How does intermittent fasting (16/8) increase metabolism? As per general notion the metabolic rate increases if you eat small meals multiple times a day, how come not eating for 16 hours increases metabolic rate.
I haven't read anywhere that it increases metabolism however I've read of people being concerned that it might slow it down due to lower caloric intake. Two things work with I.F., one is theoretical and the other has historical validity.
The theoretical part is that a person fasting will go longer periods of time not taking in food that might trigger an insulin response and so without insulin the person will use fat as an energy source instead of glucose from carbohydrates and protein. With no other source of energy the body has no other option than to use fat for energy which it does after depleting glycogen stores. Also there are additional benefits from fasting such as autophagy.
Historically I.F. is usually accompanied by reduction of calories and there's plenty of evidence that reduction of calories is an effective means of losing weight and often metabolism is also reduced eventually. The problem with reduction of calories is often people resume eating like before after losing weight, sooner or later, and a few eat more than before which, along with a lower metabolism, is a recipe for weight gain and often more than before.
Typical I.F. a person starts from eating 3 meals and small snacks of around 2000 calories a day or more and go to what I'll call Stage 1 and stop snacking which reduces calories to around 1800 calories a day and once accustomed to that go to Stage 2 of skipping 1 meal, usually breakfast, and so now 1400 to 1500 calories a day, then reducing the eating window to 6 to 8 hours and usually eating fewer calories so now 1200 to 1400 calories a day, then possibly going to O.M.A.D. (one meal a day) and only eating 1000 to 1200 calories a day. The quality of food and eating low glycemic value fruits and veggies is especially important while eating fewer meals a day along with good hydration and adequate electrolytes (minimum RDA of sodium chloride and potassium chloride).
Might seem drastic or impossible but there are many on this system and pleased with the results. It isn't something you do in a matter of days, it can be weeks or even months but once a person reaches that point it should be maintainable for as long as desired and there are additional benefits such as autophagy. It takes awhile to adjust similar to "jet lag" but for a longer period of time. People are accustomed to eating whenever they feel like it so they also become hungry around the same time of day. Supposedly after an hour or 2 the hormone that causes hunger sensation (ghrelin) subsides and doesn't return for a few hours. Some people think the hunger will increase with the passage of time but it tends to do the opposite. After awhile if a person is busy he/she might forget to eat and skip that meal.