Sodium, acid-base balance, and bone health
We’ve known for years that excessive sodium intake contributes to hypertension, and a new meta-analysis of 13 studies has confirmed that high sodium intake is associated with increased risk of stroke and overall cardiovascular disease.1 Salt consumption is also associated with kidney disease, and a new study suggests that reduced sodium intake could benefit bone health.
Women 45-75 years old with prehyptertension or stage 1 hypertension were assigned to one of two diets. Both diets supplied the same amount (800 mg) of calcium. One diet was a high-carbohydrate, low-fat diet. The other diet was a low-sodium diet (1500 mg), which included red meat but was designed to have a low acid load.2
Western diets, generally high in animal protein, produce acid in the body, forcing the body to buffer this acid in part by the release of alkalizing salts from bone (e.g. calcium citrate and calcium carbonate) – this is associated with urinary calcium loss and is thought to contribute to osteoporosis. Fruits, vegetables, and legumes have favorable effects on acid-base balance, since the acid-forming effect of their protein content, which is lower than that of animal products anyway, are balanced by their mineral content.3-4
After 14 weeks, the women on both diets increased markers of bone formation and reduced their calcium excretion – those on the low sodium diet had a greater reduction in calcium loss. The authors concluded that this diet was protecting the mineral reserves in bone, and that this could have long-term implications for bone health. Future studies will likely measure bone mineral density and fracture incidence in response to these diets.2
The average daily consumption of sodium for Americans is around 4000mg, almost double the U.S. recommended maximum of 2300mg. The low sodium diet in this study provided a maximum of 1500mg of sodium per day, but included up to six servings of red meat per week, limited the consumption of nutrient-rich legumes to 4-5 per week, and was based on high-calorie, nutrient-poor grain products - 7-8 servings per day.5 The high-carbohydrate low-fat diet was likely based on grain products as well.
Although both of these diets had favorable effects when implemented in place of a standard western diet, they both have room for improvement. By minimizing the high-protein, high-saturated fat animal products, and replacing grain products with mineral- and phytochemical-rich vegetables, fruits, and legumes as the base of the diet, both acid load and sodium would be further reduced, presumably leading to further benefits on bone health.
1. Strazzullo P et al. Salt intake, stroke, and cardiovascular disease: meta-analysis of prospective studies. BMJ 2009;339:b4567
2. Nowson CA et al. The effects of a low-sodium base-producing diet including red meat compared with a high-carbohydrate, low-fat diet on bone turnover markers in women aged 45-75 years. Br J Nutr. 2009 Oct;102(8):1161-70. Epub 2009 May 18.
3. Welch AA et al. Urine pH is an indicator of dietary acid-base load, fruit and vegetables and meat intakes: results from the European Prospective Investigation into Cancer and
Nutrition (EPIC)-Norfolk population study. Br J Nutr. 2008 Jun;99(6):1335-43. Epub 2007 Nov 28.
4. Massey LK. J Nutr. Dietary animal and plant protein and human bone health: a whole foods approach. 2003 Mar;133(3):862S-865S.