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Japanese Moms Who Stay Thin During Pregnancy Cause Kidney Problems in Babies, Study Finds

The Japanese are considered to be among the healthiest in the world, but kidney failure caused by hypertension or high blood pressure has been a major concern for many people in Japan.

According to researchers, low birth weight babies caused by pregnant Japanese women trying to stay thin may have to do with the country’s kidney problems.

Scientists Monash University’s Biomedicine Discovery Institute in Australia and the Jikei University School of Medicine in Tokyo specifically identified the problem to be nephrons — the structural unit in kidneys that regulates the concentration of water and soluble substances such as sodium salts in the body.

Jikei University School of Medicine (Photo via Flickr / Takayuki Miki (三木貴幸) (CC BY-SA 2.0))

The findings, published in the Journal of Clinical Investigation Insight, showed that the Japanese have generally fewer nephrons than normal, which in itself, has broad health implications.

Japanese people without hypertension or kidney disease had 640,000 nephrons, a far cry from the European average of 900,000 to one million. For Japanese patients with hypertension, the number is at around 392,000, while the figure further slides down to 268,000 for those with chronic kidney disease.

“In the present study, 9 Japanese hypertensive subjects with a mean age of 68 years had 37 percent fewer nephrons than 9 normotensive subjects with a mean age of 64 years,” the paper noted.

This is particularly significant since one-eighth of the population suffers from chronic kidney disease in the country, the second-highest rate in the world.

Nephron expert and lead researcher John Bertram explained that scientists have hypothesized a lower nephron count could be responsible for hypertension since the 1980s due to the role of the kidney itself, dictating the amount of sodium and renin in the bloodstream.

Bertram noted that their research, which involved the analysis of 59 kidneys from 59 individual patients collected via autopsies, established a clear correlation between nephron count and hypertension.

3D reconstruction of nephrons captured with a 63X objective on a Nikon Confocal A1R microscope (Photo via Flickr / eLife – the journal (CC BY-SA 2.0))

“This paper, the first such study in an Asian population, shows that Japanese with hypertension have significantly fewer nephrons than normotensive Japanese do – it’s a clear-cut finding,” Bertram said.

Researcher Go Kanzaki explained how low birth weight babies and kidney problems could actually be connected.

“There is a trend towards Japanese women staying thin and small during pregnancy to try to look beautiful but their babies are more likely to be born smaller and with smaller kidneys and therefore less nephrons—the number of nephrons is set at birth,” he said.

A separate study, published in Journal of Clinical Medicine Research in 2016, further posits that pregnant Japanese women’s “strong desire to be thin” has indeed contributed to a steady decrease in the average birth weight in Japan over the last three decades.

It is important to point out, however, that such a trend is not necessarily caused by women’s vanity alone.

“Japan recommends stricter limiting of weight gain during pregnancy, compared to other developed countries,” Japanese researchers pointed out in another report published in Scientific Reports.

“In this context, pregnant Japanese women are expected to weigh themselves at every perinatal check-up … Moreover, the majority of Japanese women believe that carrying a smaller baby would ensure a smooth delivery, which can lead them to avoid extra weight gain during their pregnancy.”

Bertram and his team explained that a person’s low nephron count could help identify potential hypertension or CKD, however, current methods used to count nephrons rely mainly on autopsies.

“Ultimately, you would hope that health professionals will think more about low birth weight although the idea’s not there yet,” explained Bertram.


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