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 34 prior published studies across 16 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 12 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.