Intermittent fasting: 16:8, 14:10 and 5:2 explained

Intermittent fasting means limiting when you eat rather than what you eat. For some people it is an easy way to eat less; for others it leads to evening overeating. Here is how the popular schedules work and what the evidence actually says.

The main schedules

16:8

You fast for 16 hours and eat within an 8-hour window — for example, from noon to 8 pm. In practice that usually means skipping breakfast and late-night snacks. It is the most popular version of time-restricted eating.

14:10

A gentler version: a 10-hour eating window, such as 8 am to 6 pm or 9 am to 7 pm. For many people this is simply “stop eating after dinner”, and it is a sensible place to start.

5:2

You eat normally five days a week and cut right down on two non-consecutive days — typically to about 500 kcal for women and 600 kcal for men. The low days are usually one or two small, protein-rich meals with vegetables.

Other variants exist — alternate-day fasting, one meal a day (OMAD), 24-hour fasts — but they are harder to sustain and carry more risk of overeating afterwards.

Why intermittent fasting works: the calorie deficit

Intermittent fasting does not burn fat by some special mechanism. It helps with weight loss because a shorter eating window or two low days a week usually mean eating fewer calories overall — fewer snacks, no late-night grazing, one meal fewer. If you eat as much in 8 hours as you used to in 14, your weight will not change.

Randomized trials comparing time-restricted eating or 5:2 with ordinary daily calorie restriction mostly find similar weight loss when total calories are the same. So the question is not whether fasting is better, but whether it suits you. If you are not sure what a deficit is, start with what a calorie deficit is.

Possible upsides

• Simple rules. “Nothing after 7 pm” can be easier to follow than counting every snack.
• Fewer eating occasions — useful if most of your extra calories come from evening snacking.
• Bigger meals within the window, which some people find more satisfying.

Claims about detox, “resetting” metabolism or dramatically boosted fat burning are not supported by good evidence in people.

Possible downsides

• Hunger, irritability, headaches or trouble concentrating, especially in the first weeks.
• Overeating when the window opens, which can cancel out the deficit.
• Harder to get enough protein in two meals — aim for 30–40 g per meal.
• A late eating window can clash with family meals, social life or sleep.
• For some people, rigid food rules feed an unhealthy relationship with eating.

Who should not do intermittent fasting

Do not fast, or only do it with your doctor's agreement, if you:

• are pregnant, breastfeeding or trying to conceive;
• are under 18;
• have or have had an eating disorder;
• have diabetes, especially if you take insulin or other medication that can cause low blood sugar;
• are underweight, frail or older with poor appetite;
• take medication that must be taken with food;
• have heart, kidney or other chronic conditions.

How to start sensibly

1. Start with 12:12 or 14:10, not 16:8 or longer fasts. Extend the window only if it feels easy.
2. Keep drinking. Water, plain tea and black coffee are generally fine during the fast; sweetened drinks and milky coffees break it.
3. Eat proper meals in your window: protein, vegetables, whole grains and some fat — not just whatever is quickest.
4. Track your total intake for a couple of weeks. If your weight is not moving, the window has probably not reduced your calories.
5. Stop if you feel unwell, start bingeing or obsessing about food. Plenty of people lose weight without fasting at all — see how to lose weight without starving.

Fasting and calorie tracking together

Temfy Nutri does not have a fasting timer. What it can show is the thing that actually decides your results: total calories and protein for the day against your target. Log your meals as usual — a photo of the plate is enough for an AI estimate you can correct — and after a week or two compare your intake and weight trend in Analytics and the weight chart. If your weight has stalled, the reasons are usually the same as on any diet: see weight loss plateau.