For these analyses, data from a subset of the NHANES III population including 1156 non-Hispanic white men, 702 non-Hispanic black men, 777 Mexican-American men and 1400 non-Hispanic white women, 939 non-Hispanic Black women, and 831 Mexican-American women were examined. Mean age, anthropometric measures, and biomarker concentrations for each subgroup of the study population are presented in . Mean age was significantly different across all race/ethnicity subgroup comparisons for each gender. Among men, Mexican-Americans were shorter and weighed less than non-Hispanic whites or blacks. Non-Hispanic blacks had a lower sum of skinfolds and waist-to-hip ratio (WHR) than the other two subgroups and non-Hispanic whites had the highest waist circumference. With regard to biomarker measures, Mexican-American men had the lowest levels of IGF-1. Non-Hispanic black men had among the highest levels of IGF-1 and the lowest levels of IGFBP-3, resulting in this group have the highest IGF-1:IGFBP-3 molar ratio.
Demographic and anthropometric variables of NHANES III subset
Among women, all three subgroups were significantly different from each other for height and percent body fat, while non-Hispanic black women weighed more than non-Hispanic whites or Mexican-Americans. Non-Hispanic white women had a lower BMI, sum of skin folds, WHR, and waist circumference than the other two groups of females. Similar to the males, Non-Hispanic black women had the highest IGF-1:IGFBP-3 molar ratio due to high IGF-1 levels and low IGFBP-3 levels when compared to the other groups.
Given the significant differences in anthropometric measures and IGF-1, IGFBP-3, and IGF1/IGFBP-3 molar ratio between the groups shown in , we stratified our subsequent analyses by race/ethnicity and gender, while also adjusting for age. We found that the relationships between levels of IGF-1, IGF-BP3, and IGF-1/IGFBP-3 molar ratio and anthropometric measurements did vary by race/ethnicity and gender (). BMI was inversely associated with IGF-1 levels in all population groups, while waist-to-hip ratio (WHR) and waist circumference were inversely associated with IGF-1 levels in all groups except non-Hispanic black men and Mexican-American females. Percent body fat was inversely associated with IGF-1 levels in all groups of women, as was sum of skin folds except for non-Hispanic Black women. While, among men, only Mexican-Americans had a significant inverse association with sum of skin folds and only Non-Hispanic white men exhibited a significant inverse association with percent body fat. Height was positively associated with IGF-1 levels only in Mexican-Americans, both male and female. Interestingly, none of the anthropometric measures, other than BMI, were associated with IGF-1 levels in non-Hispanic Black Men and the lower 95% confidence bound for the BMI regression coefficient was −0.09.
Age-adjusted regression coefficients for average IGF-1 related to anthropometric variables, stratified by race/ethnicity and gender
Among women, all three subgroups exhibited significant inverse associations of BMI and percent body fat with IGF-1 levels. The significant inverse association of WHR with IGF-1 in non-Hispanic black women but not Mexican-Americans is also interesting because these two groups did not have statistically different mean WHR, yet the estimates of the regression coefficients and 95% CI reflect notable differences in this relationship between the two subgroups. Height was positively associated with IGF-1 levels only in Mexican-Americans, both male and female.
The pattern of IGFBP-3 association with anthropometric variables also showed great variation by race/ethnicity and gender (). In non-Hispanic Black men, IGFBP-3 levels were positively associated with all anthropometric measures, except height. The only other statistically significant association among men was the positive relationship of height with IGFBP-3 in Mexican-American men. Among women, non-Hispanic whites showed a significant positive association of IGFBP-3 levels with BMI, WHR, and waist circumference. WHR also was positively associated with IGFBP-3 in non-Hispanic Black women, while height was positively associated in both non-Hispanic Black and Mexican-American women. In contrast, inverse associations of the IGF-1/IGFBP-3 molar ratio with anthropometric variables were mostly consistent across all race/ethnicity and gender groupings (); however, there was still a great deal of variation in the regression coefficients, especially among women when examining BMI, WHR and percent body fat.
Age-adjusted regression coefficients for average IGFBP-3 related to anthropometric variables, stratified by race/ethnicity and gender
Age-adjusted regression coefficients for average IGF-1:IGFBP-3 (Molar Ratio) related to anthropometric variables, stratified by race/ethnicity and gender
The analyses of the association of IGF-1, IGFBP-3, and the IGF-1/IGFBP-3 molar ratio with WHR by race/ethnicity and gender were repeated including BMI in the model to determine if adjustment for overall adiposity (i.e. BMI) altered the association with central adiposity or fat distribution pattern, as measured by WHR. When including both BMI and WHR in the same model, the significance of the association of WHR with IGF-1 and IGFBP-3 was largely attenuated in most groups. For BMI, the significance of the association with IGF-1 levels in non-Hispanic white and Mexican-American men and with IGFBP-3 levels in non-Hispanic white women was attenuated when WHR was added to the model. However, when examining the IGF-1/IGF-BP-3 molar ratio, the significance of the associations with both BMI and WHR remained in all populations after mutual adjustment, with the exception of WHR for Mexican-American males and females.
Prior reports of the association of IGF-1 levels with BMI suggested a peak near or in the overweight range (BMI of 24–27) in both men and women (20
). We sought to determine if this observation would be evident in this large population-based study by examining IGF-1 levels by BMI deciles (). The BMI deciles were weighted to the NHANES III population and the analyses were age-adjusted. Overall, the decrease in IGF-1 across BMI deciles was more pronounced among women. Among men, a general decrease in IGF-1 by BMI deciles was also seen but the difference across BMI deciles reflected by the p value was largely from the comparison of the highest BMI decile to the other deciles. Across all deciles of BMI, non-Hispanic Black men and women had the highest IGF-1 levels. IGFBP-3 levels slightly increased across BMI deciles for non-Hispanic white and black men, while largely remaining the same across BMI deciles for the other subgroups (data not shown). Since increasing BMI was associated, with decreasing IGF-1 and slightly increasing or little change in IGFBP-3, all groups showed a significant inverse association of the IGF-1/IGF-BP-3 molar ratio with BMI (data not shown). Consistent with the mean measures in , Non-Hispanic black men and women had the highest IGF-1/IGFBP-3 molar ratio across all deciles of BMI by race/ethnicity and gender.
Age-adjusted IGF-1 means by weighted BMI deciles stratified by race/ethnicity and gender, NHANES III