Heat stress impacts growth rates of infants up to age 2, new analysis shows

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

The study is the first of its kind to show that heat stress can 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 Lancet Planetary Health.

The research, which examined data collected from infants and their mothers during a clinical trial in The Gambia, found a slight decrease in birth weight for gestational age for every 1°C increase in heat stress average daily 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 the body for gestational age in fetuses exposed to heat stress during the third trimester, but this hypothesis was less certain.

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

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

The results were seen in both male and female infants.

The team, led by researchers from the Medical Research Council Unit in The Gambia (MRCG) at the London School of Hygiene & Tropical Medicine (LSHTM), says the impacts of heat exposure need to be considered. 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 throughout their first 1,000 days of life, consisting of 329 (49%) female infants and 339 (51%) male infants.

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

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

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 a equivalent temperature (°C) with an associated risk of developing thermal stress.

During the study, the average level of exposure to heat stress 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 at MRCG and lead author of the study, said: “Our study demonstrates that the intersecting crises of climate change, food insecurity and undernutrition disproportionately affect the most vulnerable, including including young children.

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

“It is likely that heat stress may impact appetite, food intake and availability, and we are already investigating whether there could 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 expected to experience the most heat stress and where slower growth might be recorded, to enable us to develop effective public health measures.

“As global rates of child wasting remain unacceptably high and global warming continues, these findings should spur action to improve children’s health.”

The researchers say more research is needed to assess the relationship between heat stress and health impacts in regions beyond The Gambia. The available data did not contain information on feeding practices, maternal infections, or socioeconomic status, which could also impact fetal and infant growth.