Obesity rates among U.S. adults have skyrocketed in the last 20 years. The Center for Disease Control and Prevention’s most recent numbers now classify 34 percent of adults as overweight and 30 percent as obese. And the impact can be felt both by the individual and society as a whole, as weight-related health problems account for approximately 5 to 7 percent of total health care expenditures in the United States every year.

In an attempt to isolate some of the childhood indicators of adult obesity, Associate Professor Ariel Kalil (with Harris School alumna Kathleen M. Ziol-Guest, PhD’05, of Harvard University and Greg J. Duncan of Northwestern University) examines how childhood economic status affects adult body mass index. Kalil explained, “If we can identify the specific associations between income in childhood and adult health, there may be a greater chance of implementing targeted interventions for low-income children in the United States.”

The authors analyzed data from the U.S. Panel Study of Income Dynamics (PSID), the longest-running longitudinal study of household income from 1968 to 2005. The survey also includes detailed demographic information for participants, including body mass index (BMI), which measures body fat based on height and weight. While past research has linked childhood socioeconomic characteristics to adult health, this paper by Kalil, Ziol-Guest, and Duncan is the first to connect a nationally representative sample of income data—from across the entire length of childhood—to adult BMI, measured as late as 37 years of age.

Kalil and her co-authors averaged family income over three periods of childhood: the prenatal year through the calendar year in which the child was born (a two-year period, termed “perinatal”); ages one to five; and ages six to 15. The perinatal period is key to isolate the effects of poverty on very young children. Past research has shown that low income during the perinatal period may be associated with fetal undernutrition, low birth weight, or slow growth in the first two years of life.

In fact, Kalil and her co-authors determined that low household income—below $25,000—during the perinatal years is associated with significantly higher BMIs and rates of overweight, obesity, and morbid obesity. The researchers defined overweight as a BMI of 25 kg/m2 or greater; obesity as 30 kg/m2 or greater; and morbidly obese as 40 kg/m2 or greater. They found that income during the other two childhood periods was not significantly associated with elevated adult BMI.

Income changes during all three childhood periods, such as families making more money as the years progressed, were fairly common. As for overall outcomes, the authors found that it was those income changes during the perinatal years that appeared to impact adult BMI the most. A $10,000 increase over the two perinatal years for lowest income children resulted in a significant reduction (2.51 kg/m2) in adult BMI.

The largest reduction was seen for children in the 75th percentile of the BMI distribution. This suggests that early childhood poverty has the greatest consequences on adult health for those children who start out with a higher percentage of their body mass from fat. As a child aged however, changes in income had less and less of an effect on eventual adult BMI.

“Our finding … is consistent with the hypothesis that fetal programming induced by early stimulants and insults has long-lasting implications for physiology and disease risk,” said Kalil. In particular, over- or undernutrition during pregnancy can affect development in several ways, including fat cell and pancreatic B-cell development, which causes irreversible changes to appetite regulation. Low income is also associated with exposure to environmental stressors, which can result in physiological changes that affect eating behavior.

One important fact, the authors noted, is that PSID weight and height is self-reported, which previous research suggest leads to underestimating weight and height for both men and women. So, it is possible that this study’s conclusions are lowered estimates of how early childhood poverty relates to adult BMI.

Nonetheless, the authors wrote, “These findings provide initial evidence that environmental and social influences may be especially relevant for individuals vulnerable to weight gain, suggesting that genotypes may moderate children’s sensitivity to environmental insults.”

The study indicates that government support for low-income families, especially investment in early childhood, can have critical importance for eventual adult health. And the results also show how important it is to focus on the nation’s poorest. “Targeting these transfers, or similar programs, to families with the youngest children may offer the largest benefit for health and well-being.”