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in pregnancy and are able to respond to homeostatic demands (Weinberger et al., 1977). Though disputed (Weinberger et al., 1977), pregnant women appear to handle acute and large saline loads as high as 9.5 g (410 mmol) (Chesley et al., 1958) as well as that seen in nonpregnant women. At the opposite extreme of sodium intake, it is evident that in cultures with virtually no sodium intake (e.g., the Yanomamo Indians), reproduction occurs with markedly higher levels of plasma renin activity and serum aldosterone compared with that observed in nonpregnant women; no evidence of observable adverse effects of such extreme diets on gestational outcome have been reported (Oliver et al., 1981).

Plasma Sodium Concentration. Plasma sodium concentration decreases 4 to 5 mmol/L during normal pregnancy due to the resetting of the osmotic threshold for arginine vasopressin secretion and thirst to a level ≈ 10 mOsm/kg below nonpregnant values (see Chapter 4). Thus pregnant women should not be considered hyponatremic until concentrations fall to 130 mmol/L or lower. In contrast, values exceeding 140 mmol/L should raise suspicion of hypernatremia. Finally, the propensity of pregnant women to vomit in the first trimester and the possibility that their onset of sweating at a lower temperature may mean they have greater sweat loss and thus greater sodium losses (Clapp, 1991) might also affect plasma sodium concentrations and hence sodium requirements.

Plasma sodium concentration decreases during pregnancy despite the small but positive cumulative sodium balance previously discussed (at its greatest, just a few mg/day). There are also many gestational physiological changes. They include increased glomerular filtration rate and therefore increased filtered sodium; alterations in plasma concentration of hormones that influence sodium excretion, thus labeled as natriuretic (e.g., progesterone, atrial natiuretic peptide) and antinatriuretic (e.g., angiotensin II, aldosterone, desoxycorticosterone); and even physical factors (e.g., oncotic pressure). All of these physiological changes are known to influence kidney function, but how they eventually affect renal sodium handling is still obscure.

Summary. There is a lack of evidence to suggest that sodium requirements of preganat women differ from that of nonpregnant women. The median energy intake of pregnant women (1,978 kcal/day [IOM, 2002]) falls within the range of energy consumed by young men and women (IOM, 2002). Therefore, the AI for sodium for pregnant women is equal to the AI for nonpregnant adolescent



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