How to Intermittent Fast: Methods and Safety

Intermittent fasting is a structured eating pattern that cycles between defined periods of eating and fasting, and learning how to intermittent fast safely can make a significant difference to your results. Rather than dictating which foods to eat, it focuses on when calories are consumed. Popular approaches include the 16:8 method, the 5:2 diet, and time-restricted eating. This article explains how intermittent fasting works, which method may suit you, how to start safely in the UK, and what realistic timescales to expect.

What is intermittent fasting and how does it work?

Intermittent fasting cycles between defined eating and fasting periods, lowering insulin to encourage fat use for energy. It is not suitable for pregnant women, under-18s, underweight adults, or those with eating disorders without GP advice.

Intermittent fasting is an eating pattern that cycles between defined periods of eating and fasting, rather than focusing on which foods to eat. It does not prescribe specific meals but instead structures when calories are consumed. Common approaches include daily time-restricted eating windows and alternate-day or twice-weekly fasting protocols.

The proposed mechanisms behind intermittent fasting centre on metabolic adaptation. During a fasting period, insulin levels fall, which encourages the body to draw on stored glycogen and, subsequently, fat for energy.^[8][9] Fasting states may also influence levels of glucagon and growth hormone, both of which play roles in energy regulation.^[9] Some research suggests that prolonged fasting may promote a cellular recycling process called autophagy, though the clinical significance of this in humans is still being investigated and should not be overstated.^[1]

Some evidence also suggests that aligning eating windows with earlier parts of the day may support circadian rhythm and metabolic health.^[6] This is an emerging area of research and is not yet reflected in formal UK clinical guidance.

Intermittent fasting is not suitable for everyone. The following groups should seek advice from their GP or a relevant specialist before making significant changes to their eating pattern:

• People who are pregnant or breastfeeding

• Children and young people under 18

• Adults with a BMI below 18.5 or who are otherwise underweight

• Frail older adults

• Anyone with a current or recent history of an eating disorder

• People managing certain medical conditions, including type 1 diabetes

Intermittent fasting is one dietary strategy among several, and no single approach suits all individuals. The British Dietetic Association (BDA) provides patient-facing information on intermittent fasting that may help you decide whether it is appropriate for you.

Which intermittent fasting method is right for you?

The 16:8 method is the most accessible starting point, while the 5:2 restricts intake to 500 to 600 kcal on two non-consecutive days. People taking insulin or sulfonylureas must consult their GP before changing meal timing.

The most appropriate intermittent fasting method depends on your lifestyle, health status, and personal preferences, and there is no single protocol that suits everyone. The most widely studied and practised approaches include:

• 16:8 method: Eating is restricted to an 8-hour window each day, with a 16-hour fast. This is often considered the most accessible starting point for beginners.

• 5:2 method: Normal eating occurs on five days of the week, with calorie intake reduced to around 500 to 600 kilocalories on two non-consecutive days. This figure is based on the protocol used in research by Harvie and colleagues and popularised in the UK; individual calorie targets may vary.

• Alternate-day fasting: Fasting or significant calorie restriction every other day. This is a more demanding protocol and may be less sustainable for many people.

• Time-restricted eating (TRE): A broader category that includes the 16:8 method, focusing on confining eating to a defined daily window. Some research suggests that earlier eating windows may offer additional metabolic benefits, though evidence on this point is mixed and no formal UK guidance currently endorses one timing pattern over another.

For people with busy schedules, the 16:8 method is often practical, as it can be achieved by eating an earlier evening meal or delaying the first meal of the day. However, people who take insulin, sulfonylureas, or other glucose-lowering medicines should not alter their meal timing without first discussing this with their GP or diabetes team, as changes to eating patterns can increase the risk of hypoglycaemia.^[7][10]

If you are considering intermittent fasting alongside a medically supervised weight management programme, it is worth discussing your chosen method with a healthcare professional. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where clinically appropriate options can be explored. Whichever method you choose, consistency and nutritional quality during eating periods remain central to achieving meaningful outcomes.

How do you start intermittent fasting safely in the UK?

Start by speaking to your GP, especially if you take regular medication or have a health condition. Gradually extend your overnight fast over several weeks and follow the NHS Eatwell Guide during eating periods.

Starting intermittent fasting safely begins with speaking to your GP, particularly if you have an existing health condition, take regular medication, or have a history of disordered eating. Intermittent fasting is generally not recommended for children and young people under 18, adults who are underweight or frail, or anyone with a current or recent eating disorder.

