Impact of low nitrate/nitrite diet and nitrate supplementation on litter size and fetal and placental growth in pregnant mice

Background: In pregnancy, nitric oxide (NO) plays important roles in embryo development, maternal cardiovascular function, fetoplacental blood flow and fetal growth. Whilst the importance of NO synthase (NOS)-derived NO has been studied in pregnancy, evidence for the role of NO derived from dietary nitrate is lacking. Herein, we utilised dietary manipulation strategies to investigate the effects of dietary nitrate depletion/supplementation on pregnancy outcomes in mice. Methods: Pregnant dams received either a standard control diet or a commercially available nitrate/nitrite depleted (low NOx) diet from the start of pregnancy, along with drinking water containing either 0.7 mM NaNO3 or 0.7 mM NaCl as control. Pregnancy outcomes (litter size, fetal/placental weights) and maternal and fetal plasma nitrate/nitrite concentrations were determined at gestational day 17.5. Results and discussion: Litter size was unaffected by maternal low NOx diet, but fetal and placental weights were significantly reduced (p < 0.001 for both). Maternal plasma nitrate and nitrite concentrations were significantly lower in low NOx animals (p = 0.017 and p = 0.003, respectively), with maternal 0.7 mM NaNO3 supplementation restoring nitrate, but not nitrite, levels. Whilst fetal and placental weights were unaffected by maternal NaNO3 supplementation, litter size was significantly increased (p = 0.024). Unexpectedly, maternal food/energy intake was significantly reduced in low NOx dams (p < 0.001), suggesting that the lower fetal and placental weights may be due to nutrient insufficiency, rather than nitrate depletion per se. These findings have important implications for the interpretation of studies using commercial low NOx diets to study the effects of nitrate-depletion, particularly where food intake has not been previously reported.