Intermittent Fasting: Eating Windows and Calorie Targets

Intermittent fasting is a timing pattern rather than a food list. The best known forms are time-restricted eating, such as 16:8, where all food falls inside an eight-hour window each day, and 5:2, where intake is cut sharply on two days a week and is normal on the other five. Some people fast for a whole day once or twice a week.

A review in the New England Journal of Medicine describes what is known about these schedules, including how much of the evidence comes from animal studies and short human trials. That is a reason for caution about big promises, not a reason to dismiss the pattern.

Fasting changes when you eat, but calorie balance still decides the direction of your weight. If a shorter window leads you to eat less overall, a deficit follows; if a feast at the end of the window cancels it, no deficit follows. The calorie target is the same whichever clock you use, and the TDEE and deficit calculators produce it.

The intermittent fasting calculator on this site takes that daily target and spreads it across your window or your fasting days. In 5:2, each reduced day has its own small target, and the other five days pick up the rest of the weekly budget.

A short window makes protein and fibre harder to fit, since two big meals leave less room for them than three or four small ones. Choose filling foods with a low energy density, and plan the first meal so it is not the largest bite of the day on an empty stomach.

Drinks are the other trap. Black coffee, tea and water are common during the fast, but sweetened or milky drinks carry calories, so log them rather than treating them as outside the system.