Work at intersection of economics and biology reveals public health insight

For decades scientists have been unable to determine why exactly there is a high incidence of diabetes and other metabolic diseases among individuals who are considered of normal weight in developing countries. A related puzzle is why undernutrition does not always decline with economic development.

A new study led by Yale’s Kaivan Munshi makes the case that there’s a single biological explanation for both.

In a new working paper, Munshi, a professor of economics in Yale’s Faculty of Arts and Sciences and a faculty affiliate of the Economic Growth Center (EGC), and a team of co-authors describe how an increase in food consumption can clash with an individual’s inherited metabolism, causing greater incidence of disease for a few generations in a family line. If the results hold up to further testing, they could have wide ramifications for nutrition programs and policies aimed at curbing diabetes in developing countries.

The paper was released as part of the EGC’s Discussion Paper series.

In earlier research, Munshi has explored how ties among members of social groups — including castes in India and migrant groups in the United States — function in the larger economy. In 2012, while he was at the University of Cambridge, he received funding from the National Institutes of Health for a study of the role community groups might play in tuberculosis-control programs in South India. This led to his research on metabolic disease — any disease or disorder that disrupts metabolism, the process of converting food to energy. Munshi has continued this line of research since joining the Yale faculty in 2019.

The new study reaches back in history to understand the links between current contemporary development trends and public health.

In previous studies, researchers have argued that in the pre-modern economy, people’s caloric intake was generally low, though there were wide, short-term fluctuations in the amount of food available. Over the course of centuries when human societies saw almost no economic growth, the human body adapted to both this long-term food scarcity and fluctuations through several physical processes, including by establishing and defending a “set point” for body mass.

Set point theory posits that the body has a stabilizing — or “homeostatic” — system that uses metabolic and hormonal adjustments to maintain the body’s energy balance against fluctuations in food intake. These metabolic tweaks would have compensated for temporary periods of higher or lower consumption, keeping the body at a stable — and necessarily low — body mass index (BMI), which is roughly an individual’s weight divided by height.

Homeostatic systems, however, can only self-regulate within fixed bounds; when these bounds are exceeded the system will fail. With the onset of economic growth in the modern economy, a sharp increase in available food came as a shock to the system.

Munshi and his coauthors argue that during a society’s initial period of economic development, whenever it is, the bodies of the people in that society fight to defend their biological set point, which is adapted to ancestral consumption levels. This, they argue, accounts for both of the biomedical mysteries that have been documented.

Those individuals whose bodies successfully defend the set point are able to increase their food intake without a corresponding increase in nutritional status — measured in BMI. Consequently, they also play a role in the weak relationship between income and nutritional status observed in India and other developing countries. Those individuals who escape the set point but who are not necessarily overweight contribute to the increased incidence of diabetes and other metabolic disorders because the metabolic balance their bodies had established to maintain the set point has been disrupted, the researchers say.

Populations in Europe escaped their premodern set points many generations ago, but it is happening today in many developing countries, Munshi said.

He pointed out that if there were quality data on income and health over multiple generations in any country, researchers would be able to test this theory directly. Such data is lacking.

However, available household-level data across countries that are at different stages of development offer insights into the issue. For their research, Munshi and his team used health and income data from Ghana, India, and Indonesia to test the theory, and the results were a striking reflection of where those countries are in the process of economic development. In Ghana, most of the population remains at its set point. In India, half the population has passed it, while in Indonesia that is true for three quarters of the population. Additional implications of the model linking BMI to diabetes hold up with data from many other Asian and African countries.

“I’ve never seen results fall into place so cleanly,” Munshi said, “and I suspect it’s because these forces are biological.”

Munshi sees translating his results for the wider research community as an important step in the research. Also critical will be the conducting of further tests, which his team is now doing.

He is excited that these results, which represent a turning point in his research, may yield important discoveries. “I spent 25 years working at the interface between economics and sociology,” he said, “and may spend the next 25 years working at the interface of economics and biology.”

This article is adapted from a longer profile of Munshi on the Economic Growth Center’s website.