The associations between protein intake and bone mineral density is controversial. The general mainstream consensus is that high protein diets are detrimental to bone mineral density, but this is not supported by evidence in the literature. In fact, research suggests that too little protein intake is just as detrimental to bone health as too much. The reason that high protein diets are considered detrimental to bone health is because amino acids such as cysteine and methionine are thought to lower blood pH by conversion to acidic metabolites. The response to this is a stimulation of osteoclastic bone resorption which liberates alkalising minerals from the bone matrix, in an attempt to raise blood pH and maintain homeostasis. Over the long-term this buffering of blood pH by the bones results in a reduced bone mineral density and an increase in urinary calcium loss.
In order to investigate the associations between protein intake and bone mineral density, researchers1 analysed the protein intakes and bone mineral densities of 560 female subjects aged between 14 and 40y, at baseline using a cross sectional analysis and also over time using longitudinal analysis. Protein intake was assessed by a food frequency questionnaire and bone mineral density was assessed by dual-energy X-ray absorptiometry. At baseline the mean protein intake was similar across each tertile (15.5%). Longitudinal increases in protein intake were not associated with changes in bone mineral density. However, in cross sectional analysis at baseline, lower vegetable protein intake was associated with lower bone mineral density. Higher protein intakes were associated with increased spinal bone mineral density in 14 to 19 years olds, but this was the only statistically significant stratum were an association was present.
The association between vegetable protein and bone mineral density is interesting because it supports evidence from previous studies that show an association between vegetables (and fruit) and higher bone mineral density. Vegetables and fruit contain potassium salts (base precursors) that can be metabolised to form bicarbonate which has an alkalising effect on blood pH. It therefore seems likely that those with high intakes of protein from vegetables are also consuming high amounts of potassium salts and so have higher bone mineral densities. The failure to distinguish between protein intake from meat and vegetables may explain the lack of consensus in research regarding the effect of protein on bone mineral density. Because the interest in the acidity of blood and bone mineral density has only recently increased, many older studies looking at bone health and nutrition need to be reassessed based on this new data.
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