Dietary acid load is determined by the balance of acid-inducing foods and alkaline- (base-) forming foods. Animal protein is generally more acid-forming because it tends to have higher levels of sulfur-containing amino acids, such as methionine, which produce sulfuric acid when metabolized in the body. Fruits and vegetables, on the other hand, are generally base-forming, which help neutralize acids in our kidneys.
Ancient human diets, like those of our fellow great apes, largely consisted of plants, so they likely produced more base than acid in the kidneys of our ancestors. Humans evolved eating these alkaline (base-forming) diets over millions of years. Most contemporary diets, on the other hand, produce acid in excess. This switch from base- to acid-forming diets may help explain our modern epidemic of kidney disease. A 2014 analysis of the diets and kidney function of more than 12,000 Americans found that a higher dietary acid load was associated with significantly higher risk of protein leakage into the urine, an indicator of kidney damage.
Acid-inducing diets are believed to impact the kidney through “tubular toxicity,” damage to the tiny, delicate, urine-making tubes in the kidneys. To buffer the excess acid formed by your diet, kidneys produce ammonia, which is a base and can neutralize some of that acid. Counteracting the acid is beneficial in the short term, but over the long run, all the extra ammonia in the kidneys may have a toxic effect. The decline in kidney function over time may be a consequence of a lifetime of ammonia overproduction.
The chronic, low-grade, metabolic acidosis attributed to a meat-rich diet helps explain why people eating plant-based diets appear to have superior kidney function and why various plant-based diets seem to be so successful in treating chronic kidney failure. Under normal circumstances, a vegetarian diet alkalinizes the kidneys, whereas a non-vegetarian diet carries an acid load.
If people are unwilling to reduce their meat consumption, they should be encouraged to eat more fruits and vegetables to balance out that acid load. Researchers have found that giving people fruits and vegetables not only offered similar protections to baking soda (sodium bicarbonate) pills to help neutralize excess acid but had the added advantage of lowering the subjects’ blood pressure.