The alkaline diet, the acid-ash hypothesis and bone health

Tanis R. Fenton, PhD, RD

Dietitian/Epidemiologist, Adjunct Assistant Professor, Department of Community Health Sciences, University of Calgary

Numerous webpages, books for lay persons, and alternative health practitioners espouse that diets favouring protein foods (especially meat and dairy products) and grains produce excess acid, such as phosphate, that must be buffered. They claim that this buffering mobilizes bone mineral and increases urinary calcium excretion and loss of bone mass which can lead to osteoporosis. But the scientific evidence indicates otherwise.

Highlights

Promotion of the alkaline diet to prevent osteoporosis is not justified.
Calcium balance studies have not shown that increasing the diet acid load promotes skeletal bone mineral loss or osteoporosis.

Higher phosphate intakes are associated with increased calcium balance.
Protein appears to be beneficial to bone health

Popularly known as the “alkaline diet,” the acid-ash hypothesis has had only limited critical review. The hypothesis assumes that excess urinary calcium is equal to the loss of calcium from the skeleton. But urinary calcium excretion is only a surrogate measure because differences in calcium absorption can offset the calcium excretion.

Calcium balance is a superior means of measuring calcium status compared to urine calcium because it assesses effects of an intervention on the whole-body retention of calcium, and also takes into consideration alterations of calcium absorption.

The objective of our meta-analysis was to assess the effects of changes in net acid excretion from changes in diets or “alkaline” supplements on both urine calcium and calcium balance in studies of calcium metabolism with superior methodology.1 Five of 16 studies met the inclusion criteria. These studies altered the amount and/or type of protein and reported calcium balance as an outcome. This is what we found:

• there was a significant linear relationship between an increase in net acid excretion and urinary calcium (p<0.0001), but no changes in calcium balance (p=0.38; power=94%)
• there was no association between net acid excretion and N-terminal telopeptides (p=0.95), a marker of bone metabolism.

These findings are corroborated by the population-based Canadian Multicentre Osteoporosis Study (CaMOS) in which we used multiple regression to examine associations between acid excretion (urine pH and urine acid excretion) and measures of osteoporosis.2 There were no associations between either urine pH or acid excretion and either the incidence of fractures (6,804 person years) or changes in bone mass density (over 5 years) after adjustment for confounders.2 We concluded that urine pH and urine acid excretion do not predict osteoporosis risk.2

Furthermore, in another meta-analysis, we found that higher phosphate intakes were associated with decreased urine calcium and increased calcium balance.3 As well, there is a growing body of evidence indicating that protein is important for bone health.4