Younger boys need more exercise to prevent cardiovascular disease than current recommendations suggest, says a new study published today in the journal BMC Medicine.

Researchers honed in on kids ages 9 and under, an age group that hasn’t been previously studied much as a separate subset. After analyzing the risk of cardiovascular disease by calculating the blood pressure, cholesterol levels and skinfold thickness of 3,000 kids ages 2-9, they found that 85 minutes of exercise per day for boys ages 6-9 is probably a better recommendation than the current estimation of about an hour.

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For girls, an hour appears to be sufficient, although the study didn’t examine the cause of the gender difference. And the association in kids younger than 6 was unclear.

Regardless of gender, daily physical activity should include at least 20 minutes at a vigorous level, the researchers said.

“The association between physical activity and cardiovascular disease risk factors in children has been the focus of research for over two decades,” Robert McMurray, of the University of North Carolina Chapel Hill, wrote in an accompanying commentary. “The majority of this research has focused on children over ten years of age with little information on very young children.”

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The study also points to the need for individualized exercise recommendations, the researchers note. The Centers for Disease Control and Prevention currently recommends 60 minutes of exercise for children and adolescents (ages 6-17) including vigorous activity at least three times per week.

“When clinicians recommend physical activity for children they should evaluate ‘at risk’ children on a case-by-case basis rather than using generalized guidelines,” McMurray wrote.