Think you're taking the healthy option when you choose sushi for your lunch? Think again, according to food writer Trevor Corson.

Researching the history of sushi and its rising popularity in the United States, Corson found the popular belief that sushi is better for you than pizza is not always true.

In fact a sushi takeout box at an American supermarket could easily contain as many calories as two slices of pizza, and the sushi rolls served in restaurants are often worse.

"What we think of as sushi in the United States has become Americanised and that involves more fatty ingredients while the rice tends to be sweet," Corson told Reuters in an interview.

"There is a lot of sugar, carbohydrates and sodium in the rice and then there is the issue with fish - as when you eat fish anywhere - you have to be concerned about mercury."

Exposing the myth about sushi always being healthy was just one of the surprises that Corson came across as he researched his book The Zen of Fish, which combines the history of sushi with the story of American Kate Murray who trains at the California Sushi Academy in Los Angeles.

As he traced the history of sushi in Japan, Corson found that it was not about eating raw fish with the word sushi simply referring to its essential ingredients - rice seasoned with vinegar.

"Sushi began as a way of preserving old fish and street vendors turned it into a crude snack food," said Corson, who lived in Japan for three years and has also written The Secret Life of Lobsters.

In Japan the more "authentic" or high end sushi experience has dropped in popularity alongside the rise of conveyor belt sushi that is cheaper and mass produced for regular people, returning sushi to its proletarian roots.

Sushi arrived in the United States via Los Angeles' Little Tokyo in the 1960s. It was introduced by a man named Noritoshi Kanai who worked for Mutual Trading Co. and had previously tried to introduce snake meat and chocolate covered ants to the United States as he looked for products to sell to Americans.

It has since gone from being an exotic and adventurous novelty to being offered everywhere from restaurants, supermarkets, to even sports arenas - but bears only a passing resemblance to its original Japanese incarnation.

"The meal has gone through a dramatic transformation. The story of sushi is one of endless change," said Corson.

"Sushi is seen as automatically being healthy because we think fish is healthier than beef and seaweed is good for you but it is a very mixed bag and people should be much more aware of what they are eating."