PURPOSE:
The purpose of this study was to examine the relationships among diabetes, obesity and serum insulin-like growth factor 1 (IGF-1) concentrations in a large nationally representative sample of civilian, non-institutionalized adults in the United States. More specifically, the influence of obesity on the relationship among IGF-1 and markers of diabetes was examined.
METHODS:
The sample population utilized in this study was data from the Third National Health and Nutrition Examination Survey (NHANES III), which is a nationally representative sample of civilian non-institutionalized persons conducted by the National Center for Health and Prevention from 1988 through 1994. Serum IGF-1 levels were collected from participants 20 years of age and older at a mobile examination center morning session after an average overnight fast of approximately eleven hours. Participants were excluded if they were pregnant, lactating, or had incomplete data, resulting in a study population of 2430 males (47%) and 2727 females (53%). Measures of diabetes included the examination of responses to a series of questions from the questionnaire section of the NHANES III. In addition to the responses to questions, other markers of glucose control were used in this study including blood glucose, glycated hemoglobin levels, and homeostatic measure of insulin resistance (HOMA_IR). Measures of obesity included body mass index (BMI), body weight, total body fat, and waist to hip ratio. Differences in serum IGF-I across groups categorized by fasting plasma glucose and body mass index were analyzed by ANOVA and Tukey's multiple comparison test. The relationships among markers of diabetes, markers of obesity and serum IGF-1 were examined with multiple regression models that included age, gender, race-ethnicity. The Statistical Analysis System (SAS) software (version 9.1, Cary, NC and SUDAAN (Release 5.50, Research Triangle Park, NC) were utilized for all statistical analysis.
RESULTS:
Age-adjusted serum IGF-1 were significantly less in men but not women classified as diabetic compared to participants with normal blood glucose or pre-diabetes. In both men and women, across all race/ethnic groups, age-adjusted IGF-1 concentrations were significantly lower with obesity (BMI>35). A series of regression models found that age, gender and race/ethnicity are strong determinants of serum IGF-1 concentrations. However, diabetes and obesity had significant and independent effects on IGF-1. Thus the negative relationship between serum IGF-1 and diabetes was not totally explained by the presence of obesity.
CONCLUSION:
The role of IGF-1 in the etiology and/or possible treatment of diabetes is an important topic of basic and clinical research. In the present study of a large, diverse population, the complexities of the relationships among IGF-1 adiposity and impaired glucose metabolism were evident. The presence of obesity and diabetes, independently result in lower serum IGF-1 concentrations even when accounting for the influences of age, gender and race/ethnicity. The mechanisms of these effects and the clinical implications of lower circulating IGF-1 on morbidity and mortality await further investigation.