Despite what some experts say, all my experience strongly points to the fact that losing fat boils down to energy balance i.e. calories in, calories out. There’s lots of internet discussion to the contrary about how carbs make you fat and how fat makes you skinny, but I completely disagree (I disagree mightily with guys like Gary Taubes); I don’t think any macronutrient is good or bad. I do strongly believe, and have through experience come to understand that certain types of carbohydrates stimulate some combination of physiological and psychological response that creates a desire to eat more of them, but it isn’t the calories in carbs themselves that are creating fatness.
Let’s say I want to eat a doughnut. Let’s also say it’s 400 calories, about all of which comes from flour and sugar. Should I be full? Definitely, because by the time I’m done eating one of my 400 calorie salads with meat and other stuff in it, I’m normally stuffed to my uvula. So then yes, this doughnut will fill me because it’s 400 calories, the same as the salad.
Obviously, this isn’t true (at least not for me); the salad is more filling because it’s bigger and contains a good macronutrient mix and fiber and all that good stuff. Accordingly, it doesn’t stimulate the severe insulin response that the sugar and flour in the doughnut does. Buuuut, they both contain the same energy. The real issue is that if I have the doughnut, I know I’ll have to white-knuckle the ensuing insulin-blood sugar roller coaster. It’ll make me want to eat more, but if I can hang on and not cave into doing that (pounding another doughnut), I’ll have consumed the same number of calories as I would have had I eaten the salad.
I think this pretty clearly illustrates how a calorie both is and isn’t a calorie. Energy wise, a gram of sugar and a gram of protein are the same. Similarly 36 calories from a few tablespoons of oats is the same as the 36 calories in 1/3 tablespoon of lard because each of those 36 calories requires the same amount of energy to be burned. But chemically, the foods (and molecules) with which those calories are associated can elicit wildly different hormonal responses and chemical reactions in the brain. This is directly connected with how we feel after we eat a certain food, like if it makes us feel full or slow or hungry or energetic or some combination of those.
Since my diet is part of my lifestyle, and I’m not into white-knuckling my lifestyle, I’m definitely not into white-knuckling my diet, which is why I typically prefer not to eat sweets and things from boxes and bags (unless they’re carrots). I know how I react to calories from the wrong sources. Although I have great will power, I do my best to never have to exercise it.