Consumers need to focus on the amount of calories in food, not just on the level of added sugar, a leading US nutritionist has told just-food.

The level of added sugar has come into the spotlight in the US after the American Heart Association (AHA) urged consumers there to lower their consumption.

Professor Joanne Lupton of Texas A&M University agreed that added sugar contributes to obesity but said the "real issue" was also calories from other ingredients, not just sugar.

"The real issue with added sugar is that it only provides calories, not vitamins, minerals or phytochemicals," Professor Lupton told just-food.

"This means that as you increase the amount of "added sugars" in the diet one of two things (or both) will happen - either you take in more calories than you need and thus you gain weight, or you attempt to hold calories constant and thus you dilute your diet with 'empty calories' - and at a certain point you are consuming too little vitamins, minerals, and phytochemicals," she said.

"Thus I would substitute 'discretionary calories' or 'empty calories' for 'added sugar' since in my mind the sugar is irrelevant. It's the calories that are the issue."

The AHA said US consumers were eating too much added sugar and set a limit of no more than 150 calories of added sugar a day for men - and 100 calories for women.

The association cited a survey that measured US eating habits between 2001 and 2004 that said the average American consumed 355 calories of added sugar a day.

Professor Lupton, however, insisted the AHA report was "targeting the wrong thing". She said: "The recommendation should be to maintain a healthy weight by selecting nutrient dense foods and maintaining an active lifestyle without going over one's energy requirement.  The best test to see if you are accomplishing the healthy weight advice is to get on a scale," she said.

Professor Lupton, who helped put together the 2005 Dietary Guidelines in the US, said US food manufacturers were lowering the added sugar in "many products", while consumers were armed with an "increasing amount of information" that helped them identify added sugar on food labels.

The next set of dietary guidelines are due next year and Professor Lupton said it would be interesting to see what the new criteria said on added sugar.

She warned: "Since the issue with added sugars is calories, not the sugar itself, it is important for consumers to look at the calorie level of comparable products and not just immediately pick the one with lower added sugars. Sometimes fat is substituted for sugar in a product, and if it is higher in fat and the same or higher in calories it would not be a good choice."