How do hyenas protect themselves

Extremely acidic gastric juice in scavengers - and humans

Raleigh (USA) - The high acidity of gastric juice plays an important role in the digestion of meat. The hydrochloric acid also kills microbes that were ingested with food and could get into the intestines. This protection against food infections should be stronger for scavengers than for pure herbivores, since decaying meat is more likely to contain pathogens. This is now confirmed by the results of studies reported by American biologists in the journal “PLoS One”. They found that the gastric juice of scavenging birds and mammals generally has extremely acidic, i.e. very low pH values. The fact that humans, as omnivores, have a similarly acidic gastric juice with a pH value of 1.5 as these animals, could allow conclusions to be drawn about their evolution.

"We started this project because we wanted to better understand the relationship between stomach acid, diet and intestinal microbes in birds and mammals," says DeAnna Beasley of North Carolina State University in Raleigh. The results should also provide information on the extent to which a person's stomach influences the intestinal flora and thus their health. Beasley and her colleagues have already collected published data on the acidity of gastric juice from 68 species of mammals and birds. The researchers compared these values ​​with information on the diet of the respective animal.

They found the lowest pH values ​​in animals that ate carrion. For them, the risk of infection is greatest when they eat, and their stomach acts as a filter that only a few microbes can pass through alive. Even in predatory animals that feed on related animal species, the pH levels in the stomach were often lower than in other mammals or birds that hunt insects or fish. This can also be explained by the different risk of infection, say the biologists. Pure herbivores, on the other hand, have only a very low risk of ingesting pathogens with their food. Her gastric juice had higher pH levels, mostly between 3 and 6. The beaver took a special position with a pH value of only 1.7 despite a purely vegetable diet. The researchers explain this by the fact that these animals store food underwater, which increases the risk of contamination. The reason for the similarly low pH values ​​in humans and scavengers could lie in the evolution of humans: There may have been a developmental phase in which our ancestors largely fed on carrion before becoming successful hunters.

The disadvantage of a particularly acidic gastric juice is that hardly any useful microbes can get into the intestines with food. This makes treatment with probiotic bacteria more difficult, which could help normalize an intestinal flora that has been disturbed by antibiotics. The pH in the human stomach also changes with age. In newborns it is even higher, so that initial colonization of the intestine is made easier. Values ​​above 4 were measured in premature infants. There is therefore a greater risk of gastrointestinal infections in the first few weeks of life. The same applies to old people, in whom, according to the authors, the pH value can rise to 6.6. Surgical stomach reduction and medication that neutralize the acid in the stomach or inhibit its production can also raise the pH value. This could affect both the risk of infection and the range of species in the intestinal flora.

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