A tiny saltshaker symbol that warns certain meals are high in sodium will start to appear on Tuesday on menus in chain restaurants in New York City, the first US city to take the step in an effort to combat heart disease and stroke.

Any menu item containing more than 2,300 milligrams of sodium, the daily limit many nutritionists recommend and which equals about one teaspoon of salt, must display the emblem of a salt shaker in a black triangle.

The measure unanimously approved by the New York City Board of Health in September applies only to restaurants with at least 15 establishments across the US, and concession stands at some movie theaters and sports stadiums.