have impaired cutaneous delayed-type hypersensitivity responses, mitogen-stimulated lymphocyte proliferation, and bactericidal capacity of neutrophils (12). Persons with morbid obesity have been observed to have a lower bactericidal capacity of neutrophils compared with nonobese control subjects (13). We have shown previously that obesity is related to higher blood levels of total leukocytes, neutrophils, and monocytes (14), which have been identified as independent risk factors for cardiovascular disease and cancer (15-19). The mechanisms responsible for the increased risk of infection and altered immunity in obese persons are unknown, but they may be linked to the negative effects of hyperglycemia, hyperinsulinemia, and hyperlipidemia on the function of certain immune cells (2,8). Obesity is a condition associated with altered metabolic, psychological, and physical fitness status, and immunologic alterations are likely to be mediated by one or a combination of these variables (1,2,11,14,20).

A clear picture of the influence of obesity on human immunity has not yet emerged, and findings to date must be regarded as preliminary (2). The purpose of this study was to compare a wide variety of measures of immune function in obese and nonobese subjects. To test for the influence of other factors on immunity, aerobic fitness, psychological well-being, and serum levels of glucose, triglycerides, and cholesterol were measured and included in multiple regression models to determine their comparative effects (2,8,11,14).

METHODS

Subjects and Research Design

Obese and nonobese, female, white subjects were recruited from the surrounding community through advertisements according to these selection criteria (14): between the ages of 25 and 75 years; in good health with no known diseases, including diabetes, cancer, and heart disease; body mass index (BMI, calculated as kg/[m.sup.2]) between 25 and 65 for obese subjects and less than 25 for nonobese subjects (20); not currently following a reducing diet; not using medications known to affect immune function; not using supplements in excess of 100% of the Recommended Dietary Allowance (21) on a regular basis; not experiencing chronic pain, marked sleep disturbance, serious allergies, salient emotional or mood problems; no recent history of systemic infection, bone fracture, or surgery; and not smoking cigarettes. Blood chemistry tests revealed that all subjects had normoglycemia and normal liver and kidney function. We recruited obese subjects who ranged widely in BMI (mild to severe obesity) so that the effect of different degrees of obesity on immunity could be evaluated by multiple regression analysis. Subjects also varied in age from young to old to test the influence of age on the obesity-immunity relationship. Informed consent was obtained from each subject, and the experimental procedures were approved by the institutional review board of Appalachian State University.

Measurements of body composition, aerobic fitness, psychological well-being, and immune function were conducted during January 1996 and January 1997. The same procedures were used each year; data were …