Rules requiring regular physical education for young teens stem from “good science,” researchers say.

Frequent “phys ed” classes not only improve fitness, they also encourage healthy living, finds a study from Oregon State University.

Researchers looked at more than 400 students, ages 12 to 15. They found that more than one in five received no physical education, and only about 27 percent met federal government physical activity guidelines. Nearly 40 percent were obese or overweight.

“Perhaps some were not meeting the guidelines because fewer than 35 percent actually knew what the guidelines were for their age group,” said study co-author Brad Cardinal. He’s a professor in the school of biological and population health sciences.

The federal recommendation calls for at least 60 minutes a day of moderate to vigorous physical activity five days a week, the researchers said. The study found that long-term physical education boosts fitness and teaches students how regular physical activity is associated with good health. There was a strong connection between regular physical education and students meeting the federal activity guidelines, the researchers said.

The study authors said the trend toward more lax phys ed mandates for middle-school students is detrimental to their development.

Physical activity has been shown to improve thinking and academic achievement. It also helps students develop the knowledge, interests and skills for a lifelong healthy lifestyle, according to the researchers.

“We have the physical activity guidelines for a reason, and they’re based on good science,” Cardinal said in a university news release. “With only slightly more than one in four adolescents meeting the guidelines, today’s youth are being shortchanged in terms of their holistic development. They are not being prepared to live the proverbial good life,” he added.

New guidelines are due to be released in 2018.

“Because of a growing propensity toward inactivity in daily life, such as increased media consumption and screen time, the guidelines very well may have to be ratcheted up to compensate,” Cardinal said.

The study was published recently in the American Journal of Health Promotion.