The federal government system for evaluating calories in foods is flawed, and may be off by as much as 25 percent, nutrition experts say.

The method overestimates the energy provided to the body by proteins, nuts, and foods high in fiber by as much as 25 percent, The New York Times reports.

“The amount of calories a person gets from protein and fiber are overstated,” said Geoffrey Livesey, the head of Independent Nutrition Logic, a nutrition consulting company in Britain, and a nutrition consultant to the United Nations. “This is especially misleading for those on a high-protein, high-fiber diet, or for diabetics” who must limit their intake of carbohydrates.

What this means is an adult aiming to take in 2,000 calories a day on a low-carbohydrate, high-protein diet may actually be consuming several hundred calories less. Interestingly, calorie estimates for junk foods, particularly processed carbohydrates, are more accurate.

The calorie-counting system is based on a method devised in the late 1800s by Wilbur Atwater, a scientist at the Department of Agriculture. Researchers place a portion of food in a device called a calorimeter and burn it to see how much energy it contains.

But the technique is not foolproof for certain hard-to-digest foods — such as nuts —and estimates of the calories they contain are often way off, according to recent research by David Baer, a nutrition scientist at the Department of Agriculture.

An alternate and more accurate system of counting calories has been devised by Livesey and has been presented to the United Nations Food and Agriculture Organization, which provides recommendations to member nations.