Most weight-loss diets encourage people to count calories. And our packaged foods dutifully list calories on their labels.

But where did those numbers actually come from? And can they be trusted?

No, according to an eye-opening article published this week in the London-based magazine New Scientist. In fact, the calorie counts for various foods may be off by as much as 25 percent — enough to undermine any diet and a possible factor in the growing obesity epidemic.

Writes the article’s author, science journalist Bijal Trivedi:

One reason why calorie counts are inaccurate:

By not incorporating this digestive factor, our current method of calorie counting may significantly overestimate the number of calories in foods rich in dietary fiber and protein. According to one expert cited in the article, the energy (calories) derived from dietary fiber could be 25 percent less — and that from protein, 20 percent less — than current estimates.

What this means in real life, says Trivedi, is that the amount of calories listed on the label of, say, a fiber-rich muesli (cereal) bar may be higher than that of a fiber-challenged chocolate brownie, while, in truth, the muesli bar may contain fewer calories.

The brownie is likely packing more calories than stated on its label for two other reasons: its soft texture and its refined sugar and flour. Although not currently considered when determining the calorie content of food, both these factors increase the amount of energy (calories) released by a food in the body, according to recent research.

Cooking "ramps up the caloric potential of food" as well, says Trivedi, by making it easier for the body to break up and absorb the food -- and its energy. She cites a study that found humans digested 90 percent of a cooked egg but only 51 percent of a raw egg.

Don't expect our current system of calorie counting to be overhauled soon, however. The general consensus among nutrition experts, says Trivedi, is that such an overhaul “would require a huge amount of research in both animal models and human volunteers, plus a more complicated labeling system than consumers are used to, for little real public health benefit.”

Still, some scientists would like to see some attempt to correct food labels based on the latest scientific understanding of digestion. As one expert told Trivedi: “The public should be able to apply the science. [And] if you’re not following the science you’re following something else.”

Something to chew over as you count calories this weekend.