(Patho)physiological implications of chronic dietary sodium restriction during pregnancy; a longitudinal prospective randomized study

OBJECTIVE: To study the possible pathophysiological implications of long continued dietary sodium restriction in pregnancy. DESIGN: Longitudinal prospective randomized study of the effects of a low sodium diet compared with unrestricted sodium intake in pregnancy. SETTING: Academic Department of Obstetrics and Gynaecology at Sint Radboud Hospital, Nijmegen, The Netherlands. SUBJECTS: 42 healthy nulliparous women. INTERVENTION: A low sodium diet (20 mmol sodium daily) started in the 14th week of pregnancy and stopped after delivery. MAIN OUTCOME MEASURES: Maternal weight gain, food intake, blood pressure, cardiac output, systemic vascular resistance, haematocrit and birthweight. RESULTS: Total maternal weight gain and dietary energy intake during pregnancy and weight at 1 and 6 weeks postpartum were significantly lower in the low sodium group. Blood pressure during pregnancy did not show major differences. Stroke volume and cardiac output during pregnancy were significantly lower in the low sodium group whereas systemic vascular resistance was significantly higher. Haematocrit values in the low sodium group tended to be lower during pregnancy, but were significantly lower at 1 and 6 weeks postpartum than in the unrestricted group. Placental and birthweights were not significantly different between the two groups. CONCLUSIONS: Chronic dietary sodium restriction during pregnancy is characterized by a diminished body fat accumulation and a reduction in circulating volume, due to a decrease in both plasma and red cell volume, in combination with a high systemic vascular resistance without major effects on blood pressure and birthweight.