Not everyone is going to respond to calories equally. This sounds obvious but I think most people don’t really take this into consideration. Overweight people will generally respond better to a high fat/low carbohydrate diet, where lean people can generally get away with more carbs. A person with a higher percentage of body fat is gonna shove carbohydrates to the liver and convert to fat, where a person with a lower percentage of body fat is gonna store carbs in muscle tissue.
Knowing your ancestry is important. Knowing what kind of mutations you might have and what your ancestors were likely to have eaten. A great tool for this is 23 & Me. Everything from starch digestion response/how you may handle dairy to seeing if your ancestors had a high percentage of folate leading to more of a plant based diet.
Digestibilty vs Nutrient Density. Certain foods may be healthy for you, but if you can’t absorb the nutrients it’s kind of a waste of time. So make sure foods are nutrient dense but also digestible (sprouting, soaking, fermenting, washing/rinsing).
You eat too much and too often. Restricted eating and fasting can shift you into a higher level of fat burning and fat oxidation.