What Is Intermittent Fasting? Methods and Safety

Intermittent fasting is an eating pattern that alternates between defined periods of eating and deliberate fasting, focusing on when you eat rather than what you eat. It has grown considerably in popularity across the UK, with approaches such as the 16:8 and 5:2 methods becoming widely adopted. The underlying principle involves allowing insulin levels to fall so the body can draw on stored fat for energy. This article explains how intermittent fasting works, the most common methods, how to start safely, and who should avoid it or seek medical advice first.

What is intermittent fasting and how does it work?

Intermittent fasting cycles between eating and fasting windows, allowing insulin levels to fall so the body shifts from glycogen to stored fat for energy. It is not a medically licensed treatment and should not replace clinical weight-management assessment.

Intermittent fasting is an eating pattern that cycles between defined periods of eating and deliberate periods of fasting, rather than focusing on which foods to eat. It is not a diet in the conventional sense but a structured approach to when calories are consumed. The underlying principle is that extending the time between meals allows insulin levels to fall sufficiently for the body to begin drawing on stored fat for energy, a process sometimes called metabolic switching.^[1]

During a fasting window, the body gradually depletes its glycogen stores and shifts towards burning fatty acids and producing ketones. A narrative review by Mattson et al. published in the New England Journal of Medicine (2019) summarised these mechanisms and explored their potential relevance to metabolic health, though the authors noted that long-term clinical data remain limited and that benefits depend on overall energy balance and individual adherence. Subsequent systematic reviews have broadly supported the view that intermittent fasting can improve insulin sensitivity and body weight in some adults, but have also found that outcomes are comparable to those achieved with continuous energy restriction when total calorie intake is similar.^[2][4]

It is important to understand that intermittent fasting is not a medically licensed treatment and is not the same as a clinically supervised weight-management programme. For people with significant obesity or weight-related health conditions, NICE guidance (NG246, Obesity: identification, assessment and management) recommends structured interventions that may include dietary advice, behavioural support and, where appropriate, pharmacological treatment.^[3] Intermittent fasting may complement such approaches for some individuals, but it should not replace professional clinical assessment.

What are the most popular intermittent fasting methods in the UK?

The 16:8 and 5:2 methods are the most widely used in the UK. Adherence over time is a stronger predictor of benefit than the specific protocol chosen.

Several intermittent fasting protocols are widely followed in the UK, each differing in the length and frequency of fasting windows. The most commonly practised include:

• 16:8 method: Eating is restricted to an eight-hour window each day, with fasting for the remaining sixteen hours. Many people achieve this simply by skipping breakfast and eating between midday and 8 pm.

• 5:2 method: Normal eating occurs on five days of the week, while calorie intake is substantially reduced on two non-consecutive days. A commonly cited target is around 500 to 600 kilocalories on fasting days (roughly 20 to 25 per cent of typical daily intake), though this figure originates from the protocol as originally described rather than from a single authoritative clinical guideline, and individual requirements vary. This approach was popularised in the UK following a BBC Horizon documentary in 2012.

• Alternate-day fasting: Eating days alternate with fasting or very low-calorie days. This is a more demanding protocol and is less commonly sustained long-term.

• Time-restricted eating (TRE): A broader category that includes the 16:8 method. Some research suggests that aligning eating windows with earlier parts of the day may offer additional metabolic benefits related to circadian biology, though the evidence is mixed and no single eating window has been established as optimal.^[5][6]

The 16:8 and 5:2 methods are among the most commonly used by people with regular working patterns in the UK, though this reflects reported practice rather than evidence of superiority. Individual preference, lifestyle and any underlying health conditions should guide which approach, if any, is appropriate. Adherence over time is generally a stronger predictor of benefit than the specific protocol chosen, a finding consistent with systematic reviews comparing intermittent fasting with continuous energy restriction.^[4][6] Anyone considering a structured fasting approach alongside a clinical weight-management plan should discuss this with their GP or a registered dietitian.

How do I get started with intermittent fasting safely?

