TOKYO—In a country where grilled fish is a breakfast food and many think the stinkiest soybeans are the tastiest, it stands to reason Coke is now a health drink.

Once a month, Hideaki Iwaya sits at the dinner table for pizza night with his wife and two teenage daughters. They recently revised their routine to add Coca-Cola Plus, which features a government-approved laxative ingredient, hoping it would help their bodies absorb less fat from the slices.

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