I’ve been reviewing the scientific literature on protein and bone health. As many readers know, there has been a theory that animal protein, by way of increased sulfur amino acids, causes calcium excretion leading to osteoporosis.
This theory was always on shaky ground and more recent evidence has contradicted the theory. I just read a meta-analysis on the subject and added the following (in italics) to the pre-existing article, Bones, Vitamin D, and Calcium , on VeganHealth.org :
As mentioned above, there is a theory that protein, especially through the sulfur-containing amino acids, increases the renal acid load, causing calcium to be excreted in the urine. Sulfur-containing amino acids are more prevalent in animal products, although they are also found in high amounts in many grains. But although protein causes an increase of calcium excretion in the urine, it is counteracted by increased absorption from the digestive tract ( 29 ).
Recent evidence has demonstrated that increased intake of common proteins does not necessarily affect bone health adversely. The composition of the protein source as well as co-existing factors in the total diet determines the renal acid load. In healthy individuals consuming high protein foods, in the context of typical mixed diets, the renal acid load does not seem to reach a “threshold” that affects calcium homeostasis.
A meta-analysis looking at protein intake and bone health was published in December 2009 in the American Journal of Clinical Nutrition ( 33 ). It included seven cohort studies that looked at protein intake and fracture risk and concluded:
Overall, the weight of the evidence shows that the effect of dietary protein on the skeleton appears to be favorable to a small extent or, at least, is not detrimental. However, the long-term clinical importance of the effect is unclear, and a reduction in fracture risk was not seen. More research is required to resolve the protein debate. In the meantime the protein intakes and balance of different protein sources as indicated in the current healthy eating guidelines represent appropriate dietary advice.
“Another 2009 meta-analysis found that among five well-designed studies measuring calcium balance, net acid excretion was not associated with either decreased calcium balance or a marker of bone deterioration ( 48 ).”
The paper had an interesting list of cohort studies in the discussion:
“First, during 8 yr of follow-up, fracture risk was not reduced among a cohort of 36,217 postmenopausal women who consumed either lower protein or lower NAE [net acid excretion] diets (37). Second, wrist fracture risk was highest among 1865 peri- and postmenopausal women who consumed the lowest protein intakes over 25 yr of follow-up (38). As well, a recent 2-yr trial in 276 postmenopausal women either supplemented with potassium citrate (expected to neutralize the acid of the Western diet) or encouraged to consume increased fruit and vegetables showed that these interventions did not reduce bone turnover or decrease bone loss (36).”
While it still seems possible that eating two or more times the RDA for protein might increase the risk of osteoporosis, the research indicates that in typical free-living populations, animal protein is not a significant problem for the bone health of meat-eaters or lacto-ovo vegetarians.
36. Macdonald HM, Black AJ, Aucott L, Duthie G, Duthie S, Sandison R, Hardcastle AC, Lanham New SA, Fraser WD, Reid DM. Effect of potassium citrate supplementation or increased fruit and vegetable intake on bone metabolism in healthy postmenopausal women: a randomized controlled trial. Am J Clin Nutr. 2008 Aug;88(2):465-74. ( Link ) Free article available.
37. Dargent-Molina P, Sabia S, Touvier M, Kesse E, Bréart G, Clavel Chapelon F, Boutron-Ruault MC. Proteins, dietary acid load, and calcium and risk of postmenopausal fractures in the E3N French women prospective study. J Bone Miner Res. 2008 Dec;23(12):1915-22. ( Link ) Free article available.
38. Thorpe DL, Knutsen SF, Beeson WL, Rajaram S, Fraser GE. Effects of meat consumption and vegetarian diet on risk of wrist fracture over 25 years in a cohort of peri- and postmenopausal women. Public Health Nutr. 2008 Jun;11(6):564-72. Epub 2007 Aug 9. ( Link ) Free article available.
48. Fenton TR, Lyon AW, Eliasziw M, Tough SC, Hanley DA. Meta-analysis of the effect of the acid-ash hypothesis of osteoporosis on calcium balance. J Bone Miner Res. 2009 Nov;24(11):1835-40. ( Link )
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