DIETARY-FAT AND INSULIN SENSITIVITY IN A TRIETHNIC POPULATION - THE ROLE OF OBESITY - THE INSULIN-RESISTANCE ATHEROSCLEROSIS STUDY (IRAS)

From the Insulin Resistance Atherosclerosis Study (IRAS), 1173 men and women of African-American, non-Hispanic white, and Hispanic ethnicity with no history of diabetes were included in an evaluation of the cro ss-sectional relation of habitual dietary fat intake with insulin sens itivity (S-I) as assessed by minimal-model analysis of a 12-sample, in sulin-modified frequently sampled intravenous-glucose-tolerance test. Dietary intake was measured by a food-frequency interview modified to enhance sensitivity to food choices within the three ethnic groups. Pe rcentage of energy from total fat was associated with rank of S-I (S-I (rank); r = -0.06, P = 0.03), with log fasting insulin (r = 0.10, P < 0.001), and with BMI (r = 0.10, P < 0.001). Multiple-linear-regression models included S-I(rank) as the dependent variable, dietary fat (g/d ) as the primary independent variable adjusted sequentially for total energy, other covariates, body mass index, and waist-hip circumference ratio (WHR). For all subjects combined, total fat intake was inversel y related to S-I(rank), but this association was not significant (P = 0.14) and was attenuated by adjustment for body mass index and WHR (P = 0.44). The association of total fat (g/d) with S-I(rank) differed si gnificantly (P < 0.01) for obese compared with nonobese individuals. H igher fat intake was associated with lower S-I(rank) among obese (beta = -1.40, P = 0.03) but not among nonobese persons (beta = 0.16, P = 0 .80). Among the obese (body mass index less than or equal to 63), adju stment for body mass index largely accounted for the observed associat ion of dietary fat with S-I(rank). These findings were generally consi stent for monounsaturated, polyunsaturated, and saturated fats. Among individuals already at increased risk for non-insulin-dependent diabet es mellitus because of obesity, high intake of dietary fat may worsen insulin sensitivity. This effect may be mediated by the relation of di etary fat to obesity.