Starting intermittent fasting: what to know on day one
Start at 12 hours and work up to 14 and then 16 over two or three weeks. During the fast, water, tea and black coffee are fine. IF works because it makes you eat less overall, not because the timing is magic — trials matching calories find no difference. Pregnant and breastfeeding women, anyone with a history of disordered eating, and anyone on diabetes medication should not start without medical advice.
IF means restricting when you eat rather than what. That simplicity is why people like it, and why it genuinely works for a lot of them.
How many kinds are there, and which should you pick?
- 16/8 — fast 16 hours, eat within 8. The most common by far, because it barely changes your schedule: skip breakfast or eat dinner earlier.
- 5:2 — eat normally five days, restrict heavily on two. Suits people who would rather not think about a clock every day.
- Alternate-day — the hardest to sustain and the one people abandon most.
For a beginner, 16/8 is the sensible starting point.
How many hours on day one?
Twelve. Most people already do twelve without noticing — dinner at eight, breakfast at eight. Then move to 14 in the second week and 16 in the third.
You can jump straight to 16, but the first week will be harder than it needs to be, and that is the most common reason people quit.
What can you drink while fasting?
Water, unsweetened tea and black coffee. None of them carries enough energy to change anything.
Milk, creamer, juice and anything sweetened end the fast. Diet drinks and sugar-free gum are a grey area — in energy terms they are irrelevant, but if part of your aim is to stop reaching for sweetness, that is your call to make.
What does the first week feel like?
Hungry in waves, some headaches, poorer concentration in the afternoon, and shorter temper than usual. Most of it settles within 7 to 10 days as the body adjusts.
The headaches usually come from water and salt rather than blood sugar. Drink enough, and do not cut salt in the same week you start — that alone helps a great deal.
Does IF beat counting calories?
On the trial evidence, no. TREAT (Lowe 2020) followed 116 people for 12 weeks and found the 16/8 group lost 0.94 kg against 0.68 kg in controls — not a significant difference — with a higher share of lean mass in the loss than expected. A larger study in the New England Journal of Medicine (Liu 2022) followed 139 people for 12 months comparing time-restricted eating plus calorie restriction against calorie restriction alone, and found no difference.
The honest conclusion is that IF is a tool for eating less without counting, which for many people is a tool they can actually keep using. That is worth a great deal. But it is not magic, and if the eating window becomes the window in which everything skipped gets made up, the result is zero.
Who should not do it?
Pregnant and breastfeeding women. Anyone with a history of disordered eating, because time restriction can reactivate old patterns. Children and adolescents who are still growing. And anyone taking diabetes medication — particularly insulin or sulfonylureas — where the risk of hypoglycaemia means the medication has to be adjusted with a doctor first, not reported afterwards.
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