A new study has linked low aerobic capacity to non-alcoholic fatty liver disease (NAFLD), and suggested that the resulting liver problems play a crucial step developing obesity-related illnesses.
Poor aerobic fitness is already strongly associated with obesity and its consequent risks of heart disease, strokes and diabetes.
But in case of NAFLD, sufferers 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. It also is a significant contributor to type 2 diabetes," said the study's lead author John Thyfault of the University of Missouri.
In order to test the link between fitness and fatty liver disease, the researchers selectively bred two groups of rats with very different levels of intrinsic aerobic capacity.
After 17 generations of careful breeding, the "unfit" rats could run an average of just 200m compared to over 1500m achieved by the average 'fit' rat.
However, the researchers saw that 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.
On the other hand, the 'fit' group enjoyed healthy livers throughout their lifespans - despite the fact that neither group was getting any real exercise.
The team's findings provide the first biochemical links between low aerobic fitness and fatty liver disease, and thus the authors have suggested that NAFLD could potentially be treated or prevented by a suitable exercise program.
The study has been published in The Journal of Physiology.