The short version of that answer is yes and no. I’ll explain why. Yes, you become more efficient at burning fat. Now, why that happens, I’ll explain it. Each macronutrient oxidizes or burns differently than the other macronutrients. So you have fat, carbohydrate, protein, alcohol’s technically a macro, depending on who you ask, but typically fat is easily converted to fat. It doesn’t have to go through any alternate pathways to become stored body fat.
Now, when you start overfeeding these macronutrients, your body responds to that in different ways. They’ve done studies on individuals what are following a mixed diet, so carb, fat, proteins pretty even. Nothing’s extremely high or extremely low, just a mixed diet. They’ve basically overfed them carbs and overfed them fat and looked at the responses. The classic study I like to cite is Tracy Horton’s study. I believe it was, I want to say it was done in the ’90s, maybe early 2000s. I don’t have the exact date. They basically found that when you overfeed carbs to somebody on a mixed diet, they tend to burn more carbs. They see an increase in carbohydrate oxidation, which means you’re burning more carbs because you’re eating more carbs. To put that simply, the more carbs you eat, the more carbs you burn. They stimulate their own oxidation. Just simply eating more of them makes you burn more of them.
But then something else happens. As you eat more carbs, you start burning less fat because you have to burn off those carbs. What they also looked at was the proportion of those calories that stored as fat. So basically what happens is all the fat that you’re eating gets stored as fat because you’re burning off the carbs.