Metabolic consequences of in utero metformin exposure on skeletal muscle in juvenile offspring in Rhesus macaques

Metformin is used to treat diabetes in pregnancy and to improve pregnancy outcomes in polycystic ovary syndrome and obesity. However, metformin crosses the placenta, accumulates in fetal tissues and amniotic fluid, and impacts fetal growth. These fetal adaptations may increase the risk for childhood obesity and metabolic dysfunction. The mechanisms underlying these increased risks are not known. Thus, our objective was to determine whether in utero metformin exposure, in an otherwise healthy pregnancy, influences offspring skeletal muscle metabolism and adaptations to a typical western diet (WD). In the fetal muscle, we found that metformin enhanced respiration with glucose-based substrates but reduced complex I (CI) reliance. Metformin also limited metabolic switching to fatty acids in fetal muscle when dams were fed a Western diet (WD). Therefore, we hypothesized that prior metformin exposure will lead to a persistent suppression of CI respiration and a decrease in ATP production that is worsened when offspring are fed a WD. Methods: Adult female Rhesus macaques were randomized to a metformin (MET, 10 mg/kg BID) or placebo group at 30 days of pregnancy and fed either a chow diet (CD) or a WD throughout pregnancy. Maternal-infant groups continued treatment through 3 months postpartum, when offspring are exclusively breastfeeding, thus preventing offspring ingestion of metformin. Offspring were weaned at 7months and continued with the maternal diet to 24 months. Gastrocnemius muscle (GAS) was collected at necropsy, and mitochondria were isolated. Leak, CI, and maximal (CI + CII) respiration were measured by high-resolution oxygraphy after the addition of TCA cycle intermediates with and without palmitoylcarnitine. In parallel, ATP production was measured by spectrometry using an NADPH-coupled reaction. Data was analyzed by two-way ANOVA for main effects (treatment and diet) and interactions. Results: WD compared to CD increased complex I (diet: p< 0.0001) and maximal respiration (diet: p=0.001) with non-lipid substrates. However, there was no concomitant increase in ATP production, resulting in a lower P/O ratio (diet: p=0.0005). Additionally, MET compared to placebo further increased CI respiration in WD and suppressed CI respiration in the CD group (interaction: p=0.0337). ATP production was not different in either diet group, comparing MET with placebo. With lipid substrate, MET increased fatty acid oxidation (p=0.02), CI (p=0.04) and maximal respiration (p=0.03) compared to offspring of placebo (treatment: p=0.0253), but did not increase ATP production. Discussion: In utero metformin exposure increases offspring skeletal muscle capacity for fatty acid oxidation and enhances overall oxidative capacity in response to a WD in lean juvenile offspring. However, reduced metabolic efficiency suggests increased cellular stress and may partially account for greater metabolic vulnerability in metformin-exposed offspring. Funding: NIH R01DK128187 01A1 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.