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A hypothesis that postchallenge hyperglycemia in subjects with low body weight (BW) may be due, in part, to small glucose volume (GV) was tested. We studied 11,411 nondiabetic subjects with a mean BW of 63.3 kg; 5,282 of them were followed for a mean of 5.3 yr. In another group of 1,537 nondiabetic subjects, insulin sensitivity, secretion, and a product of the two (index of whole body insulin action) were determined. Corrected 2 h-plasma glucose (2hPGcorr) during a 75-g oral glucose tolerance test in subjects with BW ≤ 59 kg was calculated as 2hPGcorr = δPG2h · ECW/[16.1 (males) or 15.3 (females)] + fasting PG (FPG), where δPG2h is plasma glucose increment in 2 h; ECW is extracellular water (surrogate of GV); FPG is fasting plasma glucose; and 16.1 and 15.3 are ECW of men and women, respectively, with BW = 59 kg. Multivariate analyses for BW with adjustment for age, sex, and percent body fat were undertaken. BW was, across its entire range, positively correlated with FPG ( P < 0.01). Whereas BW was correlated with 2hPG and δPG in a skewed J-shape, with inflections at around 60 kg ( P for nonlinearity < 0.01 for each). Nonetheless, in those with BW ≤ 59 kg, insulin sensitivity, secretion, and action were unattenuated, and incident diabetes was less compared with heavier counterparts. BW was linearly correlated with 2hPGcorr, i.e., the J-shape correlation was mitigated by the correction. In conclusion, postchallenge hyperglycemia in low BW subjects is in part due to small GV rather than impaired glucose metabolism.