Lawrence is 54 years old, and he notices that now he cannot eat as much as he did when he was in his thirties without gaining weight. why is this happening? for every decade past the age of 30, the body's basal metabolism declines by 1% to 2% due to the steady decrease of lean body mass. for every decade past the age of 30, the body's capacity for physical activity declines by 1% to 2% due to the steady decrease of lung function. for every decade past the age of 30, the body's response to the thermic effect of food declines by 1% to 2% due to the increasing internal body temperature. all of the above are reasons why lawrence cannot eat as much in his fifties as he did in his thirties without gaining weight.