New research suggests stomach acidity may have evolved as a defense against infection and food poisoning, rather than solely as a means of breaking down food.
Recent collaborative research published in PLOS ONE by scientists from several U.S. universities suggests acidity in the stomach of many bird and mammal species, including humans, evolved alongside our diet and the microbial populations in our gut.
Scientists examined available data on stomach acidity (measured as pH) and trophic level (diet) for nearly 70 birds and mammals. For example, humans are mammals, have an average stomach acidity of 1.5, and are omnivorous, whereas chickens are birds, have a stomach acidity of 3.7, and are specialized carnivores.
Stomach pH was grouped by trophic level for all species and the researchers discussed the patterns that emerged. As may be expected, animals that are scavengers tend to have the lowest stomach pH values (most acidic), while herbivorous animals have higher pH (three animals actually had basic stomachs: sloths, guanacos, and quokkas).
Though it has been known that high acidity in the stomach helps break down certain plant and animal material, the researchers suggest that pH is a property of animal evolution for different reasons.
This pattern, they say, may emerge because acidity in the stomach acts as a filter against the microorganisms that are consumed in an animal’s food. For this reason, animals that consume rotting flesh need a harsher filter to protect them from the plentiful harmful bacteria in their diet.
Similarly, their findings suggest animals that pretty on closely related species (i.e. birds consuming other birds) need more acidic stomachs in order to prevent infection by bacteria that are more adept at infecting a specific evolutionary lineage.
These conclusions, though they have not been scrutinized by molecular evolution work, show that stomach pH can be closely linked with animal trophic level and diet, and therefore with the evolution of the gut system.
A few of the beneficial microbial flora in the guts of animals have become specialized to survive through the acidity of the stomach after they hitch a ride on food. They have evolved to efficiently bypass the acidic filter and outcompete pathogenic bacteria that try to invade.
However, this vigorous filter may be a double-edged sword, making it difficult for microorganisms to properly recolonize the gut after antibiotic treatment or certain surgical interventions. Since there is growing evidence that diet, microbial gut-colonizers, and overall health are closely connected, research on the host-microbiome relationships evolution has forged is vital.