Elevated immune function during childhood results in as much as 49 percent growth reduction in Ecuador's indigenous Shuar population, researchers reported in the Proceedings of the National Academy of Sciences.
Immune function protects the body from foreign bacteria, viruses and other substances. However, it is metabolically expensive and may divert calories away from other important tasks, such as growth during childhood.
"This research provides direct evidence for tradeoffs between immune function and growth in a population that's experiencing significant environmental challenges," said co-author Josh Snodgrass, a biological anthropologist at the University of Oregon. "This helps us understand why growth faltering is happening to over 150 million children globally."
The findings came from on-going work in the UO-led Shuar Life and Health History Project, co-directed by Snodgrass and co-author Lawrence Sugiyama, a professor of anthropology. The project with the Shuar, a forager-farming population of about 50,000 in the Amazon River basin, began in 2005.
Earlier in the project, which also tracks the impact of economic development and market integration on adult health, researchers had identified chronic exposure among Shuar youth to infection by soil-transmitted parasitic worms, or helminths.
"One of the things we often see in populations in economically developing countries is that they are short," Snodgrass said. "If you look at what's causing their stunting, it's not their trunk. It's their legs. They are not growing as much in childhood."
In the project, the researchers analyzed finger-prick blood samples from 261 Shuar children, ages 4 to 11.
In the UO Global Health Biomarker Lab, directed by Snodgrass, they focused on four biomarkers of immune function, including antibodies produced in response to helminths. In the field, they measured growth in stature as well as lower-leg growth using a simplified, portable version of a knemometer developed by lead author Samuel Urlacher, a postdoctoral researcher at Hunter College in New York.
Although recurrent infection has long been suggested to produce disruptions in growth, the relationship has been difficult to test directly in humans. The study tracked children over 20 months. In doing so, the researchers were able to link variation in growth parameters to different types of immune function.
The effect of acute inflammation, a particularly costly immune response, on childhood growth was moderated by body fat. Children with greater fat stores were able to completely avoid the growth inhibition faced by thinner children during periods of elevated immune function.
"The study allows us to get at energetic challenges, where you are not consuming enough calories and your body has to make physiological choices about allocation," Snodgrass said. "Growth really doesn't take a lot of energy, but as you grow larger it takes resources to maintain it. That's a huge cost over time."