Insulin secretion and glucose tolerance were studied in 20-week-old male and female offspring of rat dams maintained on an isocaloric 20% or 8% protein diet during pregnancy and lactation after transfer to the same diet at weaning. Protein-restricted male and female offspring were also weaned onto a 20% protein diet. In males, post-absorptive insulin concentrations were suppressed by protein restriction from conception to adulthood (by 41%; P<0.001); however, basal insulin levels were 2.6-fold higher (P<0.001) if protein restriction was limited to gestation and lactation. Post-absorptive insulinaemia in females was unaffected by early or sustained protein restriction, but was lower than for males in the control group and the group exposed to protein restriction during early life alone (by 40% (P<0.001) and 52% (P<0.001) respectively). Plasma insulin/blood glucose ratios were higher in males compared with females in both control and early protein-restricted groups (1.6-fold (P<0.05) and 2.3-fold (P<0.001) respectively). A positive linear relationship existed between mean ambient insulin and glucose concentrations in males (r=1.0) and females (r=0.9), but the gradient was 12.4-fold greater (P<0.01) in males. beta-Cell function was evaluated after intravenous glucose challenge. In males, the acute insulin response and the suprabasal 30-min area under the insulin curve were dramatically higher in rats exposed to protein restriction during gestation and lactation alone (2.6- and 2.8-fold respectively; P<0.001). In contrast, these parameters were lowered by extending the exposure to protein restriction to adulthood in males, and by either early or prolonged exposure to protein restriction in females. The insulin resistance index was increased (2.5-fold; P<0.001) in male, but not female, rats exposed to protein restriction during gestation and lactation alone, and was not increased by extending the period of protein restriction to adulthood in either sex. Thus the data have demonstrated gender-specific lowering of insulin sensitivity due to protein restriction during early life only. The insulinogenic index (insulin response in relation to prevailing glycaemia) was increased in male, but not female, rats exposed to protein restriction during gestation and lactation alone (3.0-fold; P<0.001). A modest decline in insulin secretion in the female groups exposed to protein restriction until either the end of lactation or adulthood was compensated by increased insulin sensitivity, as demonstrated by significant decreases in the insulin resistance index in both groups (by 48% and 52% respectively; P<0.05). Glucose disappearance rates did not differ between the male and female control or early protein-restricted groups but were higher in both male (31%; P<0.05) and female groups (46%; P<0.001) exposed to protein restriction from conception to adulthood. Marked gender differences in glucose-stimulated insulin secretion were not associated with gender differences with respect to glucose tolerance. Our data therefore demonstrated that exposure to protein restriction during early life alone leads to relative insulin resistance and hyperinsulinaemia in adulthood, but this relationship is gender specific, observed only in males, and glucose tolerance is maintained.
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