Bern, Switzerland - Children who had low birth weights were more likely to show increased blood pressure with higher intakes of salt and to have smaller kidneys than children with normal birth weights in a new study [1]. The authors say this may explain the higher rate of hypertension and cardiovascular/renal disease in individuals with low birth weights.

The study, published in Hypertension, was conducted by a group led by Dr Giacomo Simonetti (Bern University Hospital, Switzerland).

Senior author Dr Markus Mohaupt (Bern University Hospital, Switzerland) commented to heartwire:
"We have shown that children aged between 7 and 15 who had low birth weights are more likely to be salt sensitive—have increased blood pressure with higher intakes of salt—than children of the same age who did not have low birth weights. Also, children who had low birth weights had smaller kidneys than those with normal birth weights, which may explain why they are salt sensitive."

It is already known that infants with low birth weights are more likely to develop hypertension, cardiovascular disease, and renal disease later in life than normal-weight infants, Mohaupt explained. "Our study sheds some light on why that may be. Of course, these children may have other complications that could also contribute to their higher cardiovascular risk, but I would say after our study, that smaller kidneys leading to salt sensitivity is probably one reason," he said.

Animal models have shown that low birth weight is associated with salt sensitivity, but this is the first time this association has been shown in humans, Mohaupt added. "I think we can now make a strong recommendation that care needs to be taken with the salt intake of low-birth-weight infants, and this is something that needs to be continued lifelong. These children need to avoid excessive salt intake. It is likely that fast food will be more harmful to them than to children who had normal birth weights. Infants with low birth weights frequently come from complicated pregnancies, with the mother suffering from preeclampsia, and these mothers also have an increased cardiovascular risk, so the mother can be counseled about dietary issues for both herself and her child. If the mother instigates a healthy lifestyle, both mother and child will benefit."

Mohaupt pointed out that this study has provided the first data on the association between low birth weight and salt sensitivity, and it would be good to see larger studies, but he added that these studies are difficult to perform, as testing salt sensitivity is a complicated procedure, requiring children to be compliant with specific diets.

The current study included 50 children, average age 11 years (range 7 to 15 years), 15 who had normal birth weights and 35 who had low birth weight, either because they were born prematurely or because they were born at full term but were small due to growth restriction. The children were all given a controlled-sodium diet for one week and then a high-sodium diet for one week. The researchers measured average arterial pressure during a 24-hour period at the end of each dietary phase. Salt sensitivity was defined as a change of 3 mm Hg or more in response to the high-salt diet.

Results showed that 24-hour blood-pressure readings were within the normal range for all groups at baseline, although the low-birth-weight children had higher blood pressures than those with normal birth weights. Glomerular filtration rate was within normal limits in all of the children but lower in the low-birth-weight children than those with normal birth weights.

Salt sensitivity was present in 37% of the low-birth-weight children and 47% of those who were small for gestational age. Mohaupt noted that salt sensitivity in children is normally low, rising with increasing age, and during adolescence it would be expected that about 18% to 20% of the general population would have the condition, so the results in this group of low-birth-weight children are very high.

The researchers also found that salt sensitivity was inversely correlated to kidney size, and the kidneys of low-birth-weight children tended to be shorter and hold smaller volumes than those of normal-birth-weight children when measured by ultrasound, even after correction for differences in height between the two groups.

"There's nearly a 50% chance of favorably affecting blood pressure by simply reducing salt intake in children born small for their gestational age and nearly a 40% chance for those born with low birth weight. These individuals can be determined very easily if their family physician just gets data on their births," Mohaupt concluded.