Effects of a westernized lifestyle on the association between fasting serum nonesterified fatty acids and insulin secretion in Japanese men.

The effects of the prolonged elevation of nonesterified fatty acid (NEFA) levels on insulin secretion have been controversial and thought to be sex-specific. To investigate the association between a westernized lifestyle and the effects of NEFA on insulin secretion in Japanese men, we examined 67 nondiabetic Japanese-American men and 220 nondiabetic native Japanese men who underwent a 75-g oral glucose tolerance test (OGTT). Most Japanese Americans we surveyed are genetically identical to Japanese living in Japan, but their lifestyle is more westernized. Sets of multiple regression analyses were performed to evaluate the relationship between the sum of the immunoreactive insulin (IRI) levels during the OGTT ((Sigma)IRI) and clinical parameters. Japanese Americans had higher levels of fasting IRI, (Sigma)IRI, and a higher insulin resistance index (homeostasis model assessment for insulin resistance [HOMA-IR]) than native Japanese, whereas there were no significant differences in fasting NEFA and triglyceride levels. A multiple regression analysis adjusted for age, fasting triglycerides, and body mass index (BMI) demonstrated that the fasting NEFA level was an independent determinant of the (Sigma)IRI only in Japanese-American men ( P = .001), but not in native Japanese men ( P = .054). Even when HOMA-IR was included in models instead of BMI, the NEFA level was a significant variable of (Sigma)IRI only in Japanese Americans ( P < .001), and not in native Japanese ( P = .098). In addition, a multiple regression analysis adjusted for age, fasting triglycerides, and BMI demonstrated that the fasting NEFA level was the only independent determinant of (Sigma)C-peptide in Japanese-American men ( P = .041). In conclusion, NEFA seems to be associated with insulin secretion independent of obesity or HOMA-IR. A westernized lifestyle may increase the effects of serum fasting NEFA levels on total insulin secretion after a glucose load in Japanese men.