Photo/Illutration A white trevally from the experiment (Provided by Nagasaki University)

NAGASAKI--Diet books often warn readers: “Eat at night, and you will gain weight.”

That principle is exactly what a researcher in the prefectural capital is hoping to turn into an advantage by drawing on it to raise fatty fish in short periods of time. 

Katsuya Hirasaka, a professor at the Nagasaki University Faculty of Fisheries who studies chrono-nutrition, is behind the unconventional study. 

Hirasaka, 46, explained that chrono-nutrition is a new scientific discipline that focuses on the timing of when food is eaten, unlike the conventional viewpoint of nutrition studies that emphasize “what” is ingested and “how much” of it is consumed.

In humans, many athletes consume protein at breakfast since doing so helps build muscle mass. Hirasaka is attempting, in a drastic change of perspective, to apply the same approach to fish.

He began demonstration tests for his idea in earnest in 2022 at the Nagasaki Kakuyo High School's fisheries training facility in the city; this particular location was ideal because Hirasaka needed a space that could house large tanks of seawater. 

Students also had a chance to get hands-on experience, and 11 members of the high school's fisheries club were tasked with feeding the fish and measuring their lengths and weights.

The high schoolers said the experience of participating in university-based research was academically stimulating.

“The study was fun and it gave me the feel of what it would be like to be in a university,” said Kai Yamaguchi, a third-year student who is 18. “I had a good time working on it. I hope, if it is somehow possible, to continue doing research with Mr. Hirasaka at Nagasaki University.”

In 2023, Hirasaka compared a shoal of white trevally, commonly known as striped jack, fed at 8 a.m. with another shoal fed at 7 p.m.

After around a month, fish from the school fed at night had 1.7 times more neutral fat in their blood than those with a morning feeding time. The former also had about 1.2 times more neutral fat in the muscles than the latter.

“Fish from the shoal fed at night taste so good,” Hirasaka said. “They have that flavor characteristic of fish with plenty of fat on them.” 

Analysis of variations in enzyme levels in fish muscles and other inspections indicate that white trevally likely have an approximately 24-hour biological clock that is activated by morning light.

Hirasaka believes the increase in neutral fat is likely due to a metabolic disorder caused by the irregular night meals dampening the rhythm of the fishes' biological clocks.

Separately, Nagasaki University is promoting a “Japan Buri” project about breeding yellowtail for overseas export.

Perhaps, with further development, Hirasaka's study could help in cultivating fattier versions of this species endemic to the seas surrounding Japan.

“Our approach, which draws only on a natural occurrence, could end up more acceptable to consumers compared to breeding methods that rely on genome editing or drugs,” the professor said.

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