Normal developmental changes during the teenage years leave young adult men at higher risk of heart disease than their female counterparts, researchers report in Circulation: Journal of the American Heart Association.
"Women's protective advantage against heart disease starts young," said Antoinette Moran, M.D., lead author of the study and professor and division chief of pediatric endocrinology and diabetes at the University of Minnesota Children's Hospital in Minneapolis.
In adults, a constellation of factors increases the risk of heart disease. They include high blood pressure, smoking, obesity, physical inactivity, abnormal cholesterol levels and insulin resistance (a pre-diabetic condition in which the body can't use insulin effectively).
To track the emergence of these risk factors, researchers followed 507 Minneapolis school children from ages 11 to 19, when they had all reached sexual maturity. Fifty-seven percent of the children were male, 80 percent were white and 20 percent were black.
During the study, the researchers made 996 observations on the group, noting blood pressure, insulin sensitivity (opposite to insulin resistance), body mass index and other body composition measures, blood glucose and cholesterol measurements.
"We wanted to see which risks emerge first and how they relate to one another in normal, healthy school kids without diabetes or other major illnesses," Moran said.
At age 11, boys and girls were similar in their body composition, lipid levels and blood pressure, researchers said.
Boys and girls became heavier during adolescence, increasing in body mass index and waist size. As expected during puberty, changes in body composition differed sharply between genders, with percentage of body fat decreasing in boys and increasing in girls.
During the study, changes in several cardiovascular risk factors or risk markers differed significantly between boys and girls:
- Triglycerides (a type of fat in the blood) increased in males and decreased in females.
- High-density lipoprotein (HDL or "good") cholesterol decreased in males and increased in females.
- Systolic blood pressure (the first number in the blood pressure reading, measuring the pressure when the heart contracts) increased in both, but significantly more in the males.
- Insulin resistance, which had been lower in the boys at age 11, steadily increased until the young men at age 19 were more insulin resistant than the women.
- Researchers found no gender difference in two other cardiovascular risk factors, total cholesterol and low-density lipoprotein (LDL or "bad") cholesterol.
"By age 19, the boys were at greater cardiovascular risk," Moran said. "This is particularly surprising because we usually think of body fat as associated with cardiovascular risk, and the increasing risk in boys happened at the time in normal development when they were gaining muscle mass and losing fat."
Although girls gained cardiovascular protection when their proportion of body fat was increasing, excess fat is still a cause for concern.
"Obesity trumps all of the other factors and erases any gender-protective effect," Moran said. "Obese boys and girls and men and women all have higher cardiovascular risk."
The researchers said further studies are needed to better understand the development of cardiovascular protection during adolescence.
"That the protection associated with female gender starts young is fascinating and something that we don't understand very well," Moran said. "That this protection emerges during puberty and disappears after menopause suggests that sex hormones give women a protective advantage. There's still a lot that needs to be sorted out in future studies -- estrogen may be protective or testosterone may be harmful."
Moran noted that this is normal physiology and not something that is influenced by lifestyle factors.
Co-authors are: David R. Jacobs Jr., Ph.D.; Julia Steinberger, M.D., M.S.; Lyn M. Steffen, Ph.D.; James S. Pankow, Ph.D.; Ching-Ping Hong, M.S.; and Alan R. Sinaiko, M.D.
The National Institutes of Health partly funded the study.
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