The effect of protein on bone

The amount and type of protein we eat, seems to affect how much calcium is conserved or lost from the body. Generally, a high protein intake, particularly from animal sources as is common in Western countries such as Australia and New Zealand, is thought to increase the loss of calcium from the body via effects on the kidneys. A report from an osteoporosis conference held by the American Dietetic Association together with the National Dairy Council in the US ⁵ . concluded that for every 1 gram of protein consumed, 1 milligram (mg) of calcium is lost in urine. Some studies have also linked diets high in protein with an increased rate of bone loss and bone fractures ^{6 7 8} whereas others suggest that a higher dietary protein level is needed to maintain bone mass in older people ^{9 10} .

But not all protein is created equal. A study of Japanese men and women, aged 20-79 years, confirmed that excess protein, particularly the type rich in sulphur containing amino acids (such as from meat) may increase calcium loss in the urine ¹¹ . Yet, when animal protein is replaced with soy protein, much less calcium is lost ¹² . A study of middle aged and elderly women in China with various dietary patterns, found that urinary calcium excretion increased with a higher intake of animal protein (including dairy and nondairy animal protein) but decreased with a higher plant protein intake ¹³ . Scientists at Wayne State University, Detroit, found that although a two-fold increase in protein intake (at constant levels of calcium and phosphorus intake) causes a 50% increase in the amount of calcium lost in the urine, a soy protein-based diet is able to maintain calcium balance under these conditions ¹⁴ . As mentioned, this is thought to be due to the lower content of sulphur amino acids found in soy protein, as compared to animal proteins. Sulphate, produced by the metabolism of sulphur containing amino acids in the body, is thought to have a negative effect on the reabsorption of calcium by the kidneys back into the bloodstream.

Despite these findings, more research is required to establish the optimal level and type of protein for bone health. Once seemingly simple, the protein-bone picture now appears to be more complex, and is probably dependent on the level of other nutrients and phytonutrients in the diet, including but not limited to calcium. However, a balanced diet with ample fruit and vegetables (including legumes such as soy) appears to be important.