There's always someone in your circle giving it a go. A friend who skips breakfast and doesn't eat until midday, a colleague announcing a 'juice week', an uncle convinced that three days on plain water transformed him. So, fasting: good or bad idea? In practice, it depends on your health, how long you plan to fast and what support you have. Here's what the current evidence suggests, without drama or hype.

What fasting really changes in your body

The principle is simple. During a set window, you don't eat, or you greatly reduce your intake. With no external fuel coming in, your body draws on its reserves: first glycogen stored in the liver and muscles, then fat. This switch explains the weight loss in the first few days — a lot of water and glycogen, relatively little fat. When you return to normal eating, some of that weight comes back.

Young mixed-race woman filling a glass of water in a bright kitchen, with no food visible

The idea of a grand internal clean-out deserves to be put back in its place. Your body already clears waste continuously, via the liver, kidneys, lungs, skin and gut. No eating pattern replaces these organs, and no claim that fasting 'purifies' the body has been approved by European health authorities. What fasting actually does is give the digestive system a rest and temporarily shift your blood sugar, insulin and lipids. Nothing spectacular, but not nothing either.

What we know, what we don't know yet

What we know. The most common formats, 16:8 or 5:2, can reduce calorie intake, with modest weight loss similar to that of a standard diet over the same period. Current evidence suggests a broadly favourable effect on blood pressure and fasting blood glucose in some people.

Matte kitchen scales and fresh vegetables on a linen cloth, soft natural light

What we don't know yet. These studies are short, small, and say nothing about cardiovascular risk over ten years. No one can claim that regular fasting protects the heart long term.

Long fasts, however, are not harmless. Beyond 48 to 72 hours without supervision, the risks are documented: dehydration, low blood pressure, electrolyte imbalance, hypoglycaemia. Add the ordinary discomforts — headaches, feeling cold, irritability, broken sleep, constipation — which often ease, but can stop you working normally.

The situations that need medical advice

Some situations rule out fasting or require prior medical advice: pregnancy and breastfeeding, children and teenagers, frail or malnourished older people, diabetes treated with insulin or sulfonylureas, a history of eating disorders, kidney or liver failure, chronic illness under treatment, severe fatigue or major stress. If in doubt, speak to your GP before you start, not during.

Young North African woman at a consultation in a bright clinic, with a healthcare professional

In practice, if you want to try

Don't go abruptly from everything to nothing: reduce portions gradually for a few days beforehand. Choose a quiet period, with no travel or exams, when you can rest. Drink water regularly. Pay attention to how you feel: persistent weakness, palpitations or dizziness are not stages to push through. When you start eating again, reintroduce food in stages, beginning with soft textures and cooked fibre, then return to a balanced plate and regular physical activity. That's where weight stability is built, not during restriction.

If you have health goals — blood pressure, blood sugar, cholesterol — fasting is never first-line. Prevention comes through diet, sleep, physical activity and, if needed, support from a professional. Very rapid weight loss is a risk factor for weight regain and eating disorders, not a trophy.

Budget

The figures below are rough UK ranges; they vary by city, practitioner and type of support.

  • GP appointment: free at the point of use on the NHS; private GP appointments usually cost around £50–£150.
  • Blood test, if prescribed by a GP: free on the NHS; private tests vary depending on the panel.
  • Appointment with a registered dietitian: around £50–£100 per session privately; NHS referral is free if you are eligible.
  • Psychological support: NHS talking therapies are free; private therapy costs around £40–£80 per session, depending on the practitioner and city.
  • Apps and online coaching: £10–£40 per month, available in app stores or on the publisher's website; be wary of numerical promises.
  • Supervised fasting retreat: from several hundred to over £1,000 per week, depending on location and level of supervision.

Sources