Small birth weight linked to 'alarming' reduced cardiorespiratory fitness in later life

Author: Ingrid Torjesen

Babies born with low birth weights are more likely to have poor cardiorespiratory fitness later in life than their normal-weight peers, suggest the findings of a study*. That is according to a study by researchers at Karolinska Institutet in Sweden published in the Journal of the American Heart Association (JAHA).

Cardiorespiratory fitness is declining globally, both for youths and adults. Given its implications for public health, there has been a growing interest in understanding the underlying causes of poor cardiorespiratory fitness and researchers have identified a range of factions including physical inactivity, genetic predisposition, and low birth weight association with preterm delivery that are linked to low cardiorespiratory fitness later in life.

Swedish researchers wanted to examine if low birth weights played a role for cardiorespiratory fitness in individuals born after pregnancy of 37-41 weeks. They followed more than 280,000 males from birth to military conscription at age 17-24 using Swedish population-based registers. At conscription the men underwent a physical examination that included an evaluation of their maximal aerobic performance on a bicycle ergometer. The researchers found that those born with higher birth weights performed significantly better on the cardiorespiratory fitness test. For every 450g of extra weight at birth, in a baby born at 40 weeks, the maximum work capacity on the bicycle increased by an average of 7.9 watts.

The association was stable across all categories of body mass index (BMI) in young adulthood and was largely similar in a subset analysis of more than 52,000 siblings, suggesting that BMI and shared genetic and environmental factors alone cannot explain the link between birth weight and cardiorespiratory fitness.

"The magnitude of the difference we observed is alarming," said Daniel Berglind, researcher at the Department of Global Public Health at Karolinska Institutet. "The observed 7.9 watts increase for each 450g of extra weight at birth, in a baby born at 40 weeks, translates into approximately 1.34 increase in metabolic equivalent (MET) which has been associated with a 13% difference in the risk of premature death and a 15% difference in the risk of developing cardiovascular disease. Such differences in mortality are similar to the effect of a 7-centimeter reduction in waist circumference."

The researchers said the findings have significant public health implications given that around 15% of babies born globally weigh less than 2.5 kilos at birth and cardiorespiratory fitness have important implications for adult health.

*Ahlqvist VH, Persson M, Ortega FB, et al. Birth Weight and Cardiorespiratory Fitness Among Young Men Born at Term: The Role of Genetic and Environmental Factors. Originally published 31 Jan 2020, Journal of the American Heart Association. 2020;9:e014290