Parents with more than two children have a greater risk of coronary heart disease (CHD), British scientists report in the Feb. 18 issue of Circulation. Oddly, that risk was also increased in people who had only one child or were childless, suggesting that two might be the magic number when it comes to children and your health.
"Quite a lot of studies have looked at the number of children and CHD just in women and have shown similar results to what we showed, which is that women with a larger number of children tend to have more heart disease later on in life," says Dr. Debbie Lawlor, the study's author and a lecturer in epidemiology in the department of social medicine at the University of Bristol in the United Kingdom. "The assumption was that it was something to do with what was happening during pregnancy."
Previous research had found an association between pregnancy and heart disease risk in women, possibly due to the increased insulin resistance that can occur during gestation.
Another potential explanation, Lawlor says, is that women with more children tend to be from poorer socioeconomic backgrounds and follow less healthy lifestyles. For that theory to have legs, men with lots of children should also have higher levels of heart disease.
To test the hypothesis, Lawlor and her colleagues looked at 4,286 women and 4,252 men between the ages of 60 and 79 who had anywhere from zero to five or more children.
They found a "J-shaped" association between the number of children and prevalence of CHD in both men and women. In other words, those parents with one child or less and those with more than two offspring had a higher risk of CHD, compared to parents with an even two.
With each additional child, both parents were more likely to be obese. Worse yet, mothers of more than two children were also more likely to show signs of insulin resistance, have elevated blood sugar levels and diabetes, and have low levels of HDL, the "good" cholesterol, and increased levels of triglycerides.
The heart disease risk kept multiplying for each child after two -- a 30 percent increase with each additional child for women and a 12 percent increase for men.
"We have this very mixed picture," Lawlor says. "There's a similar relationship with men and women in terms of heart disease, although it was stronger in women than men."
One expert says it's hard to know exactly what factors are influencing the risk for heart disease.
"I don't think there's any way to tease out exactly what it is," says Dr. Dan Fisher, a cardiologist with the New York University Medical Center in New York City. "As you get older, you're more likely to have high blood pressure. We know you get a little bit more sedentary. Is it the stress of having more kids? That you don't have time in your life now for exercise and diet? I don't know if they have the answers."
Lawlor believes there may be two things at play. Both men and women who have more children tend to be from poorer socioeconomic groups, which tend to have other risk factors, such as obesity, associated with heart disease.
In addition, women who have had a larger number of pregnancies have an added biological risk.
As a normal physiological response during pregnancy, women become insulin-resistant so they can deliver more nutrients to the growing fetus, Lawlor explains. When the baby is delivered, the mother should go back to normal, but that may not always happen.
"We think that if you keep on doing something that changes the normal hormonal system, after a while it gets worn out," she speculates.
Then what explains higher heart disease among people who have no kids or only one?
That may point to one factor that contributes to apparent infertility and heart disease, she says.
This last point probably does not apply to parents and potential parents of today, Lawlor notes.
"The people we studied are from a generation where it would be very unusual that people with one or less children would have actively chosen this," Lawlor points out.
Admittedly, Lawlor says, the number of children is a crude measure of heart disease risk. She next plans to look at diet, fat and insulin levels and obesity during child-rearing years to pinpoint the exact pathways involved in this process.
In the meantime, the study reinforces the age-old advice to eat a balanced diet and stay active.
"I agree with the real take-home message of encouraging mothers and fathers to maintain good health," Fisher says. "The controllable part is, when you're a parent you have less interest or time or availability to really focus on diet and exercise. Maybe that's one of points we need to be getting at."