(CNN) -- And that's not such a good thing. Today's kids are less fit than their parents were at the same age, according to new research presented at the American Heart Association's Scientific Sessions, and that could mean health problems later on.
The fact that younger kids are huffing and puffing more than their parents did at their age doesn't just mean they aren't in shape. It's also a red flag that they may more vulnerable to certain chronic diseases as adults.
Researchers from the University of South Australia's School of Health Sciences analyzed and compared 50 studies on children's running fitness between the years 1964 and 2010.
The studies included about 25 million kids between the ages 9 and 17 in 28 different countries. The researchers recorded running times over a specific time frame -- typically about five to 15 minutes -- or over a certain distance -- a half mile to two miles.
They found that over the 46 years, the cardiovascular endurance -- measured by the times it took to run the various distances -- among kids declined significantly.
From 1970 to 2000, U.S. children's cardiovascular endurance fell about 6% per decade, and among all countries, it fell about 5% each decade. In other words, today, kids are 15% less fit than their parents were, and in the mile run, kids are about a minute and a half slower than children were 30 years ago.
The researchers say that cardiovascular fitness is one of the most important factors for good health, not just in childhood but in adulthood as well.
"If a young person is generally unfit now, then they are more likely to develop conditions like heart disease later in life," said lead study author Grant Tomkinson in a statement.
Currently, the Centers for Disease Control recommends that children get at least an hour of daily activity that involves the entire body, such as running, biking or swimming. Not only does the exercise benefit the body, but research shows that kids who are more physically active tend to do better in school too.
This story was initially published on TIME.com.