Fifteen and 16 year olds are already showing signs of heart disease due to poor fitness.
New research shows that teenage boys with low fitness levels had unhealthy blood vessels.
The research from Dublin City University also found that these boys were more likely to be overweight, have higher blood pressure and cholesterol than fitter teens.
Professor Niall Moyna from the Centre for Preventive Medicine at DCU says this has been a long time coming:
