The challenge of accurate calorie counting
Source: Scientific American
Counting calories is not as simple as scanning a food label. Multiple factors affect the ways in which our bodies receive calories from food, including the structure of the food itself (such as a vegetable's cell walls), variances in digestive process (some complex foods such as almonds require our bodies to do more "work" than other, less complex foods such as honey) and the fact that certain foods engage the body's immune system in order to combat pathogens, a process that involves calories.
How a food is processed via cooking or grinding also affects calories available from the food. For instance, studies found that cooked sweet potatoes have more calories available for digestion than raw potatoes. This suggests that the more processed a food product, the more accurate the estimated calories per serving on the label will be.
In addition to these considerations, bodies process foods differently due to numerous factors, including the of length of one's intestine, enzyme production, and the presence of bodily microbes.
Modern diets have maximized the number of calories available at each meal. From an evolutionary stand point this is a good thing, but in a contemporary setting this contributes to a high amount of readily available, low-quality calories, leading to individual weight gain and rising obesity rates. Weight loss strategies that focus primarily on calorie counting are not likely to adequately factor in these many calorie-related considerations.