Certain medicines require particular care when meal timing changes. Insulin and sulfonylureas (such as gliclazide) carry a significant risk of hypoglycaemia if doses are not adjusted to reflect altered eating patterns; meglitinides carry a similar risk.^[10][11] Most antihypertensive medicines are not meal-timed, but fasting can cause dehydration, which may affect blood pressure. Any medication review must be carried out under medical supervision before you begin.

Once you have confirmed that intermittent fasting is appropriate for you, a gradual approach is generally advisable. Rather than immediately adopting a strict 16-hour fast, you might begin by extending the overnight fast by one to two hours and progressively adjusting your eating window over several weeks. This allows the body to adapt and reduces the likelihood of side effects such as headaches, fatigue, or difficulty concentrating, which are common in the early stages.

During eating periods, prioritising nutritionally balanced meals is essential. The NHS Eatwell Guide provides a practical framework, recommending a diet rich in vegetables, wholegrains, lean proteins, and healthy fats, with limited ultra-processed foods and added sugars.^[13] Adequate hydration is particularly important during fasting windows. Water, plain herbal teas, and black coffee are commonly permitted in fasting protocols and are generally considered unlikely to meaningfully affect the metabolic state of a fast, though individual protocols may vary.^[14]

Key safety considerations include:

• Avoid fasting if pregnant, breastfeeding, underweight, or under 18

• Discuss medication adjustments with your GP before starting, particularly if you take insulin, sulfonylureas, or other glucose-lowering medicines

• Monitor for symptoms of hypoglycaemia if you have diabetes or take glucose-lowering medication; Diabetes UK provides patient guidance on recognising and managing hypoglycaemia

• Do not fast on days of strenuous physical activity until your body has adapted

• Contact your GP if you experience persistent dizziness, fainting, or significant mood changes

For urgent concerns, NHS 111 can provide advice. Attend A&E or call 999 only if symptoms are life-threatening.

How long does it take to see results from intermittent fasting?

Results vary, but systematic reviews show intermittent fasting produces broadly similar weight loss to continuous calorie restriction over three to twelve months. Review your approach with a GP or dietitian if no change occurs after eight to twelve weeks.

The time it takes to notice changes from intermittent fasting varies considerably between individuals and depends on the method used, calorie intake during eating windows, individual metabolism, and baseline health. Early changes in body weight often reflect a reduction in water retention and glycogen stores rather than fat loss, so initial changes may not fully represent longer-term progress.

Systematic reviews and meta-analyses comparing intermittent fasting with continuous calorie restriction suggest that both approaches can produce broadly similar weight loss over periods of three to twelve months, with adherence being a key factor in both cases.^[2][3] Some studies have also reported improvements in fasting blood glucose, blood pressure, and lipid profiles in people following intermittent fasting protocols, though the evidence base continues to develop.^[2][5] NICE has not issued specific guidance endorsing intermittent fasting as a standalone clinical intervention for weight management.^[12]

Some individuals report improvements in energy levels, appetite regulation, and sleep quality after several weeks, once the initial adaptation period has passed. These subjective benefits may support long-term adherence, though they are not consistently reported across studies and should be regarded as variable rather than expected outcomes.

It is worth setting realistic expectations. Intermittent fasting is not a rapid solution, and outcomes are most meaningful when the approach is combined with nutritious eating, regular physical activity, and adequate sleep, in line with NHS weight management guidance. If you have not noticed any change in weight or wellbeing after eight to twelve weeks of consistent practice, it is reasonable to review your approach with a GP or registered dietitian. For those with obesity or weight-related health conditions, a structured medical weight management programme may offer more comprehensive support than dietary change alone.

Scientific References

1. The effect of fasting or calorie restriction on autophagy induction: A review of the literature.
2. Intermittent fasting and health outcomes: an umbrella review of systematic reviews and meta-analyses.
3. Comparison of Different Intermittent Fasting Patterns or Continuous Energy Restriction.
4. Intermittent fasting for adults with overweight or obesity.
5. The impact of intermittent fasting on body composition and cardiometabolic outcomes in overweight and obese adults: a systematic review and meta-analysis of randomized controlled trials.
6. Early, midday, and late time-restricted eating impact on anthropometry and cardiometabolic health: a network meta-analysis of RCTs.
7. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? A Review of Intervention Studies.
8. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes.
9. Endocrine regulation of the hepatic fasting response: cues, cooperation and consequences.
10. Gliclazide | Drugs | BNF — NICE.
11. Gliclazide: medicine for type 2 diabetes.
12. Overweight and obesity management — Physical activity and diet. NG246.
13. The Eatwell Guide.
14. The effect of black coffee on fasting metabolic markers.

Frequently Asked Questions