Poor aerobic fitness is strongly associated with obesity and its consequent risks of heart disease, strokes and diabetes - now considered worldwide epidemics.
But the underlying link has long puzzled scientists. New research in The Journal of Physiology connects low aerobic capacity to another serious condition - non-alcoholic fatty liver disease (NAFLD) - and suggests that the resulting liver problems play a crucial step developing obesity-related illnesses.
Sufferers of NAFLD accumulate fat in their livers and have high levels of fat in their blood, amplifying the risk-factors of obesity. The disease leads to a form of liver damage called fibrosis, similar to the results of alcohol abuse. "Fatty liver disease will be the next big metabolic disorder associated with obesity and inactivity," said the study's lead author John Thyfault of the University of Missouri. "It also is a significant contributor to type 2 diabetes."
To test the link between fitness and fatty liver disease, Dr Thyfault's team selectively bred two groups of rats with very different levels of intrinsic aerobic capacity. After 17 generations of careful breeding, their 'unfit' rats could run an average of just 200m compared to over 1500m achieved by the average 'fit' rat.
The effect on the rats' livers was devastating. At 25 weeks old, the unfit group were displaying clear symptoms of NAFLD - weakened mitochondria (the cell's powerhouses), poor fat processing power, high fat retention and other abnormalities. By the end of their natural lives, the rats' livers had sustained damage including fibrosis (the precursor to cirrhosis) and unexpected cell death.