caloric intake vs caloric use is literally mathematics
if you consume more calories than your body uses, you will gain weight. yes, some people use different amounts of calories for the same level of activity, and some people absorb calories from eaten food more readily than others, but that doesn't change the equation
if you consume (ie absorb from food) more calories than you expend, you will gain weight over time
if you consume fewer calories than you expend, you will lose weight over time.
it is mathematical reality, it is metaphysical certitude. it's not fucking bro science, it's biology 101
if you fill your car with less petrol every day than you use to drive, your car will eventually run dry
if you fill your car with more petrol every day than you use to drive, your tank will eventually fill up