How can you store fat if you are in a calorie deficit? If you burn more than you take in, what is your body using to make fat; does it just turn muscle into fat? Doesn't make sense to me.
"A calorie is a calorie"
The old school of nutrition, which often includes most nutritionists, is a calorie is a calorie when it comes to gaining or losing weight. That weight loss or weight gain is strictly a matter of "calories in, calories out." Translated, if you "burn" more calories than you take in, you will lose weight regardless of the calorie source and if you eat more calories than you burn off each day, you will gain weight, regardless of the calorie source.
This long held and accepted view of nutrition is based on the fact that protein and carbs contain approx 4 calories per gram and fat approximately 9 calories per gram and the source of those calories matters not. They base this on the many studies that finds if one reduces calories by X number each day, weight loss is the result and so it goes if you add X number of calories above what you use each day for gaining weight.
However, the "calories in calories out" mantra fails to take into account modern research that finds that fats, carbs, and proteins have very different effects on the metabolism via countless pathways, such as their effects on hormones (e.g., insulin, leptin, glucagon, etc), effects on hunger and appetite, thermic effects (heat production), effects on uncoupling proteins (UCPs), and 1000 other effects that could be mentioned.
Even worse, this school of thought fails to take into account the fact that even within a macro nutrient, they too can have different effects on metabolism. This school of thought ignores the ever mounting volume of studies that have found diets with different macro nutrient ratios with identical calorie intakes have different effects on body composition, cholesterol levels, oxidative stress, etc. Translated, not only is the mantra "a calorie us a calorie" proven to be false, "all fats are created equal" or "protein is protein" is also incorrect. For example, we now know different fats (e.g. fish oils vs. saturated fats) have vastly different effects on metabolism and health in general, as we now know different carbohydrates have their own effects (e.g. high GI vs. low GI), as we know different proteins can have unique effects.
