University of Sydney researchers have discovered that children whose mothers smoke during pregnancy had lower levels of the type of cholesterol known to protect against heart disease in later life – high-density lipoprotein (HDL) cholesterol.
By the age of eight children born to mothers who smoked while they were pregnant had HDL cholesterol levels of about 1.3 millimoles per litre (mmol/L) compared to the more normal level of 1.5 mmol/L in children whose mothers had not smoked.
This effect was independent of whether children were exposed to cigarette smoke after birth, suggesting prenatal exposure had the most impact on subsequent development. The research, published online today in the European Heart Journal, was led by David Celermajer, Scandrett Professor of Cardiology at the University of Sydney.
“Our results suggest maternal smoking 'imprints' an unhealthy set of characteristics on children while they are developing in the womb, which may well predispose them to later heart attack and stroke,” Professor Celermajer said
“This imprinting seems to last for at least eight years and probably a lot longer.”
Professor Celermajer and colleagues examined the effects of maternal smoking during pregnancy on the thickness of the arterial wall and the levels of lipoproteins in a group of 405 healthy eight-year-olds born between 1997 and 1999.
Data was collected before and after birth, including information on mothers' smoking habits before and after pregnancy, children's exposure to passive smoking, and height, weight, waist circumference and blood pressure measurements. Ultrasound scans were used to measure arterial wall thickness and they took blood from 328 children to measure lipoprotein levels.
Although smoking had no effect on the thickness of the children's arterial wall, the difference on HDL cholesterol levels attributable to mothers' smoking was about 0.15 mmol/L. This difference remained statistically significant after adjusting for factors such as post-natal smoke exposure, breastfeeding duration, physical inactivity and body mass index.
The researchers believe lower levels of HDL cholesterol at this age could lead to serious impact on health in later life.
“Cholesterol levels tend to track from childhood to adulthood,” Professor Celermajer said. “Studies have shown that for every 0.025mmol/L increase in HDL levels there is an approximately 2 to 3 percent reduction in the risk of coronary heart disease.
“If we extrapolate this we can suggest the difference of 0.15mmol/L between children of smoking mothers compared to non-smoking mothers might result in a 10 to 15 percent higher risk for coronary disease. This is an approximation only, but the best one we have.
“Children born to mothers who have smoked during pregnancy will need to be watched particularly carefully for other coronary risk factors such as high blood pressure, 'bad' cholesterol (LDL) levels, and especially cigarette smoking themselves.
“The only ways to increase HDL levels are regular exercise and the use of certain medications such as Niacin. We will have to do long-term follow-up to see if these particular children continue to have lower HDL cholesterol levels than normal, but one should presume this risk factor might indeed be persistent.
“One of the advantages of studying this in eight-year-old children is the usual factors that drive down HDL cholesterol, such as obesity and diabetes, are absent, and so we can infer a more or less direct effect of smoking on HDL levels,” concluded Professor Celermajer.