Relationships of plasma C-peptide and gender to the urinary excretion of inositols in older people

Purpose: The urinary excretions of myo-inositol and D-chiro-inositol are el evated in diabetes, and have been suggested as possible markers or effecter s of insulin action. The aim of the present study was to measure the urinar y excretion of these compounds, and to assess possible relationships with t he metabolic control of glucose, in older, non-diabetic men and women. Subj ects: 32 older (age range 54-71 yrs), moderately overweight (body mass inde x 29.1 +/- 0.4 kg/m(2), mean +/- SEM), nondiabetic men (n = 17) and women ( n = 15). Methods: 75 g oral glucose tolerance testing was done the day afte r all subjects had consumed nutrient-defined menus for five days. Plasma sa mples were analyzed for the concentrations of glucose, insulin, and C-pepti de, and the 180-minute area under the curve (AUC) for each of these compoun ds was calculated. Samples from 24-hour urine collections were analyzed for the concentrations of myo-inositol, D-chiro-inositol, L-chiro-inositol, an d pinitol. Results: The fasting glucose, insulin, and C-peptide, and the AU C for glucose and insulin, were not different between men and women. C-pept ide AUC was greater in the men versus the women (p < 0.001). The median uri nary excretions (<mu>mol/g creatinine) of myo-inositol (p < 0.001), D-chiro -inositol (p < 0.001), L-chiro-inositol (p < 0.05), and pinitol (p < 0.001) were higher, and the myo-inositol: D-chiro-inositol ratio was lower (p < 0 .001), in the men versus women. For all subjects combined, C-peptide AUC wa s positively correlated with the urinary excretion of each of the measured inositols, as well as the myo-inositol: D-chiro-inositol ratio. The correla tions between C-peptide AUC and these inositols were strongly influenced by the co-linear relationship between C-peptide AUC and gender. Conclusions: Collectively, these data show that older, moderately overweight, non-diabet ic men and women with gender-related differences in glucose-stimulated C-pe ptide AUG, an indirect indicator of insulin secretion, also display differe nces in the urinary excretion of myo-inositol, D-chiro-inositol, L-chiro-in ositol, and pinitol. The gender-related difference in the myo-inositol: D-c hiro-inositol ratio suggests that, while the urinary excretion of all of th e inositols measured were higher in the men than the women, the difference was more pronounced for D-chiro-inositol.