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 450 grams 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.

‘Alarming difference’

“The magnitude of the difference we observed is alarming,” says Daniel Berglind, researcher at the Department of Global Public Health at Karolinska Institutet and corresponding author. “The observed 7.9 watts increase for each 450 grams 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 percent difference in the risk of premature death and a 15 percent 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.”