Special Section on Bone and NutritionDoes a High Dietary Acid Content Cause Bone Loss, and Can Bone Loss Be Prevented With an Alkaline Diet?
Introduction
In this work we examine evidence that a change in acid-base balance through dietary means can affect bone health. This concept is sometimes termed the “acid-ash hypothesis” and is often promoted as an important factor in the development of osteoporosis. This hypothesis suggests that foods high in “acidic” content (e.g., animal protein and grains) cause a chronic acidemia because of their sulfate and phosphate content, whereas fruits and vegetables create a more alkaline environment because of their greater potassium-organic anion content and may even prevent age-related bone loss and osteoporosis. It proposes that the acidic anions provide an increased acid load on the kidney and that the increased net acid excretion (NAE) is accompanied by increased calcium loss in the urine. Furthermore, proponents of the acid-ash hypothesis suggest that, to maintain a normal acid-base homeostasis, there is an increase in bone resorption to buffer the excess dietary “acid.”
Further credence was given to this concept when the Dietary Reference Intake recommendation for potassium in 2005 by the Institute of Medicine (1) made a specific reference to increasing alkaline sources of potassium, that is, fruits and vegetables, to enhance bone health. However, recommendations that restrict protein intake to reduce NAE, and restriction of otherwise-healthy foods because of their “acidity.” may also have implications that could be detrimental to bone health. A recent review of the impact of acid-base balance on bone suggests the proponents of the acid-ash hypothesis of osteoporosis have underestimated the ability of the kidney and respiratory system to deal with dietary sources of acid (2). The purpose of this work is to examine the different types of evidence for an acid-base effect on bone and to provide recommendations for dietary intakes that are in-line with the evidence.
Section snippets
Acid–Base Effects on Bone: Observational Studies
A role for dietary vegetables and fruit on bone health has emerged in the literature. Population-based studies have reported that increased potassium intake through vegetables and fruits is associated with increased bone mineral density. For example, in 1999 Tucker et al, by using cross-sectional and prospective Framingham data of subjects 69–97 yr, showed that potassium and magnesium intakes, as well as fruit and vegetable intake, were significantly associated with greater bone mineral density
The Alkaline Potassium Hypothesis
Advocates of the Alkaline Potassium Diet Hypothesis use the Paleolithic diet as being “ideal” for preserving bone integrity (8). The Paleolithic diet of hunter–gatherers is high in potassium and bicarbonate precursors from vegetables and fruit, which advocates believe counteracted the very high protein intakes of those early humans. Herein dietary potassium at levels of 400 mEq per day (close to 3 times the current adequate intake for potassium) would be essential to maintain bone integrity. In
The Acid-Ash Hypothesis
The Acid-Ash Hypothesis is a variant of the Alkaline Potassium Hypothesis. Both are premised on the argument that acidic compounds in the diet (high protein, high phosphate, or other “acidic” anions like sulfate) have an adverse effect on bone, whereas alkaline components, such as fruit and vegetables, will counter this effect. Early work centered on measuring NAE after consumption of diets varying in protein, with or without variations in alkaline salts or alkaline foods (2). However, it has
Systematic Reviews of Acid-Base and Bone
A moderately large amount of literature exists on the influence of dietary acid or alkali on bone. Although the common theme is that a diet high in protein or phosphate would be detrimental to the skeleton, recent systematic reviews of this literature have not supported these premises.
The proponents of the Acid-Ash Hypothesis have suggested that a diet providing high phosphate content would be acidic and that this would result in negative calcium balance and bone loss. However, a recent
Recent Randomized Controlled Trials
To support the Alkaline Potassium Hypothesis, a number of randomized controlled trials have been published. Only those of 2 years' duration that included direct measures of bone density are examined here. There have been 3 such studies in adult women who were given alkaline potassium as either the citrate or bicarbonate salt alone with calcium and vitamin D; 2 studies published bone data together in one report (13) provided null evidence for an effect, whereas a third study (14) provides
Conclusion
In this paper, attention has been given to the concept that an alkaline diet or alkaline potassium salts promote calcium retention. As demonstrated in the systematic reviews and meta-analyses, there is not enough evidence to support a causal relationship between the acid/alkali composition of a diet and bone health. However, some studies do support a bone benefit of a “healthy” diet with increased vegetable and fruit content or supplements containing potassium bicarbonate or citrate: (1)
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