Toddlers' TV-viewing habits have a direct impact on their fitness and fatness in later childhood.
Each hour per week of television watched by a two-year-old corresponded to a reduced level of long jump performance by the age of eight to 10, researchers found.
Every extra hour of weekly TV between the ages of two and four also led to almost half a millimetre increase in waist circumference.
For children watching an average 8.82 hours a week, this amounted to a 0.41cm fatter waistline.
Youngsters exposed to more than 18 hours of TV a week -- around two and a half hours a day -- were almost a centimetre wider around the middle by 10 years of age.
Lead researcher Dr Linda Pagani, from the University of Montreal, said: "The bottom line is that watching too much television -- beyond recommended amounts -- is not good.
Physical fitness was measured by means of a standard long jump test, which provides a good indication of overall athletic ability. The scientists found that each weekly hour of TV at two years of age was associated with a reduced jump distance of about a third of a centimetre by the fourth grade.
Dr Pagani said that across the Western world both children and adults had experienced "dramatic increases in unhealthy weight" in recent decades.
He added: "Watching more television not only displaces other forms of educational and active leisurely pursuits, but also places them at risk of learning inaccurate information about proper eating."
Co-author Dr Caroline Fitzpatrick, from New York University, said: "TV is a modifiable lifestyle factor, and people need to be aware that toddler viewing habits may contribute to subsequent physical health.
"Further research will help determine whether the amount of TV exposure is linked to any additional child health indicators, as well as cardiovascular health."