Brain scans of more than 2,000 preteens suggest that exposure to cold and heat in the first few years of life can have lasting effects on the brain’s white matter microstructure, especially when living in poor neighborhoods. The study, published in Nature Climate Change, highlights the vulnerability of fetuses and children to extreme temperatures. This research has been led by the Barcelona Institute for Global Health (ISGlobal) and IDIBELL, in collaboration with the Erasmus University Medical Center Rotterdam (ERASMUS MC) and the Center for Biomedical Research Network (CIBER): areas of Epidemiology, Public Health (CIBERESP), and Mental Health (CIBERSAM).
In the current climate emergency, the impact of extreme temperatures on human health is one of the main concerns of the scientific community and society. Children are especially vulnerable to temperature changes, as their thermoregulation mechanisms are still immature.
“We know that the developing brains of foetuses and children are particularly susceptible to environmental exposures, and there is some evidence that exposure to cold and heat can affect mental well-being and cognitive performance in children”, says Mònica Guxens, researcher at ISGlobal, Erasmus MC and CIBERESP. “However, there is a lack of studies that assess the possible changes in brain structure as a result of these exposures,” she adds.
In this study, the team of scientists analyzed the structure of white matter in the brains of preadolescents to identify windows of susceptibility to exposure to cold and heat in the first years of life. The analysis included 2,681 girls and boys from the Generation R Study, a Rotterdam birth cohort, who underwent magnetic resonance imaging (MRI) between the ages of 9 and 12. The MRI protocol assessed brain connectivity by measuring the magnitude and direction of water diffusion into the brain’s white matter. In more mature brains, water flows more in one direction than in all directions, giving lower values for one marker called mean diffusivity and higher values for another marker called fractional anisotropy. The research team used an advanced statistical approach to estimate, for each participant, exposure to monthly mean temperatures from conception to 8 years of age, and its effect on these magnetic resonance connectivity parameters measured at 9-12 years of age.
Window of susceptibility between pregnancy and three years
The results show that exposure to cold during pregnancy and the first year of life, and exposure to heat from birth to 3 years of age were associated with greater mean diffusivity in preadolescence, which points to slower white matter maturation.
“The fibers of the white matter are responsible for connecting the different areas of the brain, which allows communication between them. As white matter develops, this communication is faster and more efficient. Our study is like a photograph at a certain moment in time, and what we see in that image is that the participants most exposed to cold and heat show differences in a parameter – mean diffusivity – that is related to a lower level of white matter maturity,” explains Laura Granés, researcher at IDIBELL and ISGlobal and first author of the study. “In previous studies, alteration of this parameter has been linked to poorer cognitive function and certain mental health problems,” she adds.
“The greatest changes in connectivity parameters are observed in the first years of life,” says Carles Soriano, co-leader of the study and researcher at IDIBELL, the UB and CIBERSAM. “Our results suggest that it is during this period of rapid brain development that exposure to cold and heat can have long-lasting effects on the microstructure of white matter.”
No association was found between temperature exposure in the first years of life and fractional anisotropy at 9-12 years. The scientific team argues that one possible explanation is that these two metrics reflect different microstructural changes, and that mean diffusivity may be a stronger indicator of white matter maturation, compared to fractional anisotropy.
Poverty increases risk
An analysis stratified by socioeconomic conditions showed that children living in poorer neighborhoods were more vulnerable to exposure to cold and heat. In these groups, the windows of susceptibility to cold and heat were similar to those identified in the general cohort, but started earlier. These differences may be related to housing conditions and energy poverty.
An important mechanism that could explain the effect of ambient temperature on neurodevelopment could be related to poorer sleep quality. Other possible mechanisms are the alteration of placental functions, the activation of the hormonal axis that leads to increased cortisol production or inflammatory processes.
“Our findings help raise awareness of the vulnerability of fetuses and the child population to temperature changes,” says Guxens. The results also underscore the need to design public health strategies to protect the most vulnerable communities in the face of the looming climate emergency.