Does animal protein in our diet result in higher risk of bone fractures and osteoporosis? It appears so. A broad array of clinical epidemiological studies has found a strong, positive association between animal protein intake and risk of bone fractures. The protein causes acidosis in our bodies when we metabolize it, and one of the mechanisms used to neutralize that acidity appears to be leaching calcium from our bones.
Harvard’s School of Public Health explains very nicely the mechanism by which protein can cause problems for bone health. It says, “as your body digests protein, it releases acids into the bloodstream, which the body neutralizes by drawing calcium from the bones. Following a high-protein diet for a few weeks probably won’t have much effect on bone strength. Doing it for a long time, though, could weaken bone.”
Now we’ve known for a very long time that meats, including fish, are acid forming in our body. Scientists from Columbia University, back in 1912, analyzed acid and base forming elements in food, and noted that, “all the meats (including fish)…show a decided excess of acid-forming elements”; all the “meats (including fish) show [a] decided predominance of acid-forming elements.”
Back in 1920, Columbia’s Department of Chemistry also reported that adding meat to one’s diet results in increase of calcium loss in urine, thought to be because “the added meat gave to the diet as a whole an excess of acid-forming over base-forming mineral elements[.]”
And what have we seen from the results of the consumption of animal protein with regards to bones? Researchers from Yale University’s School of Medicine looked back across a broad array of thirty-four prior published studies across sixteen countries, and they found these studies over time showed “a strong, positive association” between dietary animal protein and female bone fracture rates.
So, we’ve known for some time that this association exists. We know eating a diet high in animal protein results in acidity, and that our body leaches calcium from our bones to buffer the acid. One of the mechanisms behind this phenomenon is that animal protein has a higher amount of sulfur-containing amino acids, and the “sulfur-containing amino acids from animal protein lower blood pH”.
So animal proteins — including meat, fish, dairy, poultry or eggs — have higher amounts of sulfur-containing amino acids than plant foods. As a result, when we eat diets high in animal proteins, our body produces sulfuric acid, which increases the acidity in our bodies. One of the body’s mechanisms to neutralize this acidity is to draw calcium from our bones (similar, for example, to when we take antacids that are made with calcium to neutralize the acidity in our stomach associated with heartburn). The problem is that constantly leaching calcium from our bones can reduce bone mass, making our bones weak and more prone to fractures and osteoporosis. Moreover, the chronically higher calcium excreted in our urine can also lead to the development of kidney stones.
This Harvard study published in the American Journal of Epidemiology followed over 80,000 women over twelve years, and found that animal protein was associated with increased risk of forearm fracture, but no increase in risk was observed with higher intakes of vegetable protein. It found that women who consumed the largest amount of animal protein in the study had a 22% higher risk of fracture. Evidence suggests that “higher protein intakes in young…women have a negative impact on radial bone measurements”, meaning that women who consumed higher protein diets have been found to lose bone mineral content and bone density.
So we know that our body uses base stores (including calcium from our bones) to neutralize the acidity we get from our diet. This article from The Journal of Nutrition explains the same thing. It says, “diets that are net acid producing…induce and sustain increased acidity of [our] body fluid. With increasing age, the kidney’s ability to excrete daily net acid loads declines, invoking increased…utilization of base stores ([from our] bone [and] skeletal muscle) on a daily basis to mitigate the otherwise increasing baseline metabolic acidosis, which results in increased calciuria and net losses of body calcium. Those effects of net acid production and its attendant increased body fluid acidity may contribute to…osteoporosis[.]” What this is saying is the neutralizing mechanism in our bodies of taking calcium from our bones to deal with the higher acidity caused by our diet can contribute to osteoporosis.
This journal review in The European Society for Clinical Nutrition and Metabolism further discusses the issue, noting how problems associated with acidity become even worse with age. “The modern Western-type diet…contains excessive animal products, generating a state of metabolic acidosis, whose magnitude increases progressively with aging due to the physiological decline in kidney function.”
As explained in this peer-reviewed study from the Journal of Nutrition, “The average American diet, which is high in protein and low in fruits and vegetables, generates a large amount of acid, mainly as sulfates and phosphates. The kidneys respond to this dietary acid challenge with net acid excretion, [and] the skeleton supplies buffer by active resorption of bone.” The study concludes, that, “Overall, the evidence leaves little doubt that excess acidity will create a reduction in total bone substance…. An acid-ash diet [means] a diet that creates acid in the process of its metabolism[,]” and this article concludes, “[m]odern peoples are now eating high protein, acid-ash diets and [are] losing their bones.”
This study, published in the official journal of the Council on Renal Nutrition of the National Kidney Foundation and the International Society of Renal Nutrition and Metabolism, compared the net acid secretions among vegans (who eat no animal products), lacto-ovo vegetarians (who eat no meat, but do eat dairy and eggs) and omnivores (who eat a traditional Western diet of meat, dairy and eggs). The study found that net acid secretion was higher in both lacto-ovo vegetarians and omnivores than in vegans, suggesting “that higher protein intake results in more renal net acid excretion and more acidic urine.” In addition to increased incidents of kidney stone development, higher protein intake was also found to be associated with “increased urinary calcium excretion.” It noted, “higher protein intake, especially animal protein, was linked to an increased rate of bone loss and increased risk of hip and forearm fractures in women.” The study concludes, “Because acid-base balance has significant implications for renal and bone health, it is important…to introduce appropriate dietary modifications to prevent or treat the conditions, including kidney stones, bone loss and/or [bone] fractures, and possibly osteoporosis.”
So to recap, eating animal proteins increases body acidity, and one of the body’s mechanisms to neutralize this acidity is to leach calcium from the bones. Doing this for a long time can lead to loss of bone mass and weakened bone, increased risk of fractures and osteoporosis, as well as kidney (or renal) stones. And, an easy way to avoid the increased risk of these problems is avoiding animal protein.
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• Harvard T.H. Chan, School of Public Health: Calcium and Milk: What’s Best for Your Bones and Health? Available here (accessed Feb. 11, 2016).
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• LM Ausman, LM Oliver, BR Goldin, MN Woods, SL Gorbach and JT Dwyer. Estimated Net Acid Excretion Inversely Correlates with Urine pH in Vegans, Lacto-ovo Vegetarians, and Omnivores. J Ren Nutr. 2008 Sep; 18(5):456-65. Available here (accessed Feb. 11, 2016).
This presentation was written and narrated by Sofia Pineda Ochoa, MD, and edited by Bob Rapfogel.
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