Plain and simple, weight loss has nothing to do with calories in and calories out. Those who count calories and centralize their nutrition around it, pay heed these words.
Heres why:
1) It violates the second law of thermodynamics. This isn’t a chemistry lesson, but energy is dissipated when reactions occur. There are lots of reactions that happen when you digest food, so to say a calorie is a calorie isn’t exactly true.
2) Food interacts with your body. For a simple example: fructose vs. glucose. Glucose will cause a mass of reactions to happen in your body, is readily usable and digestible, and will cause shifts in satiety (the feeling of being satisfied or full). Fructose has to be broken down in the liver. Pound for pound, these two sugars are not the same thing.
3) As I mentioned before with respect to thermodynamics, metabolic pathways vary in efficiency. The less efficient the pathway, the more energy is dissipated along the way. Protein takes a hell of a lot of energy to metabolize, where as fat metabolizes fairly well. You can actually take advantage of this: higher protein diets will increase your metabolism.
4) These reactions control how hungry you feel. If you feel full, you’re less likely to over-consume.
5) The gylcemic index discusses spikes in sugar levels in the blood. A more sustained release into your system keeps insulin levels down, which (for most people) keeps the sugar from being converted readily to fat. Those sugars will get used instead of stored. Good stuff.
6) Multitudes of studies have shown that lower carbohydrate diets lead to weight loss, which happens for a multitude of reasons. Really, there is no room for opinion here, over 20 very important studies by reputable universities and scientists have been published.
In the big picture, food interactions are not as straight forward as saying «calories in vs calories out.» This is actually completely false for the above reasons — and more! But to keep it simple, there are many other things to focus on. Primarily, rules of thumb should be: keep away from anything processed, avoid sugar like the plague (you can have fruit in moderation), eat meat, and keep away from too many carbs (particularly grains).
