The decades-old dogma that the acid-forming quality of animal protein leads to bone loss has been called into question.
For most of the last century, a prevailing theory within the field of nutrition was that by eating acid-forming foods such as meat we were, in essence, at risk of peeing our bones down the toilet. And no wonder! Experiments dating back to 1920 showed over and over that if you add meat to the diet you get a big spike in the amount of calcium being lost in the urine.
And this made total sense. We had known since 1912 that meat was acid-forming within the body, and how do you buffer acid? What are in antacid pills, anti-acid pills like tums? Calcium.
Meat and eggs have a lot of sulphur-containing amino acids, 2-5 times more than grains and beans) that are metabolized into sulphuric acid, which the body buffers with calcium. That's why the antacids like Tums are made out of calcium, calcium compounds can buffer acid. And where is calcium stored in the body? The skeleton. So the thinking was that every time we ate a steak our body would pull calcium from our bones, bit by bit, and over time this could lead to osteoporosis. Based on 26 such studies, for every 40 grams of protein we add to our daily diet we pee out an extra 50mg of calcium. And look, we only have about 2 pounds of calcium in our skeleton, so the loss of 50 a day would mean losing close to 2% of our bone calcium every year. By the end of the 20th century, there was little doubt that acid-forming diets would dissolve our bones away.
But if you actually look at all the studies done on protein intake and bone health, that's not what you find. So, where's the flaw in our logic? Meat leads to acid, which leads to calcium loss, which leads to bone loss, right?
Well, it's uncontroversial that protein results in greater calcium excretion, but we've just been assuming it's coming from the bone—I mean where else could the extra calcium dumped in our urine be coming from but from our bones?
This is the study that appeared to solve the mystery. An intrepid group of researchers tried feeding a group of volunteers radioactive calcium and then put them on a high protein diet. What happens when you put people on a high protein diet? The amount of calcium in their urine shoots up, and indeed that's just what happened. But here's the big question, was that extra calcium in their urine radioactive or not, and to everyone's surprise? It was radioactive. Meaning, that the excess calcium in their urine was coming from their diet—remember they were feeding them radioactive calcium. So the excess calcium in their urine wasn't coming from their bones, but from what they were eating.
What seemed to be happening is that the excess protein consumption boosted calcium absorption, from down around 19% up to 26%. So all of a sudden there was all this extra calcium in the blood so presumably the kidneys are like "whoa what are we going to do with it all?" So they dump it into the urine. 90% of the extra calcium in the urine after eating a steak doesn't appear to be coming from our bones but from our diet. We're not sure why protein boosts calcium absorption. Maybe the protein increases the solubility of calcium by stimulating stomach acid production? Whatever the reason, yes, more calcium lost, but more calcium gained such that in the end most of that extra calcium is accounted for. So in effect more calcium is lost in the urine stream, but may be compensated by less loss of calcium through the fecal stream.
This was repeated with even more extreme diets— an acid-forming five burgers a day worth of animal protein diet that limited fruits and vegetables versus an alkaline diet emphasizing fruits and vegetables. More calcium in the urine on burgers, but significantly greater calcium absorption such that at the end it was pretty much a wash.
Other studies have also since supported this interpretation. Here's an ingenious one. Feed people a high animal protein diet, but with an alkali salt to neutralize the acid. The old thinking would predict that then there would be no calcium loss since there is no excess acid to buffer, but no, even though the acid load was neutralized there was still the excess urinary calcium, consistent with the radioactive isotope study, "challenging the long-standing dogma that animal protein consumption results in an acidosis that promotes the increased excretion of calcium….
So if our body isn't buffering the acid formed from our diet with our bones, how is it neutralizing the acid? Maybe with our muscles--alkaline diets may protect our muscle mass—all covered in my next video, entitled Testing Your Diet with Pee and Purple Cabbage—stay tuned !
To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Ariel Levitsky.
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Note the boost in calcium absorption described in the video can only compensate if you’re taking enough in. For example, dietary acid load may be associated with lower bone mineral density in those getting under 800mg a day.
I previously touched on this topic in my video Is Protein Bad to the Bone? But I promised I’d take a deeper dive, and here it is! If there are other topics you’d like me to cover in greater depth please note them below in the comment section.
Plant Protein is Preferable to animal protein for a variety of reasons (tends to have less methionine, is less IGF-1 promoting, etc.), but it’s not clear how much of an advantage it has when it comes to bone health.
Note to chemistry geeks: of course it’s the calcium salt anions that really do the buffering (carbonate in Tums and phosphate in bones), but I’m trying my best to simplify for a largely lay audience! Get ready for some kitchen chemistry (actually bathroom chemistry!) in my next video Testing Your Diet with Pee & Purple Cabbage.
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