Twin Conception in Sheep Leads to Impaired Insulin Sensitivity and Sexually Dimorphic Adipose Tissue and Skeletal Muscle Phenotypes in Adulthood.

Twins are often born small and early and have increased risk of obesity and diabetes later in life. Twin conception in sheep, regardless of whether the pregnancy continues as twins or is reduced to singleton in early gestation, alters offspring growth trajectory and body composition in young adulthood. We hypothesized that twin conception would result in insulin resistance in adulthood, with insulin-resistant adipose tissue and skeletal muscle phenotypes. At 3 years of age, body weight was not different among singletons, twins, and reductions; females weighed less than males. Singletons were leaner than reductions, with twins intermediate. Twins and reductions had decreased insulin sensitivity compared with singletons (singletons: mean [standard error of the mean]: 4.75 [0.4], twins: 3.34 [0.3], reductions: 3.67 [0.2] mg·I μU·kg·min, P < .01). There were no group differences in adipocyte size, adipose tissue, or circulating tumor necrosis factor α, monocyte chemoattractant protein 1, or interleukin 6 concentrations. In males, omental and subcutaneous adipose SLC2A4 was 1.5- to 2.0-fold greater in twins and reductions than in singletons ( P < .01) and SLC2A1 was greater in reductions than in singletons. Skeletal muscle IRS-1 was decreased in male twins but increased in female twins, compared to singletons ( P ≤ .01), with no effect on reductions in either sex. Skeletal muscle SLC2A4 was decreased in female twins and reductions but elevated in male twins and reductions compared to singletons ( P ≤ .01). We conclude that adult twin insulin resistance is not due to adipose tissue phenotype, but potentially phenotypic effects in skeletal muscle, and obesity is a result of twin conception per se with its origins in early gestation.