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The level of a digestive enzyme increases in babies when their mothers look at them with no expression on their face, causing them stress, according to a new study.

The research arm of Unicharm Corp., a major manufacturer of disposable diapers, said that on such occasions the babies react with a frown and have a high density of amylase in their saliva.

Kiyoshi Miyazawa, chief of Unicharm’s Life Science Research, said the ability to gauge when and to what extent babies feel stress can be useful in child-rearing.

His group conducted tests in cooperation with 11 mothers of babies aged 18 months to 21 months to find how the density of amylase changes according to how the mothers interacted with their babies.

Researchers compared the changes in the density of the digestive enzyme when the mothers smiled at them for a minute and praised them, and then when they gazed at them impassively.

Miyazawa said he started working on changes in amylase in saliva to learn whether knowing babies’ feelings could be reflected in the development of new products.

He used a device for analysis developed by Masaki Yamaguchi, a professor of medical engineering in medical and biological engineering at Iwate University.

Yamaguchi had earlier used the device in tests involving adults and found that the density of amylase in saliva rose in accordance with their level of excitement.

Yamaguchi expressed hope that future research will look at changes in the reactions of infants as they grow up.

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