Japan manages a rare feat for a developed country when it comes to feeding its children -- high scores for nutrition but very low obesity rates. One major key? School lunches.

A landmark report by the U.N.'s children agency UNICEF released Tuesday shows Japan topping the charts for childhood health indicators, with low rates of infant mortality and few underweight children.

But it also manages the lowest incidence of childhood obesity among the 41 developed countries in the Organization for Economic Cooperation and Development and European Union.

Experts say there are various factors at work, including a health-conscious society and regularly mandated check-ups for children, but a nationwide school lunch program also plays a key role.

"School lunches with menus that are created by nutritionists are provided to all primary schools and the majority of junior high schools throughout Japan," Mitsuhiko Hara, a pediatrician and professor at Tokyo Kasei Gakuin University, told AFP.

The lunches are mandatory -- no packed lunches allowed -- and while they are not free for most, they are heavily subsidised.

Each meal is designed to have around 600-700 calories balanced between carbohydrates, meat or fish and vegetables.

One sample meal served to children in Japan's Gunma gives a flavor: rice with grilled fish and a spinach and sprout dish, served with miso soup with pork, alongside milk and dry prunes.