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 we add meat to our diet we 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 we buffer acid? What are in antacid (anti-acid) pills like Tums? Calcium compounds.
Meat and eggs have a lot of sulphur-containing amino acids (two to five times more than grains and beans) that are metabolized into sulphuric acid, which the body buffers with calcium compounds. 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 50 mg of calcium. We only have about two pounds of calcium in our skeleton, so the loss of 50 grams 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 we actually look at the studies done on protein intake and bone health, that's not what we find. So, where's the flaw in the 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--where else could the extra calcium dumped in our urine be coming from but our bones?
One study 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 we 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? To everyone's surprise, it was radioactive. This meant that the excess calcium in their urine was coming from their diet, not from their bones.
What seemed to be happening is that the excess protein consumption boosted calcium absorption, from down around 19% up to 26%. 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 protein increases the solubility of calcium by stimulating stomach acid production? Whatever the reason, there was indeed more calcium lost, but also more calcium gained such that in the end, most of that extra calcium is accounted for. In effect, more calcium is lost in the urine stream, but it 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 add in an alkali salt to neutralize the acid. The old thinking would predict that 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 a mild acidosis promoting 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! I cover that in my video Testing Your Diet with Pee and Purple Cabbage.
Now the boost in calcium absorption can only compensate if we're taking enough in. For example, dietary acid load may be associated with lower bone mineral density in those getting under 800mg a day. 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.
-Michael Greger, M.D.
Note to chemistry geeks: Yes, I know it's the calcium salt anions that actually do the buffering (carbonate in Tums and phosphate in bones), but I'm trying my best to simplify for a largely lay audience. I'll make it up to you with some kitchen chemistry (actually bathroom chemistry!) in my Testing Your Diet video.
Image Credit: PD Art / Wikimedia Commons