How long does intermittent fasting last depends on the protocol chosen, ranging from 12 hours in basic time-restricted eating to around 36 hours in alternate day fasting. Intermittent fasting (IF) is an eating pattern that cycles between defined fasting and eating windows, and has attracted growing interest as a strategy for weight management and metabolic health. This article explains the most common IF schedules, how long results typically take to appear, what UK clinical guidance says, and when it is important to speak to a GP before starting or stopping a fast.

What Is Intermittent Fasting and How Does It Work?

Intermittent fasting alternates defined fasting and eating periods, working by depleting glycogen stores and shifting the body towards fat burning (lipolysis), while also reducing circulating insulin levels.

Intermittent fasting (IF) is an eating pattern that alternates between defined periods of fasting and eating. Unlike conventional calorie-restricted diets, it focuses primarily on when you eat rather than what you eat. It has gained considerable attention in recent years as a strategy for weight management, metabolic health, and general wellbeing.

From a physiological standpoint, intermittent fasting works by depleting the body's glycogen (glucose) stores during the fasting window. Once glycogen is sufficiently depleted — the timing of which varies considerably depending on an individual's diet, activity level, and metabolic rate, but may begin after roughly 12 hours or longer — the body starts to shift towards burning stored fat for energy, a process known as lipolysis. This metabolic shift is also associated with reductions in circulating insulin levels, which may improve insulin sensitivity over time.

Fasting periods are also thought to promote autophagy — a cellular 'self-cleaning' process in which the body breaks down and recycles damaged components. However, most evidence for autophagy in humans comes from small or short-term studies, and much of the foundational research is derived from animal models. Long-term effects in diverse human populations remain under investigation.

It is worth noting that intermittent fasting is not specifically recommended by NICE as a first-line treatment for weight management. UK clinical guidance focuses on achieving a sustained energy deficit within a multicomponent approach. IF may be one way some individuals achieve this, but it is not a medically prescribed intervention. The British Dietetic Association (BDA) provides evidence-based information on IF for patients and clinicians.

Common Intermittent Fasting Schedules and Their Duration

Fasting windows range from 12 hours in basic time-restricted eating to approximately 36 hours in alternate day fasting; common protocols include 16:8, 5:2, 24-hour fasts, ADF, and OMAD.

One of the most frequently asked questions is: how long does intermittent fasting last? The answer depends entirely on the specific protocol chosen. There are several well-recognised schedules, each with a different fasting duration:

• 16:8 method — Involves a 16-hour fast followed by an 8-hour eating window each day. For example, eating between 12:00 and 20:00, then fasting overnight and through the morning. Many people find this approach practical to fit around daily life, though individual preferences vary.

• 5:2 diet — Eating normally for five days of the week and restricting calorie intake to approximately 500–600 kcal on two non-consecutive days. This is a form of intermittent energy restriction rather than a complete fast; food is not eliminated entirely on low-calorie days. This approach was popularised in the UK partly through the work of Dr Michael Mosley and is discussed on the NHS website.

• 24-hour fasts — Some individuals fast for a full 24 hours once or twice per week, consuming only water, black coffee, or herbal teas during the fasting period.

• Alternate day fasting (ADF) — Alternating between normal eating days and very low-calorie or complete fasting days. Because a fasting day and the following eating day together span approximately 36 hours without normal food intake, ADF involves some of the longest fasting windows of any common protocol.

• OMAD (One Meal A Day) — A more restrictive approach involving a single daily meal, effectively a fast of approximately 23 hours.

Fasting windows across these protocols typically range from around 12 hours (as in basic time-restricted eating) up to approximately 36 hours in ADF. Longer fasts carry greater risks and are generally only appropriate for those with prior experience and, ideally, medical or dietetic supervision.

A brief note on drinks during fasting periods: staying well hydrated with water is important. Black coffee and plain herbal teas are commonly consumed, but those with caffeine sensitivity, acid reflux, or who take medicines affected by caffeine should exercise caution. Always check whether any prescribed medicines need to be taken with food.

How Long Until You See Results From Intermittent Fasting?

Initial appetite and energy changes may occur within one to two weeks, with modest reductions in body weight and waist circumference typically observed after four to eight weeks of consistent practice.

The timeline for noticeable results from intermittent fasting varies considerably between individuals and depends on factors such as starting weight, dietary quality during eating windows, physical activity levels, and overall metabolic health.

In the first one to two weeks, many people report changes in appetite regulation and energy levels, though some also experience initial side effects such as headaches, irritability, or difficulty concentrating — often referred to informally as the 'fasting adjustment period'. These symptoms are largely reported anecdotally and typically resolve as the body adapts.

