 And this is what the surface of a kidney stone looks like under a microscope. Imagine that scraping down your urinary canal. Kidney stones affect approximately 1 in 11 people in the United States, though 20 years ago it was only 1 in 20, a dramatic increase in the prevalence of this disease that started going up after World War II. Our first clue as to why was published in the 1970s. A striking relationship was found between stone incidents and the consumption of animal protein. But this was a population study, though. You can't prove cause and effect. So next, researchers in Britain did an interventional study. Added animal protein to their diet, like an extra can of tuna fish to their daily diet, and measured stone-forming risk factors in their urine, how much calcium they were peeing out, the concentration of oxalate and uric acid in their urine before, and after the extra tuna. Their overall probability of forming stones increased 250% during those days. They were eating that extra fish. And that so-called high animal protein diet? That was just enough to bring intake up to that of the average American. So the average American intake of meat appears to markedly increase risk of kidney stones. So what about no meat? Well, even by the late 70s, we knew that the only dietary factor consistently associated with kidney stones was animal protein. And not just getting your first kidney stone, the higher the intake of animal protein, the more likely an individual is to have multiple stones, rather than just one single stone episode. Not protein in general. It seems specifically high in animal protein. Conversely, a diet low in animal protein may reduce the overall probability of forming stones to become very low indeed, which may explain the apparent low incidence of stones in vegetarian society. So it may be worthwhile advocating a more vegetarian form of diet as a means of reducing the risk. But it wasn't until 2014 when actual vegetarians were studied in detail. Using hospital admissions data, they found that vegetarians were indeed at a lower risk of being hospitalized for kidney stones compared to those that ate meat. And among meat-eaters, increasing meat intake was associated with a higher risk of developing kidney stones, whereas a high intake of fresh fruit, fiber, and magnesium, may reduce the risk. We can use this information to advise the public about prevention of kidney stone formation. What advice should we give in terms of which animal protein is the worst? Despite compelling evidence that excessive animal protein consumption enhances the risk of stone formation, the effect of different sources of animal protein had not been explored until this study was published in 2014. People who form kidney stones are commonly advised to restrict the intake of red meat to decrease stone risk, but what about chicken and fish? We didn't know until now. Salmon and cod were compared to chicken breast meat and burger and steak. They found that gram per gram fish may actually be worse in terms of uric acid production. However, the overall effects were complex. Basically, stone formers should be counseled to limit the intake of all animal proteins. And not just a little bit, only those who markedly decrease their animal protein intake may expect to benefit from dietary recommendations.