Southampton, UK - A new study indicates that smaller size at birth is associated with sex-specific alterations in cardiac physiology [1]. In one of the few studies to look at this phenomenon in prepubertal children, British researchers have found that lower birth weight in both boys and girls seems to be indicative of hypertension in later life, but that this appears to be mediated via different mechanisms, depending on gender.

Dr Alexander Jones (Southampton General Hospital, UK) and colleagues report their findings online July 22, 2008 in the European Heart Journal. Jones, who conducted the work while at Southampton, is now working at Great Ormond Street Hospital, in London. He told heartwire that it is already known that low birth weight increases the risk of heart disease and diabetes in later life, "but we wanted to show that there was some intermediate step between this very early phenomenon that epidemiologists have found and the situation when you are 50 or 60. Is there something already going on in early childhood that can show that this process is already in play?

"What's really interesting about this study is that we found good associations between size at birth and biology that might lead to hypertension, but it's very sex-specific," he continued. "If we are able to untangle this biology in humans at this sort of age in greater detail, then we should then be able to target these processes much earlier in life than we currently do. If you or I go to the doctor aged 30 or 40, we're unlikely to impress upon them that we need to have anything done to modify our risk of heart disease. But it's possible that in the future that view will change."

But in the meantime, says Jones, it's vital to reiterate already well-known health messages: "This adds weight to the general health advice about good maternal nutrition, which we know is crucial in determining fetal size, and we know that mothers not smoking is also very important."

If we are able to untangle this biology in humans at this sort of age in greater detail, then we should then be able to target these processes much earlier in life than we currently do.

In their study, Jones et al studied 140 healthy boys and girls aged seven to nine years, born at term and followed prospectively. Jones stressed to heartwire that all of the children were within the usual range of weight at birth.

The children's blood pressure, ECGs, and cardiac impedance were recorded before, during, and 10 minutes after a stress test, in which they were asked to perform a public-speaking task involving storytelling followed by mental arithmetic.

"In boys, we found that the lower their birth weight was, within the normal range, the more likely they were to have a higher vascular resistance and higher blood pressure, particularly 25 to 30 minutes after the start of the stress test. This probably represents a prolongation of the vascular stress response in these boys," Jones said.

"In contrast, girls who were smaller at birth did not demonstrate a specific response to stress. They consistently (whether under stress or at rest) showed evidence of greater activity in the sympathetic nervous system, which could have adverse effects later in life."

Jones said prior work in adolescents in Australia has shown increased sympathetic activity among teenage girls who had low birth weights, and that this had eventually led to reduced baroreflex activity, something that can increase the risk of hypertension. "We didn't test the baroreflex activity in the children, but this is another mechanism by which people who are smaller at birth could go on to develop hypertension in later life," he noted.

If you're exposed to certain environmental triggers at the right time in your development, then your physiology is changed . . . permanently, or at least dramatically.

"This is the first evidence in children of relationships between size at birth and the later function of the heart and blood vessels. The sex differences in these relationships were striking and may eventually lead to a better understanding of why men and women tend to develop high blood pressure and heart or vascular disease at different times in their lives," Jones reiterated.

"I think that's quite exciting because it might make us look much more carefully at things like sex hormones that could drive this and make it a sex-specific phenomenon."

Jones says this has been dubbed "programming," whereby "fetal growth and development are important in determining the way your physiology works throughout life. If you're exposed to certain environmental triggers at the right time in your development, then your physiology is changed and an important part of this is that it appears to be changed permanently, or at least dramatically, so that even if you could modify it, you can't change it back to where it might have been."