Cecilia Lee

After noticing the high rate of metabolic diseases in developing countries, professor of economics Kaivan Munshi looked to explain the phenomenon by investigating the intersection of economics and biology. 

Munshi looked at populations in the eastern developing world and found a threshold income value at which rates of metabolic diseases skyrocket, despite the low average population body weight. Using principles of biological adaptation, he explained that populations have an ancestral body mass index value that their body regulates to —  called the set point — which is lower for those in the developing world. Higher food consumption, which is correlated with increased income in developing countries, past a certain point can lead to metabolic disease, escaping the homeostatic bounds that the body is able to regulate.

“Beyond some threshold, the body will no longer be able to defend the set point, so the homeostatic system that was maintaining the energy balance breaks down,” said Munshi.

Normal body mass index is between 18.5 and 25. Body mass index, or BMI, is the value derived by dividing a person’s mass by the square of their body height. In the United States, according to Munshi, 9.5 percent of type 2 diabetics fall within this normal range. 

However, in India, even when changing the upper bound from 25 to 23, 55 percent of type 2 diabetics fall within the normal range of BMI. 

“Diabetes is a pretty substantial issue in populations from low and middle-income countries, especially in South Asia, meaning India, Pakistan and Bangladesh, and it has implications on not just eventual mortality directly due to diabetes, but also increases the risk of several other things, like cardiovascular disease,” Saad Omer, director of the Yale Institute for Global Health, explained.

Munshi found that, up until the median income level, there is no relationship between increasing income and an increase in metabolic disease, even though those with a greater income are able to eat more. However, as soon as the approximate median income is surpassed, there is a strong correlation between an increase in income and metabolic disease, even though the population’s BMI is typically around 22 –– well within the established normal range. This change is discontinuous, meaning it occurs abruptly and intensely.

To explain these results, Munshi turned to a mathematical model with three main points: adaptation, mismatch and set point. Adaptation refers to the concept that a fetus adapts to an environment that has an insufficient supply of nutrients. The body does this by adjusting the homeostatic systems that it uses to preserve a stable energy level. With an insufficient amount of energy in the womb, the body adapts to need less energy to function. This adaptation not only persists for that fetus’s life, but can remain for generations. 

This persistence for generations ties into the next component of the model, mismatch. 

“With economic development, you have a very substantial increase in income as well as food consumption, and there is going to be a mismatch between the level of food consumption … these populations are adapted for and the current level of consumption that they have due to economic development, and this is what is supposed to give rise to diabetes and other metabolic disorders in these populations,” Munshi said. 

According to Munshi, the analyzed populations are adapted for a nutrient supply that is lower than what is now available to them. Since these countries are still developing, the population has not yet adjusted to what is called their set point.

In this study, the set point refers to the BMI that a person’s body homeostatically adjusts to. To make this more clear, Munshi compared this to the homeostatic process of regulating body temperature. The body regulates its processes so that it maintains a temperature of 98.6 ºF, which is the set point body temperature. However, if the body strays too far from this set point, it can no longer regulate its temperature to the set point, resulting in disorders arising from extreme body temperature conditions like hypothermia or hyperthermia. 

When the threshold income level is reached, which was found to be approximately the median income in the analyzed developing countries, food consumption, and therefore BMI, surpass this set point. When the set point is surpassed, glucose levels cannot be regulated, resulting in metabolic diseases such as diabetes. Those in the population with a normal BMI have metabolic diseases because their lower set point cannot cope with the increase in glucose consumption. 

This same phenomenon happened about 200 years ago in many European populations, according to Munshi.

In order to combat the prevalence of metabolic disease, Munshi proposes reducing food consumption slightly. He said this would be much easier for diabetics in developing countries because the set point BMI is not too far from the typical BMI of type 2 diabetics in those populations, as opposed to the majority of type 2 diabetics in America. 

Omer emphasized that countries should invest in early prevention and make “sure that access is equitable.”

“We know that lower to middle-income countries … are the ones where now the majority of the world’s poor live,” said Rohini Pande, director of the Economic Growth Center at Yale. “The ability of the world’s poor to escape poverty is going to, in part, depend on their health conditions. The more that we can understand about the interplay of economic development and human biology, the better place we are going to be to identify interventions that can help ensure that economic development can help improve people’s health.”

The Economic Growth Center at Yale was founded in 1961, focusing on the quantitative study of lower-income economies.