Nutritional Factors Affecting Calcium Requirements - Sodium

FAO/WHO  and other Nutritionists...verbatim

The calculations of calcium requirements proposed above were based on data from developed countries (notably the United States and Norway) and can only be applied with any confidence to nations and populations with similar dietary cultures. Other dietary cultures may entail different calcium requirements and call for different recommendations.
In particular, the removal or addition of any nutrient that affects calcium absorption or excretion must have an effect on calcium requirement. Two such nutrients are sodium and animal protein, both of which increase urinary calcium and must be presumed therefore to increase calcium requirement. A third candidate is vitamin D because of its role in calcium homeostasis and calcium absorption.

Sodium

Walser, M. 1961. Calcium clearance as a function of sodium clearance in the dog. Am. J. Physiol., 200: 769-773.

Nordin, B.E. & Need, A.G. 1994. The effect of sodium on calcium requirement. Advances in Nutritional Research. Volume 9 Nutrition and Osteoporosis. Draper, H.H. ed. p.209-230. New York: Plenum Press.

It has been known at least since 1961 that urinary calcium and that sodium administration raises calcium excretion, presumably because sodium competes with calcium for re-absorption in the renal tubules. Regarding the quantitative relationships between the renal handling of sodium and calcium, the filtered load of sodium is about 100 times that of calcium (in molar terms) but the clearance of these two elements is similar at about 1 ml/min, which yields about 99 percent re-absorption and 1 percent excretion for both.

Goulding, A. & Lim, P.E. 1983. Effects of varying dietary salt intake on the fasting excretion of sodium, calcium and hydroxyproline in young women. NZ Med. J., 96: 853-854.

Sabto, J., Powell, M.J., Breidahi, M.J. & Gurr, F.W. 1984. Influence of urinary sodium on calcium excretion in normal individuals. Med. J. Aust., 140: 354-356.

Kleeman, C.R., Bohannan, J., Bernstein, D., Ling, S. & Maxwell, M.H. 1964. Effect of variations in sodium intake on calcium excretion in normal Humans. Proc. Soc. Exp. Bio. (NY), 115: 29-32.

Epstein, F.H. 1968. Calcium and the kidney. Am. J. Med., 45: 700-714.

However, these are approximations, which conceal the close dependence of urinary sodium on sodium intake and the weaker dependence of urinary calcium on calcium intake. It is an empirical fact that urinary sodium and calcium are significantly related in normal and hypercalciuric subjects on freely chosen diets.

Nordin, B.E.C. & Polley, K.J. 1987. Metabolic consequences of the menopause. A crosssectional, longitudinal, and intervention study on 557 normal postmenopausal women. Calcif. Tissue Int., 41: S1-S60.

Sabto, J., Powell, M.J., Breidahi, M.J. & Gurr, F.W. 1984. Influence of urinary sodium on calcium excretion in normal individuals. Med. J. Aust., 140: 354-356.

The slope of urinary calcium on sodium varies in published work from about 0.6 percent to 1.2 percent (in molar terms); a representative figure is about 1 percent – that is, 100 mmol of sodium (2.3 g) takes out about 1 mmol (40 mg) of calcium.

Goulding, A. & Campbell, D. 1983. Dietary NaCl loads promote calciuria and bone loss in adult oophorectomized rats consuming a low calcium diet. J. Nutr., 113: 1409-1414.

McParland, B.E., Goulding, A. & Campbell, A.J. 1989. Dietary salt affects biochemical markers of resorption and formation of bone in elderly women. Br. Med. J., 299: 834-835.

Need, A.G., Morris, H.A., Cleghorn, D.B., DeNichilo, D., Horowitz, M. & Nordin, B.E.C. 1991. Effect of salt restriction on urine hydroxyproline excretion in postmenopausal women. Arch. Internal Med., 151: 757-759.

The biological significance of this relationship is supported by the accelerated osteoporosis induced by feeding salt to rats on low-calcium diets and the effects of salt administration and salt restriction on markers of bone resorption in postmenopausal women. Because salt restriction lowers urinary calcium, it is likely also to lower calcium requirement and, conversely, salt feeding is likely to increase calcium requirement.

Elliott, P., Stamler, J., Nichols, R., Dyer, A.R., Stamler, R., Kesteloot, H. & Marmot, M. 1996. Intersalt revisited: further analyses of 24 hour sodium excretion and blood pressure within and across populations. Br. Med. J., 312: 1249-1253. 

This is illustrated in Figure 18, which shows that lowering sodium intake by 100 mmol (2.3 g) from, for example, 150 to 50 mmol (3.45 to 1.15 g), reduces the theoretical calcium requirement from 840 mg (21 mmol) to 600 mg (15 mmol). However, the implications of this on calcium requirement across the world cannot be computed because information about sodium intakes is available from very few countries.