The lower risk of heart disease enjoyed by women compared to that for men is already evident in the teenage years, a new study shows.

Among 507 Minneapolis students followed from middle school to age 19, researchers found that after puberty, boys showed an increase in their triglycerides (a type of blood fat) and a dip in their levels of “good” HDL cholesterol. The opposite pattern was seen in girls, however.

By age 19, males were also, on average, more resistant to the hormone insulin than females were. Insulin resistance is a precursor to type 2 diabetes, a major risk factor for heart disease later in life.

The findings, published in the journal Circulation, are in line with the fact that women are relatively protected from heart disease before menopause. What’s new is that the study shows this sex difference emerges quite early - by the end of puberty.

“We were somewhat surprised that this occurred so early, because it occurred at the very time that boys were becoming more lean and girls were gaining body fat,” lead researcher Dr. Antoinette Moran, of the University of Minnesota Children’s Hospital in Minneapolis, told Reuters Health.

As is expected with normal development, boys in the study generally gained muscle mass and lost body fat after puberty, while girls generally put on more body fat. The fact that boys’ heart risk factors increased while girls’ declined shows that “there is more to the story than just body fat,” Moran noted. Instead, she explained, hormonal differences between the sexes are at work.

This does not, however, negate the importance of maintaining a healthy body weight through the teen years, regardless of sex. “When either boys or girls were overweight,” Moran emphasized, “that overwhelmed normal physiology and led to insulin resistance and cardiovascular risk in either sex.”

SOURCE: Circulation, May 6, 2008.