The precise model of explanation shifts about every decade, so I'll give you the prevalent model. The standard paradigm is that excess calories are stored as fat. Additional body fat secretes a number of messenger molecules that set up the body for various dysregulations: hypertension, hyperglycemia, hyperlipidemia etc. that in turn create in the long run all the beetus diseases such as diabetes, coronary artery disease including heart attacks, chronic kidney disease, peripheral artery disease, dementia, even some types of cancer, etc. What has become disputed the last 15 years is why people overeat in the first place. The standard paradigm says it's because of overeating (which is a useless dictum and not a scientific explanation). The vegans claim it's because of animal products, the low-carb/keto folks claim it's because of too much (processed) carbohydrates esp. sugar, the clean eaters claim it's because of processing in general, the Paleo people claim it's because of eating foods that we aren't evolutionarilly adapted for. Et cetera. The basic message is: eat real unprocessed food, not too much, a good part of it plants, cook for yourself and your family instead of eating fast food, get a good sleep, exercise, live a life in peace with yourself, your peers and with meaning - maybe even with a mission. Whatever next year's paradigm will be, this will always be true.