Begin gradually by delaying breakfast and moving the evening meal earlier, extending the fasting window over one to two weeks. Maintain a balanced diet during eating windows and stay well hydrated throughout.

Starting intermittent fasting safely involves a gradual approach, realistic expectations and attention to nutritional quality during eating windows. Rather than immediately adopting a strict sixteen-hour fast, many people find it helpful to begin by delaying breakfast by one to two hours and moving their evening meal slightly earlier, progressively extending the fasting period over one to two weeks.

During eating windows, it remains important to consume a balanced diet that meets nutritional requirements. The NHS Eatwell Guide (available at nhs.uk) provides a practical framework, emphasising vegetables, wholegrains, lean proteins, dairy or dairy alternatives, and healthy fats. Severely restricting calories within an already narrow eating window can lead to nutrient deficiencies, fatigue and difficulty sustaining the approach. Very low-calorie regimens should be undertaken only under clinical supervision.

Hydration is particularly important during fasting periods. Water and plain herbal teas are generally considered acceptable during a fast. Black coffee is also commonly used, though people sensitive to caffeine may wish to limit intake. Drinks containing milk, sugar or calories will break a strict fast; whether this matters depends on the individual's goals and the protocol they are following.

It is advisable not to fast when acutely unwell. People managing long-term conditions, particularly diabetes, should follow their clinician's sick-day rules and seek advice before changing meal timing, as fasting can affect how medicines work and when they should be taken.

For those exploring weight management more broadly, it is worth knowing that clinically supervised options exist alongside lifestyle approaches. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where a range of options may be considered based on individual clinical need. Keeping a simple food and fasting diary during the first few weeks can help identify patterns, highlight any symptoms of concern and support a conversation with a GP or dietitian if needed.

Is intermittent fasting safe for everyone?

Intermittent fasting is not suitable for everyone; those with insulin-treated diabetes, people taking SGLT2 inhibitors, pregnant women, children and those with eating disorder histories should avoid it or seek medical advice first.

Intermittent fasting is not appropriate for everyone, and certain groups should avoid it or seek medical advice before starting. People for whom intermittent fasting is generally not recommended include:

• Those with type 1 diabetes or insulin-treated type 2 diabetes, as fasting can increase the risk of hypoglycaemia and may require medication adjustment under medical supervision. Insulin and sulfonylureas (such as gliclazide) are among the medicines most associated with hypoglycaemia risk during fasting, and dose changes should only be made with clinician guidance.^[7]

• People taking SGLT2 inhibitors (such as dapagliflozin or empagliflozin): the MHRA has issued Drug Safety Updates warning that restricted food intake, dehydration or acute illness in people taking these medicines can increase the risk of diabetic ketoacidosis (DKA), which may occur even when blood glucose is not markedly elevated.^[8] Anyone taking an SGLT2 inhibitor should discuss fasting plans with their diabetes team before making any changes.

• Pregnant or breastfeeding women, for whom adequate and consistent calorie and nutrient intake is essential.

• Children and adolescents, whose nutritional needs during growth make calorie restriction potentially harmful.

• People with a history of eating disorders, including anorexia nervosa, bulimia nervosa or binge eating disorder, as structured fasting may reinforce disordered patterns.

• Those who are underweight or have significant nutritional deficiencies.

• People taking certain medications, including anticoagulants, corticosteroids or blood pressure medicines, where timing relative to food intake may affect efficacy or safety. Some medicines, such as rivaroxaban at higher doses, require food for adequate absorption.^[9][10]

Even in otherwise healthy adults, some people experience side effects when beginning intermittent fasting, including headaches, irritability, difficulty concentrating and disrupted sleep. These symptoms often improve within one to two weeks as the body adapts, but persistent or severe symptoms warrant a conversation with a GP.

Anyone with a long-term health condition should speak to their GP before starting any fasting regimen. If symptoms such as fainting, severe dizziness, chest pain or signs of hypoglycaemia occur during a fast, contact NHS 111 for urgent advice or call 999 if the situation is life-threatening. Suspected adverse effects from any medicine taken alongside a fasting approach can be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

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