Exposure to heat and cold in early life may affect development of white matter in the brain

In the current climate emergency, the impact of extreme temperatures on human health is a major concern for the scientific community and society. Children are particularly vulnerable to temperature changes, as their thermoregulation mechanisms are still immature.

We know that the developing brain of fetuses and children is particularly susceptible to environmental exposures, and there is some preliminary evidence suggesting that exposure to cold and heat may affect mental well-being and cognitive performance in children and adolescents.

In this study, a research team  looked at the white matter structure in the brains of preadolescents to identify windows of susceptibility to cold and heat exposure in early life. The analysis included 2,681 children from the Generation R Study, a birth cohort in Rotterdam, who underwent magnetic resonance imaging (MRI) between 9 and 12 years of age. The MRI protocol assessed brain connectivity by measuring the magnitude and direction of water diffusion within the brain's white matter.

In more mature brains, water flows more in one direction than in all directions, which gives lower values for a 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 until 8 years of age, and their effect on these MRI connectivity parameters (mean diffusivity and fractional anisotropy) measured at 9-12 years.

Susceptibility window between pregnancy and age three

The results show that exposure to cold during pregnancy and the first year of life, and exposure to heat from birth until 3 years of age were associated with higher mean diffusivity at preadolescence, pointing to slower white matter maturation. Cold and heat, in this case, are defined as those temperatures that are at the lower and upper end, respectively, of the temperature distribution in the study region.

The fibers of the white matter are responsible for connecting the different areas of the brain, enabling communication between them. As the white matter develops, this communication becomes faster and more efficient. This study is like a photograph at a particular moment in time and what scientists see in that image is that participants more exposed to cold and heat show differences in a parameter—the mean diffusivity—which is related to a lower level of maturation of the white matter.

In previous studies, the alteration of this parameter has been associated with poorer cognitive function and certain mental health problems.

The largest changes in connectivity parameters are observed in the first years of life.

These  results suggest that it is during this period of rapid brain development that exposure to cold and heat can have lasting effects on the microstructure of white matter.

Granés L, Cold and heat exposure in early life and white matter microstructure in preadolescents, Nature Climate Change (2024). DOI: 10.1038/s41558-024-02027-w