The body uses different levels of energy to digest foods (thermic effect), as well as certain foods only being partially digested. The fact that the degree to which we chew some foods (e.g. nuts and seeds) can affect caloric absorption means that we will never have a truly "accurate" caloric reading. Your basal metabolic rate is severely affected by this.
Also, to Pooch's point, it should be noted that foods are not merely raw calories with nutrients along for the ride. Our metabolism of various macronutrients is tailored to how our body uses them, and their effect on our body's complicated interaction of systems is likewise varied. Not only does it require more energy to digest protein, but protein tends to be more satiating than carbohydrate. Our digestive system, and how it interacts with our brains, our endocrine system, and our other vital organs, is far more complex than an energy-burning calorimeter.
Yes, caloric consumption in excess of metabolic needs leads to weight gain; the opposite leads to weight loss. But there is much more to it than just running a debit and credit spreadsheet. How do you account for BMR changes when caloric restriction becomes the norm? Looking up BMR on a chart is no longer going to be utile. I may expend 500 kcal jogging; a man of equal body weight and rate and duration of exertion may expend 400. Etc.
Also, to Pooch's point, it should be noted that foods are not merely raw calories with nutrients along for the ride. Our metabolism of various macronutrients is tailored to how our body uses them, and their effect on our body's complicated interaction of systems is likewise varied. Not only does it require more energy to digest protein, but protein tends to be more satiating than carbohydrate. Our digestive system, and how it interacts with our brains, our endocrine system, and our other vital organs, is far more complex than an energy-burning calorimeter.
Yes, caloric consumption in excess of metabolic needs leads to weight gain; the opposite leads to weight loss. But there is much more to it than just running a debit and credit spreadsheet. How do you account for BMR changes when caloric restriction becomes the norm? Looking up BMR on a chart is no longer going to be utile. I may expend 500 kcal jogging; a man of equal body weight and rate and duration of exertion may expend 400. Etc.