Protein derived from animal or vegetable sources can have quite different effects on bone metabolism because of the way the protein affects the acid base balance of the plasma. Sulphur containing amino acids in protein can be metabolised to sulphuric acid which can lower the pH of the blood by increasing hydrogen ion concentration. In contrast, vegetables and fruits contain substantial amounts of base precursors such as organic potassium salts (citrate, malate and gluconate) that accompany the acid forming protein. These potassium salts can be metabolised to bicarbonate that then has a neutralising effect on the acidic components in the plasma. Generally grains are acid forming like animal proteins. Chronic acidic plasma results in the mobilisation of the minerals in bone in order to maintain the correct pH of the blood. This can result in increased calcium excretion and detrimental bone tissue loss.
Research1 has measured the ratio of animal to vegetable protein in the diet of white women aged >65 years and compared it to the bone density of the subjects. The bone density was assessed by dual-energy X-ray absorbtiomentry, and protein intakes were measured using food-frequency questionnaires. The results showed that bone mineral density was not significantly associated with the ratio of animal to vegetable protein intake even when adjusted for possible confounding variables. However, Women that had high ratios of animal to vegetable protein in their diets did have a higher rate of bone loss at the femoral neck and a higher rate of hip fractures compared to those with a lower ratio. These results are supportive of previous studies that show that high intakes of animal protein are associated with a higher risk of hip fractures in women aged over 50 years.