Intermittent Fasting for Weight Loss — Methods, Results and Risks
Intermittent fasting is not a diet in the usual sense — it is a schedule. It does not tell you what to eat; it tells you when. The idea is simple: by restricting eating to certain hours or days, you eat less overall, and the calorie deficit that follows produces weight loss. The evidence for that is real, but it is also narrower than the claims made for it. This guide covers the main methods, what the research actually shows, who should not try it, and how to do it safely if you decide to.
The three most-studied methods are 16:8, 5:2, and alternate-day fasting. In 16:8, you fast for 16 hours and eat within an 8-hour window — for example, skipping breakfast and eating between noon and 8 pm. In 5:2, you eat normally five days a week and restrict to about 500–600 kcal on two non-consecutive days. In alternate-day fasting, you alternate a normal day with a very low-calorie day. All three produce weight loss when they produce a calorie deficit. None of them has been shown to produce more fat loss than ordinary calorie restriction when calories are matched — which is the most important fact in this whole guide.
Use the calculators for your own numbers
Work out your maintenance calories with the TDEE Calculator and set your target with the Weight Loss Calculator — whatever schedule you choose, the deficit is what matters. If you train, use the Protein Calculator to make sure your eating window contains enough protein. The worked example below uses a 85 kg man, 178 cm, age 38, moderately active, to show what a 16:8 day looks like in calories and protein.
Related reading: Calorie Deficit Explained — How It Works and How Big It Should Be, How Fast Should You Lose Weight? Safe Rates, Explained, What Are Macros?, Weight Loss Calculator, Protein Calculator, full guides section
The formula, shown and cited
Fasting does not change the energy maths. Using Mifflin-St Jeor (Mifflin et al., American Journal of Clinical Nutrition, 1990) for our example — a man, 85 kg, 178 cm, age 38:
BMR = 10 × 85 + 6.25 × 178 − 5 × 38 + 5 = 850 + 1,112.5 − 190 + 5 = 1,777.5 kcal a day
At moderately active (1.55):
TDEE = 1,778 × 1.55 = 2,756 kcal a day
A 500 kcal deficit means eating about 2,256 kcal a day. On 16:8, that is 2,256 kcal inside an 8-hour window — roughly two large meals and a snack. At 1.6–2.2 g/kg while dieting (Morton et al., 2018), the protein target is 136–187 g a day, which has to fit into the same window. That is the practical challenge of fasting: not the hunger, but the logistics of eating enough protein in fewer meals.
| Method | Fasting pattern | Typical weekly deficit | Who it suits |
|---|---|---|---|
| 16:8 | 16 hours fast, 8 hours eating, daily | Depends on intake | People who skip breakfast anyway |
| 5:2 | 2 days at 500–600 kcal, 5 days normal | ~3,000–4,000 kcal | People who prefer structure to daily restraint |
| Alternate-day | Alternate normal and ~500 kcal days | ~4,000–5,000 kcal | Rarely — hard to sustain |
| OMAD (one meal a day) | 23 hours fast, 1 hour eating | Varies widely | Very few people; hard to get enough protein |
Randomised trials comparing intermittent fasting with ordinary calorie restriction, matched for calories, have consistently found similar weight loss — typically 3–8% of body weight over 3–12 months. Fasting is not a shortcut around the deficit; it is one way of creating it. Some people find it easier to skip a meal than to eat smaller meals all day. Others find it miserable. The method that works is the one you can sustain.
How to do 16:8 safely
Start gently: a 12-hour overnight fast (for example, 8 pm to 8 am) is already what most people do without thinking about it, and extending it to 14 or 16 hours is a small step. Drink water, black coffee and unsweetened tea during the fast — they do not break it in any meaningful sense, and they make it far more comfortable. Break the fast with a proper meal containing protein, not with a snack that leaves you hungry an hour later. Keep protein high across the eating window, and keep resistance training in your week, because fasting days make it easier to under-eat protein and lose muscle along with fat.
There are people who should not fast, or should only do so with medical advice: anyone with diabetes, especially on medication that can cause low blood sugar; anyone with a history of eating disorders; pregnant or breastfeeding women; children and teenagers; people who are underweight; and anyone on medication that must be taken with food. If any of those apply to you, do not start a fasting schedule without talking to a clinician first. The safety floors still apply: no target below 1,200 kcal a day for women or 1,500 kcal a day for men without medical supervision, whatever the schedule.
What fasting does NOT do
The most common overclaims about fasting are that it "detoxes" the body, that it burns fat preferentially, or that it raises metabolism. None of those holds up well. The liver and kidneys handle detoxification continuously, and fasting does not improve on them. Fat loss from fasting is the same fat loss as from any deficit — the body does not know or care what the clock said. And prolonged fasting, if it leads to muscle loss, lowers metabolic rate rather than raising it. There is genuine research interest in fasting and cellular repair processes such as autophagy, but the human evidence is early and mostly from short studies, and it is not a reason to choose fasting over a simpler diet that you will actually follow.
Frequently asked questions
Does intermittent fasting work better than counting calories?
No — when calories are matched, randomised trials find similar weight loss from fasting and from ordinary calorie restriction. Fasting works when it helps you eat less overall; it does not work by a separate mechanism. Choose it if the schedule suits your life, not because it promises extra fat loss.
What can I drink during a fast?
Water, black coffee and unsweetened tea. These contain essentially no calories and do not meaningfully break a fast. Avoid anything with calories — including milk in coffee, juice, and sugary drinks — and be cautious with artificial sweeteners, which some people find increase hunger even though they add no calories.
Will I lose muscle with intermittent fasting?
You can, if protein is too low or if you stop training. At 1.6–2.2 g/kg of protein a day and two or three resistance sessions a week, muscle loss during a fast is similar to muscle loss during any other diet. The risk is practical: fitting 150 g of protein into an 8-hour window takes planning, and people who eat one small meal a day often fall well short.
Is 16:8 safe for women?
For most healthy women, yes, but women report more difficulty with fasting than men in some studies, including greater hunger and more disruption to sleep and mood. Women with irregular cycles, low energy availability, or a history of disordered eating should be especially cautious and should talk to a clinician before starting. There is no evidence that women need a different fasting schedule — only that they should pay closer attention to how they feel.
Can I exercise while fasting?
Yes, and many people train in a fasted state without problems, especially for moderate exercise. For hard sessions, performance is usually better if you have eaten beforehand, so consider scheduling intense training inside your eating window. If you feel dizzy, nauseous or unusually weak while training fasted, stop, eat, and adjust the schedule.
How long should I try intermittent fasting before deciding if it works?
Give it four weeks. Track your weight daily and look at the weekly average, track your intake honestly, and note your energy, sleep and training performance. If the average is falling at 0.5–1% of body weight a week and you feel well, it is working. If you are miserable, constantly hungry, or losing strength, the schedule is not for you — and an ordinary calorie deficit will work just as well.
Estimates only, not medical advice. Minimum safe intakes: 1,200 kcal/day women, 1,500 kcal/day men — consult a healthcare professional before a very-low-calorie diet.