Declines in aerobic fitness: are they only due to increasing fatness?

This study quantifies the cross-sectional and longitudinal relationships between young people's fatness (BMI, skinfold thickness) and fitness (performance on tests of aerobic fitness).

Over the last 20-30 years, young people have become fatter and less fit. It is likely that the decline in fitness is largely due to increases in fatness. There are strong mechanistic connections; within cohorts, variability in fatness accounts for about 20% of variability in running performance; there is a strong correlation between overweight prevalence and relative fitness across specific cohorts from different countries; and secular declines in fitness coincide temporally with increases in BMI.

Australians aged 10-12 years tested in 1985 were matched for age, sex, BMI and triceps skinfold thickness with their counterparts tested in 1997 (n = 279 matched pairs), and 12-15 year-old tested in 1995-1996 were matched with their counterparts tested in 1999-2000 (n = 2,834 matched pairs). Performance differences on running tests in the matched datasets were compared with performance differences in the complete (unmatched) datasets.

Performance differences persisted even when young people were matched for fatness. Matching for fatness reduced overall performance differentials by 29-61%. Other factors such as reduced physical activity and subsequent training effect are likely to have contributed to the decline.