Low birth weight linked to salt sensitivity, blood pressure and kidney issues

Nearly half of children who were small at birth had a significant rise in blood pressure when they ate a high-salt diet, according to a Swiss study published in Hypertension: Journal of the American Heart Association.

Results of the study suggest that restricting salt in these children could improve blood pressure and supports the theory that some causes of heart disease begin in the womb.

“This study indicates that disturbances during pregnancy that affect intrauterine growth could compromise the function of organs in adulthood,” said Markus G. Mohaupt, M.D., co-author of the study and head of the Division of Hypertension in the Department of Nephrology/Hypertension at the University of Bern in Switzerland.

The study is the first to find that low-birth-weight (LBW) and small-for-gestational-age (SGA) children – those weighing about 5.5 pounds or less – are more likely than those of normal birth weight to show blood pressure increases in response to a high-salt diet when tested in late childhood or adolescence. This response is known as salt sensitivity.

“Salt sensitivity in children is low and rises with increasing age through adulthood,” said Giocomo D. Simonetti, M.D., the study’s lead author.  “During adolescence about 18 percent to 20 percent of adolescents in the general population have the condition. However, in the study, salt sensitivity was present in 37 percent of all low-birth-weight children and in 47 percent of the children in our study who were small for gestational age.”

The researchers studied 50 children, average age 11 years. Fifteen had normal birth weights and 35 had been LBW or SGA babies – either because they were born prematurely or because they were born at full term but were small due to growth restriction inside the womb.

Causes of intrauterine growth restriction include maternal high blood pressure, a pregnancy complication called preeclampsia or maternal smoking during pregnancy, said Simonetti, a fellow in the division of paediatric nephrology at the Children’s Hospital, University of Bern.

In the study, children ate a controlled sodium diet for one week and then a high sodium diet for a week. The researchers measured the participants’ average arterial pressure – a measurement that combines the blood pressure when the heart beats and pressure between beats – during a 24-hour period. Salt sensitivity was defined as a change equal or above 3 millimetres of mercury (mm Hg) in response to the high-salt diet.

Other key findings include:

• Salt sensitivity was inversely correlated to kidney size, i.e., smaller kidney size was associated with higher blood pressure. However, the kidney’s filtering abilities did not correlate to kidney size.
• The kidneys of LBW children tended to be shorter and hold smaller volume compared to those of normal birth weight children when measured by ultrasound, even after correcting for differences in height between the two groups.
• LBW children tended to be shorter in stature than children of normal birth weight.

Though 24-hour blood pressure readings were within the normal range for all groups at baseline, the LBW/SGA children had higher office blood pressures than those whose size was appropriate for gestational age at birth, researchers said.

The salt sensitivity findings are potentially important because blood pressure tends to rise with increasing age. High blood pressure can damage the kidneys, the brain and the heart so controlling blood pressure is essential to the preserving of healthy kidneys and other organs.

Mohaupt said evidence from studies in adults points to a worse long-term outcome for kidney function if the kidney is smaller during childhood or adolescence. Previous studies have reported associations between LBW and both kidney disease and cardiovascular disease in adulthood.

If the findings are confirmed by future research, it could help physicians find a way to protect individuals from high blood pressure in part through dietary changes, Mohaupt said.

“These children should be followed for signs of reduced renal function and also for an elevated blood pressure,” he said. “There’s nearly a 50 percent chance of favourably affecting blood pressure by simply reducing salt intake in children born SGA and nearly a 40 percent chance for those born with LBW. These individuals can be determined very easily if their family physician just gets data on their births.”

(Source: Hypertension: Journal of the American Heart Association: American Heart Association: August 2008)