Fast food usually packs a salty punch -- and that's dangerous for kids.

Research by the World Action on Salt and Health found popular fast food chains including McDonald's (MCD), KFC (YUM)and Subway are serving up "worryingly high" amounts of salt in children's meals.

Companies like McDonald's have been trying to focus on healthier food to adapt to changing consumer tastes and boost sales.

But WASH says salt levels need to come down.

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Eight out of 10 meals contained over one gram of salt -- that's more than a child aged between 4 and 6 should eat in one sitting. Kids in that age group shouldn't consume more than 3 grams of salt each day. Younger children need even less. And adults are advised to stay under 6 grams, or roughly one teaspoon, every day.

Too much salt has been linked to high blood pressure, heart disease, strokes and obesity. And loading up on the white stuff early in life can set up bad habits, causing health problems later in life.

"The more salt you eat as a child, the more likely you are to have serious health issues in later life," said WASH chairman Graham MacGregor, who is also professor of cardiovascular medicine at Queen Mary University of London.

"That is why it is vitally important that children do not get used to the taste of salt," MacGregor said.

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The study, which examined fast food in 37 countries, also found the amount of salt in kids meals varies widely around the world.

Take a McDonald's chicken nugget meal. In Turkey, the meal contained 2.4 grams of salt, compared to 1.4 grams in China, and just over a gram in the U.S.

KFC had similar ranges. A child's serving of fries in Canada has 1.9 grams of salt. That dropped to 0.9 grams in Germany, and half a gram in Australia.

And American kids are better off with Subway's turkey breast meal over kids in Germany. In the U.S., it has just under one gram of salt, compared to 1.5 grams in Germany.