Within four to eight weeks, research suggests that consistent intermittent fasting can lead to modest reductions in body weight and waist circumference. A 2020 systematic review and meta-analysis published in Obesity Reviews (Harris et al.) found that IF protocols produced broadly comparable weight loss to continuous calorie restriction over periods of 8 to 24 weeks, with typical reductions in the range of 0.8–13% of body weight depending on the protocol and population studied. Dietary quality and physical activity remained key determinants of outcomes.

Beyond three months, some studies have reported improvements in lipid profiles, blood pressure, and markers of inflammation, though evidence remains mixed and effect sizes modest. It is important to maintain realistic expectations:

• Results are not universal and depend heavily on overall lifestyle

• Intermittent fasting is not a rapid weight-loss solution

• Sustainable results require consistency over weeks and months, not days

If no meaningful change is observed after 8 to 12 weeks of consistent practice, it may be worth reviewing dietary habits during eating windows or consulting a registered dietitian. The NHS provides general guidance on realistic weight-loss timelines and healthy eating.

NHS and NICE Guidance on Fasting-Based Diets

Neither the NHS nor NICE endorses intermittent fasting as a first-line clinical intervention; NICE CG189 recommends a multicomponent approach centred on a sustained calorie deficit, physical activity, and behavioural support.

In the UK, neither the NHS nor NICE currently endorse intermittent fasting as a first-line clinical intervention for weight management. NICE CG189 (Obesity: identification, assessment and management) and the associated NICE Quality Standard QS127 recommend a multicomponent approach to weight management, encompassing dietary changes to achieve a sustained energy deficit, increased physical activity, and behavioural support. Calorie-deficit diets remain the primary dietary recommendation within this framework. Intermittent fasting may be one way to achieve a calorie deficit if it is safe and sustainable for the individual, but NICE does not endorse specific branded dietary approaches.

The NHS does acknowledge intermittent fasting — including the 5:2 diet — as one of several dietary approaches that some people find helpful, and provides information on its suitability and limitations on the NHS website. The NHS notes that fasting-based diets are not suitable for everyone and advises that they should not be undertaken by:

• Pregnant or breastfeeding women

• Children and young people under 18

• Individuals with a history of eating disorders

• People with type 1 diabetes or those taking insulin or sulphonylurea medicines

• Those who are underweight or have certain chronic health conditions

The MHRA and EMA have not issued specific regulatory guidance on intermittent fasting as a therapeutic intervention, as it is a dietary practice rather than a medicinal product. However, clinicians are advised to consider potential interactions between fasting and prescribed medicines — particularly those requiring food intake or affecting blood glucose — and to adjust prescribing accordingly where necessary.

If you think you have experienced a side effect from a medicine or medical device, you can report it via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

When to Speak to a GP Before Starting or Stopping a Fast

Medical advice should be sought before fasting if you have diabetes managed with insulin or sulphonylureas, take medicines requiring food, are pregnant, have an eating disorder history, or are over 65.

Whilst intermittent fasting is generally considered safe for healthy adults, there are important circumstances in which medical advice should be sought before beginning any fasting protocol. Speaking to a GP is strongly recommended if you:

• Have type 1 or type 2 diabetes, particularly if managed with insulin or sulphonylureas (such as gliclazide or glibenclamide), as these medicines carry a significant risk of hypoglycaemia during fasting and dose adjustments may be required. Other glucose-lowering medicines, including metformin, carry a much lower risk of hypoglycaemia on their own, but a GP or diabetes team should still be consulted before making dietary changes. Diabetes UK provides specific guidance on fasting with diabetes.

• Are taking prescribed medicines that should be taken with food to reduce gastrointestinal irritation — for example, NSAIDs (such as ibuprofen or naproxen), aspirin, or corticosteroids. The Specialist Pharmacy Service (SPS) provides UK-specific guidance on medicines and food requirements.

• Are taking lithium, where consistent fluid and salt intake is important to maintain stable blood levels; changes to eating and drinking patterns should be discussed with your prescriber.

• Have a history of disordered eating, as structured fasting may trigger or exacerbate restrictive behaviours.

• Are pregnant, planning a pregnancy, or breastfeeding.

• Have a chronic health condition such as kidney disease, liver disease, cardiovascular disease, or an immune disorder.

• Are over 65, as older adults may be at greater risk of muscle loss, dehydration, and nutritional deficiencies during fasting.

You should also contact your GP or seek prompt medical attention if, during a fast, you experience severe dizziness, fainting, chest pain, palpitations, confusion, or persistent vomiting. These may indicate hypoglycaemia, dehydration, or another underlying condition requiring assessment. Do not drive or operate machinery if you feel dizzy, faint, or suspect low blood sugar.

When stopping a prolonged fast (particularly one lasting more than 24 hours), reintroduce food gradually, beginning with small, easily digestible meals, to minimise gastrointestinal discomfort and reduce the risk of refeeding symptoms. If you are unsure whether intermittent fasting is appropriate for your individual health circumstances, a referral to a registered dietitian via your GP can provide personalised, evidence-based guidance.

Frequently Asked Questions