Animal stomach acid evolved for protection

An analysis published today in PLOS ONE has discovered an evolutionary link between diet and stomach acidity.

It’s well known that stomach acid is used to chemically break down food and to protect animals from food poisoning. But acidity varies widely between species in the animal kingdom, especially in birds and mammals.

“Yet, while the idea that the stomach serves as a barrier to pathogens has often been discussed, no study appears to have formally compared the stomach pH [the measurement of acidity] in birds or mammals as a function of their biology in general or their likely exposure to foodborne pathogens in particular,” wrote the authors.

This lack of knowledge drove the researchers to examine the relationship between stomach acidity and diet. They hypothesized that the differences in gut pH across animal species related to what they ate, as some diets have a higher risk of pathogen infection than others. Carnivores, for example, run a higher risk than herbivores, as meat carries more pathogens than plants.

To test this, they gathered all the existing literature on the stomach acidity of birds and mammals—68 species in total—along with data on the natural feeding habits of each species. Then, they ran an analysis to see how diet related to acidity levels, with lower pH levels corresponding to stronger acidity.

Scavengers—who run the highest risk of food contamination—had the most acidic stomachs, allowing them to effectively filter microbes. Obligate scavengers (which rely on scavenging alone to eat) had the “strongest” stomachs, with an average pH of 1.3; facultative scavengers (who scavenge opportunistically) were a close second, at 1.8.

Next highest were generalist carnivores (who don’t prey on specific animals) at 2.2, followed by omnivores (2.9), carnivores that specialize in specific prey type (3.6), and herbivores (4.1 and 6.1 for different gut locations).

“The finding confirms our hypothesis, but you have to get that confirmation before moving forward,” said DeAnna Beasley, a co-author of the paper.

Humans are a little weird

One surprise of the research was that humans (who are classified as omnivores) have an average stomach pH of 1.5, an acidity on the level of scavengers. While there is no definitive reason why, the authors had a few hypotheses.

“One explanation for such acidity may be that carrion feeding was more important in humans (and more generally hominin) evolution than currently considered to be the case,” explained the authors. “Alternatively, in light of the number of fecal-oral pathogens that infect and kill humans, selection may have favored high stomach acidity, independent of diet, because of its role in pathogen prevention.”

The authors also made note that when human pH levels change, people are more at risk for illnesses. For example, elderly humans more prone to bacterial infections in the stomach and gut had pH levels of 6.6 in one study.

Further, patients of gastric bypass also experience a drop in acidity, with pH levels around 5.7 to 6.8, along with people who take stomach acid reducing agents like proton-pump inhibitors and young children. This means they are more likely to experience infections from food.

However, the authors did have a suggestion on how to avoid illness due to low acidity: “[T]his risk may be reduced if such individuals tend to avoid foods in which pathogen risk is elevated, which include (as for birds and mammals more generally) foods that resemble carrion (raw fish, raw mammal meat, etc…), and perhaps even meat in general.”

Despite the results, the study still leaves many things unclear: “This raises significant questions about how humans have evolved, our species’ relationship with food over time, and how modern changes in diet and medicine are affecting our stomachs, our gut microbes and – ultimately – our health,” Beasley said. “Those are questions the research community is already exploring, and the answers should be interesting.”

(Image credit: Thinkstock)