Dietary phosphate is supplied through meat, grains and dairy products, and increasingly as food additives. Phosphate is a fundamental mineral component of hydroxyapatite, the main structural element of bone. However, the acid-ash hypothesis postulates that dietary phosphate, a marker of the metabolic production of acid, is detrimental to bone.

According to the acid-ash hypothesis, acidic ions such as phosphate contribute to the diet acid load, causing demineralization of bone and excretion of bone calcium in the urine, resulting in osteoporosis. Consumers have therefore been advised to reduce their consumption of foods with high phosphate contents, such as meats, fish and dairy produce, and to follow an alkaline diet to lower their acidic ion intakes.

However, there have been few critical reviews of the acid-ash hypothesis. Recent findings have suggested that acid-generating diets may not be detrimental to whole body calcium balance, questioning the validity of the acid-ash hypothesis. In addition, other variables such as calcium intake may also influence the relationship between dietary phosphate intakes and bone health.

A study by Fenton et al.¹ used a meta-analysis to quantify the potential contribution of phosphate to bone loss in 269 healthy adults, determined by surrogate and direct measures of osteoporosis. The influence of calcium intake and degree of protonation of the phosphate supplements on urine calcium, calcium balance and markers of bone metabolism was assessed. The results contradicted the acid-ash hypothesis, with higher phosphate intakes being associated with decreased urine calcium and increased calcium retention. No evidence was found that higher phosphate intakes contributed to bone demineralization and were detrimental to bone health.

¹ Fenton, TR; Lyon, AW; Eliasziw, M; Tough, SC; Hanley, DA (2009). Phosphate decreases urine calcium and increases calcium balance: a meta-analysis of the osteoporosis acid-ash hypothesis. Nutrition Journal 8 (Sept.) 15 pp.

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