A team of Japanese researchers found that social loneliness decreases the levels of oxytocin, known as the "happy" or "love" hormone, in mice, resulting in lipid metabolism disorders in the liver and worsening arteriosclerosis.

The team from Keio University and others said that loneliness without social connections contributes to the development of mental illness, obesity and Type 2 diabetes.

Clinical studies conducted on humans have shown that loneliness increases an incidence of myocardial infarction. But the detailed mechanism of how that happens had not been fully understood.

Atsushi Anzai, a lecturer at the university, and his colleagues divided sibling mice born from the same mothers into groups of four or five, then observed them for 12 weeks.

They analyzed the risk of developing arteriosclerosis and lipid metabolism disorders, as well as the effect of oxytocin on the liver. Oxytocin, produced in the hypothalamus in the brain and released into the bloodstream, not only facilitates childbirth but also affects emotions.

The team found that mice kept alone secreted less oxytocin than those in groups, suffering increases in neutral fat and bad cholesterol levels in the blood and progression of arteriosclerosis.

It also discovered that oxytocin regulates lipid metabolism in the liver and helps release bad cholesterol into the intestine.

The government stepped up measures against the problem of social loneliness during the COVID-19 pandemic as social interactions decreased in addition to an increasing number of single-person households.

"It's been reported that people with little physical contact with others have lower levels of oxytocin. Enriching social connections is important to prevent dyslipidemia, which causes arteriosclerosis," Anzai said.

The team's study was published in the U.S. journal Circulation Research.