It’s normally assumed that the highly acidic environment of the stomach evolved to break down an animal’s food, aiding the process of digestion. But new research seems to show that the low pH might have a second function, helping protect the body from gut infections.
Researchers found a general pattern in which birds and mammals that are scavengers and feed on decaying flesh, or carrion, had the most acidic stomach acid. They suggest that strong stomach acid might help to protect these animals from harmful bacteria and disease, acting as an ecological filter and preventing them from reaching the gut where they could cause illness.
“We started this project because we wanted to better understand the relationship between stomach acidity, diet and the microbes that live in the guts of birds and mammals,” says DeAnna Beasley, co-author of the paper published in PLoS One. “Our idea was that this could offer some context for looking at the role of the human stomach in influencing gut microbes, and what that may mean for human health.”
The researchers from North Carolina State University first examined all the existing literature on the stomach acidity of birds and mammals, eventually compiling a list of 68 species. Working off this, they then collected data on the feeding habits of the animals, and eventually ran an analysis to see how these are related to stomach acidity.
The results showed that the scavengers' stomach acid had a much lower pH when compared to carnivores and herbivores – exactly as the researchers had predicted. However, their investigation raised some interesting questions about the eating behavior of humans. The results showed that the stomach acidity of humans was much lower than would be expected when looking at other primates. They say that this evidence suggests that carrion feeding might have been more important to early humans than previously thought, though the lack of data on the stomach pH of other apes makes it difficult to predict when low pH stomach acid evolved.
“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,” said Beasley. “Those are questions the research community is already exploring, and the answers should be interesting.”
They suggest that some medical interventions of the stomach, such as gastric bypass surgery in which 60% of a person’s stomach is removed, could have dramatic implications in making patients more susceptible to gastrointestinal infections. In fact, recent work supports a pattern between those who have gone under the knife and an increased chance of microbial overgrowth. They also predict that people who have altered their stomach pH, through the use of antacids for example, are also at a higher risk.
“The next step will be for scientists to examine the microbial ecosystems in the guts of these animals to see how these ecosystems have evolved,” says Beasley. “Do animals with high stomach acidity have smaller or less diverse populations of gut microbes? Or do they simply host microbes that can survive in acidic environments?”