the overall impact on calcium balance is unclear, as is the role of sodium intake on bone mineral density (Table 6-20). Although some epidemiological studies have reported an inverse effect of sodium intake on bone mineral density (Devine et al., 1995; Martini et al., 2000), this relationship was not apparent in other studies (Jones et al., 1997; Matkovic et al., 1995). The effects of a reduced sodium intake in preventing bone fractures has not been tested.
Hypercalciuria is a common risk factor for the formation of renal stones (Strauss et al., 1982). Individuals who were found to form calcium stones were reported to have a higher sodium chloride intake (14 g [239 mmol]/day) compared with healthy subjects (8 g [136 mmol]/day) (Martini et al., 1998). A prospective cohort study showed a significant trend (p < 0.001) for the risk of renal stones with increased sodium intake (Curhan et al., 1997). The risk of renal stones has been reported to increase with an increased sodium:potassium ratio (Stamler and Cirillo, 1997).
Several studies have examined the relationship between sodium intake and bronchial responsiveness to agents (e.g., histamines) that cause airway constriction. In two surveys, bronchial reactivity was strongly and directly related to urinary sodium excretion after adjusting for age and cigarette smoking (Burney et al., 1986; Tribe et al., 1994). In analysis of NHANES III data (Schwartz and Weiss, 1990), bronchitis was positively associated with the dietary sodium:potassium ratio. However, other cross-sectional studies have not found a relationship (Britton et al., 1994; Zoia et al., 1995).
A low salt diet (3.75 g/day, containing 1.5 g [65 mmol] of sodium) improved while a high salt diet (13.75 g/day, containing 5.5 g [239 mmol] of sodium) worsened postexercise pulmonary function in subjects with exercise-induced asthma (Gotshall et al., 2000). When asthmatic patients were given 4.6 g (200 mmol)/day of dietary sodium, all measures of severity of asthma were adversely affected (Carey et al., 1993). Furthermore, salt loading (6.1 g/day, containing 2.4 g [105 mmol] of sodium) was found to worsen the symptoms of asthma (Medici et al., 1993).
It has been hypothesized that high doses of salt can result in destruction of the mucosal barrier of the stomach such that the mucus membrane is easily invaded by carcinogens (Correa et al., 1975).