Determined at Birth? Kidney makeup may set hypertension risk

In the kidneys, clusters of capillaries and tubes called nephrons filter gallons of blood every day and direct the impurities to the urine. According to a current theory, people lacking a full complement of nephrons are at increased risk of developing high blood pressure.

A study from Germany now finds hard evidence for this phenomenon and suggests that the number of nephrons in each person’s kidney is set at birth.

The theory linking nephron number and blood pressure has long been championed by Barry M. Brenner of Harvard Medical School and Brigham and Women’s Hospital in Boston. He holds that when nephrons are in short supply, and therefore overworked, there’s a release of hormones and retention of sodium by the kidneys. Both contribute to hypertension. Since Brenner first proposed his theory in 1988, research in animals has found that a shortage of nephrons correlates with high blood pressure.

In the study reported in the Jan. 9 New England Journal of Medicine, researchers compared kidneys taken from people between the ages of 35 and 59 who had died in accidents. The scientists examined 10 kidneys from people who had had hypertension and 10 from similar people who had had normal blood pressure. By counting nephrons in uniform-size slices of kidney tissue from each cadaver, the scientists found that the average number of nephrons in the people with high blood pressure was fewer than half that in the people with normal blood pressure.

The scientists also sought to find damaged nephrons or ones that had fallen out of use. “We looked very hard . . . but did not find evidence that hypertensive patients had lost nephrons over time,” says study coauthor Kerstin Amann, a pathologist at the University of Erlangen-Nürnberg in Erlangen.

This result suggests that a person is endowed with a set number of nephrons at birth, Brenner says.

Amann says that she and her colleagues designed the study “to disprove or prove” Brenner’s hypothesized link between hypertension and nephron count. “We were astonished by the results,” she says.

Although the cause of high blood pressure isn’t completely understood, scientists have noted that low-birth-weight babies are more likely to develop health problems–including high blood pressure–later in life than normal-birth-weight babies are, Brenner notes. Low birth weight has been associated with low nephron number in rats, and there’s indirect evidence of the link in people, he says.

Scientists have also shown that restricting protein intake by pregnant rats yields offspring with fewer nephrons in their kidneys and more subsequent hypertension (SN: 12/09/00, p. 382:

Such findings suggest that some cases of high blood pressure can be prevented by good prenatal nutrition that boosts a newborn’s nephron supply, Brenner says.

What’s more, the new findings could have implications for organ transplantation.

Screening of kidney donors to reveal which ones began life as low-birth-weight newborns might indicate who would provide a kidney that’s short on nephrons, Brenner says.

Also, he says, the study suggests that physicians should carefully watch kidney donors’ long-term health, since each one has lost half of his or her original nephron endowment.


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