IntroductionRenal functional response (RFR) is the acute increase in glomerular filtration rate (GFR) after a protein load. Low RFR is a marker of single nephron hyperfiltration. Low birth weight (LBW) is associated with reduced number of nephrons, lower kidney function, and smaller kidneys in adults. In the present study, we investigate the associations among LBW, kidney volume, and RFR.MethodsWe studied adults aged 41 to 52 years born with either LBW (≤2300 g) or normal birth weight (NBW; 3500–4000 g). GFR was measured using plasma clearance of iohexol. A stimulated GFR (sGFR) was measured on a separate day after a protein load of 100 g using a commercially available protein powder, and RFR was calculated as delta GFR. Kidney volume was estimated from magnetic resonance imaging (MRI) images using the ellipsoid formula.ResultsA total of 57 women and 48 men participated. The baseline mean ± SD GFR was 118 ± 17 ml/min for men and 98 ± 19 ml/min for women. The overall mean RFR was 8.2 ± 7.4 ml/min, with mean RFR of 8.3 ± 8.0 ml/min and 8.1 ± 6.9 ml/min in men and women, respectively (P= 0.5). No birth-related variables were associated with RFR. Larger kidney volume was associated with higher RFR, 1.9 ml/min per SD higher kidney volume (P= 0.009). Higher GFR per kidney volume was associated with a lower RFR, −3.3ml/min per SD (P< 0.001).ConclusionLarger kidney size and lower GFR per kidney volume were associated with higher RFR. Birth weight was not shown to associate with RFR in mainly healthy middle-aged men and women.