There has been evidence for well over a hundred years that animal protein decreases bone health. The explanation of animal protein causing excess metabolic acid was first suggested in the 1880s (1) and was documented as long ago as 1920s (2). It is also known that animal protein increases more the metabolic acid load in the body than plant protein (3,4). This is due to animal protein including more of the sulphur containing amino acids than plant protein, that when digested and metabolised, produce acid forming sulphate ion, which must be excreted by the kidneys.
When animal protein increases metabolic acid, the amount of calcium in the urine also increases. This effect has been established for over eighty years (2) and has been studied since the 1970s. Summaries of this studies were published in 1974 (5), 1981 (6), 1990 (7). Each of these summaries clearly shows that the amount of animal protein consumed by many of us on daily basis is capable of causing substantial increases in urinary calcium.
Chart 10.1 is taken from the 1981 publication (6), and shows that doubling protein intake (mostly animal based) from 35 to 78 grames per day, causes an alarming 50% increase in urinary calcium. This effect occurs well within the range of protein intake most of us consume, which in average is around 70-100g/day.
Incidentally, a 6 month study funded by the atkins centre found that those people who adopted the atkins diet excreted 50% more calcium in their urine after six months on the diet (8).
Worldwide, the incidence of osteoporosis correlates directly and strongly with animal protein intake. The highly acidic nature of animal protein is the major cause of bone loss (9).
1-Brazel U.S. “Acid loading and osteoporosis” J.AM. Geriatr Soc. 30 (1982): 613
2-Sherman HC. “Calcium requirement for maintenance in man” J. Biol. Chem. 39 (1920) 21-27.
3-Brosnan JT, and Brosnan ME. “Dietary protein, metabolic acidosis, and calcium balance” In: H.H. Draper (ed.), Advances in Nutrition Research, pp. 77-105. New York:Plenum Press, 1982
4-Frassetto LA, Todd KM, Morris RC, Jr., et al. “Estimation of net endogenous noncarbonic acid production in humans from diet potassium and protein content.” Am. J. Clin. Nutri. 68 (1998): 576-583
5-Margen S, Chu J-Y, Kaufmann NA, et all. “Studies in calcium metabolism. I. The calciuretric effect of dietary protein.” Am. J. Clin. Nutr. 27 (1974): 584-589
6-Hegsted M, Schuette SA, Zemel MB, et all. “Urinary calcium and calcium balance in young men as affected by level of protein and phosphorus intake.” J.Nutr. 111 (1981): 553-562
7-Kerstetter JE, and Allen LH. “Dietary protein increases urinary calcium.” J. Nutr. 120 (1990): 134-136
8-Westman EC, Yancy WS, Edman JS, et all. “Carbohydrate Diet Program.” Am. J. Med. 113
9-Barzel US, Massey LK. Excess dietary protein can adversely affect bone. J Nutr. 1998;128:1051-3(2002): 30-36