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Infant Diet Has Lasting Effects on Monkey Immune System

The effects of breast milk on the infant immune system may persist for at least half a year after breastfeeding has stopped, new research in monkeys suggests.

In the 3 September issue of Science Translational Medicine, Dennis Hartigan-O'Connor of the University of California, Davis and colleagues examined the effects of both breastfeeding and formula feeding on monkeys' immune systems during the first six months of life. The researchers were surprised to see two distinct kinds of immune systems develop and persist in animals that had received either breast milk or formula as infants.

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The diet of a newborn macaque has lasting effects on its immune system, through diet's influence on the bacteria that grow in the monkey gut. | Kathy West/California National Primate Research Center

The differences persisted even six months after weaning occurred and the animals began receiving identical diets. Hartigan-O'Connor and his team traced these immune differences to commensal or "friendly" bacteria in the gut, populations of which may be shaped by early diet.

Researchers think that some commensal gut bacteria produce immune-boosting compounds as a byproduct of their metabolism. In general, six- to 12-month-old breast-fed infant monkeys in the new study had richer and more diverse commensal bacterial populations than their formula-fed counterparts.

The researchers observed that breast-fed animals developed more of an immune cell type (Th17 cells) thought to be important for protection against invasive Salmonella infection. In theory, individuals who did not have gut bacteria that supported the development of these cells early in life might be less able to resist systemic Salmonella infection later.

There were also differences in the kinds of bacteria that thrived in each group. Prevotella bacteria, for example, were significantly more abundant in breast-fed animals at one year of age, while Clostridium bacteria were less abundant.

"We will be interested to find out how long these immunologic differences last," said Hartigan-O'Connor.

"Monkeys and people continue to generate new T cells throughout childhood," he continued, "and these new cells are almost certainly influenced by the microbiota as well, so I suspect the differences will become less profound over time."

The results hint that commensal gut bacteria — which are shaped by early diet — leave a lasting imprint on the immune system that may affect a person's vulnerability to infections or autoimmune disorders later in life.

"The most important next step is to find out if the developmental variability we describe has an impact on humans' immune response to infections or vaccinations. If this variability is important then we will be able to attack the question of how to make sure that children acquire a microbiome that best supports health," Hartigan-O'Connor said.