The underlying question seems to be this; is slowing down one’s metabolism beneficial or not?
Everyone has their own personal basal metabolic rate. This is the amount of energy required by our body when completely at rest and doing nothing. The lower the basal metabolic rate the less we have to eat to provide our resting life requirements.
Eating requires that the food is processed to convert it to energy to fuel our cells, trillions of them. This processing involves every cell but especially those directly involved like, stomach, intestinal, duodenum, colon, pancreas, liver, kidney, lymph, blood, adipose, nervous system cells and more. Without going into the chemistry of cellular metabolism one can simply state a basic principle of physics. The more work done by a machine the more ware and tare will accrue to it, and the shorter it’s useful life will be. Our cells are biological machines and the less work they have to do the longer they will last.
In general it is safe to say the lower ones resting energy consumption (basal metabolic rate) the longer the life of their cells before they die or are killed by the immune system and require replacement, or become permanently poor functioning (senescent) aged cells. In other words it is likely that the lower resting energy requirement the lower the rate at which we age. Without question the biggest risk factor associated with nasty chronic illnesses like heart attack, cancer, alzheimer’s etc is our age. Not our actual ‘birthday’ age but our ‘biological’ age.
Generally speaking eating just enough to maintain a healthy body weight and get adequate nutrition (vitamins and minerals) will lead to achieving a lower metabolic rate and improve the chances of being in good health for longer.
Anything that works for an individual to permanently maintain and or achieve a healthy lean body weight, whether that be intermittent fasting, eating less, or exercise, will likely reduce that person’s resting metabolic rate and improve their chances of living healthier for longer.