β-cell function in black South African women: exploratory associations with insulin clearance, visceral and ectopic fat.

The role of ectopic fat, insulin secretion and clearance in the preservation of β-cell function in black African women with obesity who typically present with hyperinsulinemia is not clear. We aim to examine the associations between disposition index (DI, an estimate of β-cell function), insulin secretion and clearance and ectopic fat deposition. This is a cross-sectional study of 43 black South African women (age 20-35 years) with obesity (BMI 30-40 kg/m2) and without type 2 diabetes that measured the following: DI, insulin sensitivity (SI), acute insulin response (AIRg), insulin secretion rate (ISR), hepatic insulin extraction and peripheral insulin clearance (frequently-sampled intravenous glucose tolerance test); pancreatic and hepatic fat, visceral adipose tissue (VAT) and abdominal subcutaneous adipose tissue (aSAT) volume (magnetic resonance imaging), intramyocellular (IMCL) and extramyocellular fat content (EMCL) (magnetic resonance spectroscopy). DI correlated positively with peripheral insulin clearance (β 55.80, p=0.002). Higher DI was associated with lower VAT, pancreatic fat and soleus fat, but VAT explained most of the variance in DI (32%). Additionally, higher first phase ISR (p=0.033) and lower hepatic insulin extraction (p=0.022) associated with lower VAT, independent from SI, rather than with ectopic fat. In conclusion, peripheral insulin clearance emerged as an important correlate of DI. However, VAT was the main determinant of a lower DI above ectopic fat depots. Importantly, VAT, but not ectopic fat, associated with both lower insulin secretion and higher hepatic insulin extraction. Prevention of VAT accumulation in young black African women should therefore be an important target for beta cell preservation.