Most U.S. high school students engage in behavior that puts them at an increased risk of dying, according to the U.S. Centers for Disease Control and Prevention.

In a new study, “Youth Risk Behavior Surveillance – United States, 2009,” CDC researchers analyze trends in youth and young adult risk behaviors from 1991 to 2009.

For youths ages 10 to 24, the four most common causes of death have been motor vehicle crashes, unintentional injuries, homicide and suicide. The contrast with the list of causes of mortality for older Americans is stark: for Americans over 25, the leading causes of death are cardiovascular disease and cancer

The researchers found that 9.7% of the students surveyed reported regularly failing to wear seat belts when riding in a car driven by someone else, and 28% said they ride in cars with drivers who have been drinking alcohol.

In the 30 days before the survey, 17.5% of the students had carried a weapon to school.

During the 12 months before the survey, 31.5% had been in a physical fight.

At some point, 6.3% had attempted suicide.

Only 2.1% of surveyed students said they had ever injected an illegal drug, but 19.5% admitted to smoking cigarettes, 21% admitted that they had used marijuana, and 42% said they had drunk alcohol. (According to the Center on Alcohol Marketing and Youth, Baltimore, people who begin drinking before the age of 15 are four times more likely to develop alcohol dependence than if they wait until they are 21.)

Just over a third of the students surveyed (34%) said they were sexually active. Of those, nearly 40% had not used a condom during their last sexual intercourse.

But the most common risk factors involve fitness and nutrition.

In the week before the survey, 78% of the students had not eaten fruits or vegetables more than 5 times per day, and 29% of them had consumed soda at least once per day. Some 82% of students were not physically active for at least an hour a day each day for the week before the survey. Only a third attended physical education classes daily, and 12% were obese.

The U.S. Department of Health and Human Services, the parent of the CDC, points out in its Physical Activity Guideline for Americans that regular physical activity helps children and adolescents improve cardiovascular health, muscle fitness, bone health and body composition, and that getting fit can even reduce symptoms of depression. A compelling point, considering that suicide remains a leading cause of death for Americans under the age of 25.