Heat stress impacts growth in infants up to 2 years old, new analysis finds

Exposure to high levels of heat may impact the growth of fetuses during pregnancy and infants up to 2 years of age, a new analysis suggests.

The study is the first of its kind to show that heat stress may impact the development of babies after birth and adds to the team's previous research showing the impact of heat stress on fetal development. The results are published in The Lancet Planetary Health.

The research, which looked at data from infants and their mothers collected during a clinical trial in Gambia, found a small decrease in birth weight for gestational age for each 1°C increase in average daily heat stress during the first trimester.

No effects on growth were observed for heat stress experienced during the second trimester. Research has suggested that there may be an increase in head circumference relative to body size for gestational age in fetuses exposed to heat stress during the third trimester, but this was less certain.

The results also show that infants up to 2 years of age exposed to high heat in their environment may have lower weight and height for their age. The greatest decreases were seen in infants aged 6 to 18 months who had experienced higher average daily levels of heat stress in the previous three months.

At 12 months, infants exposed to an average heat stress value equivalent to 30°C were more likely to have reduced weight for their height and age, compared with those who had experienced heat stress equivalent to 25°C.

The results were found in both male and female infants.

The team, led by researchers from the Medical Research Council Gambia (MRCG) unit at the London School of Hygiene & Tropical Medicine (LSHTM), say the impacts of heat exposure need to be urgently considered in public health interventions to reduce the impact of climate change on pregnant women and their children.

Data were initially collected as part of the Early Nutrition and Immunity Development (ENID) randomized controlled trial conducted in West Kiang, The Gambia, between January 2010 and February 2015. A total of 668 infants were followed during their first 1000 days of life, including 329 (49 %) girls and 339 (51 %) boys.

The trial analyzed the relationship between heat stress and fetal growth based on clinically recognized weight, length, and head circumference for gestational age scores. It also assessed the effect of heat stress on infant growth based on weight and length scores from 0 to 2 years.

At birth, 66 (10 %) infants weighed less than 2.5 kg, which is described as low birth weight, 218 (33 %) were small for gestational age, and nine (1 %) were born preterm.

Heat stress occurs when the way our body controls its internal temperature is compromised by external factors such as weather or physical activity. In the study, heat stress was defined using the Universal Thermal Climate Index, which takes into account factors such as heat, humidity, wind speed and solar radiation, and assigns an equivalent temperature (°C) to an associated risk of developing heat stress.

During the study, the average level of heat stress exposure was 29.6 °C. The highest daily maximum was 45.7 °C and the highest daily minimum was 28.9 °C.

Dr Ana Bonell, Assistant Professor based at the MRCG and lead author of the study, said: “Our study demonstrates that the intersecting crises of climate change, food insecurityand undernutrition disproportionately affects the most vulnerable, including young children.

"These findings build on previous evidence showing that the first trimester is a vulnerable period for heat exposure and it is important that we now examine factors that may contribute to this relationship.

“It's likely that heat stress may impact appetite, food intake and availability, and we are already investigating whether there may be direct effects on cellular and inflammatory pathways, adding to the already reduced ability of pregnant women and infants to regulate their own body temperature.

“We need to explore which populations are likely to experience the most heat stress and where growth retardation may be occurring, to enable us to develop effective public health measures.

“With global rates of child wasting remaining unacceptably high and ongoing planetary warming, these findings must spur action on improving child health. »

Researchers say further research is needed to assess the relationship between heat stress and health impacts in areas beyond The Gambia. Available data did not include information on feeding practices, maternal infections or socioeconomic status, which may also impact fetal and infant growth.