Because the “simple” calories out depends on your body. Whether older or younger, individual people have different metabolisms and are also varyingly susceptible to external factors. The hormonal affect of aging can affect nutrient digestion, muscle mass, resting body activity, and a bunch of other things. Even for younger people, there is entire subsets where losing weight is harder because their systems are different and the “calories in” that work for other people with their dimensions may not be lower than their functional “calories out” for their unique body.
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