Intermittent Fasting Explained: UK Guide 2026 | Complete Nutrition

Understanding Fasting

Intermittent fasting explained

Intermittent fasting describes any pattern of alternating eating and fasting periods on a regular schedule. The main protocols are time-restricted eating (daily windows of 12 to 18 hours fasted), the 5:2 diet (two low-calorie days weekly), alternate day fasting (every other day) and one meal a day. All work through similar mechanisms with different patterns. Weight loss is typically modest. Choose the protocol you can sustain. No protocol is biologically superior to another at matched calorie deficits.

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Updated:
May 2026

Written by:
Dominic Walton, MD

Reading time:
7 min

The full answer

The four main intermittent fasting protocols

Intermittent fasting is not one approach. It is a family of patterns with different structures suited to different lifestyles.

1. Time-restricted eating (12:12, 14:10, 16:8, 18:6)

Daily fasting windows lasting 12 to 18 hours with eating windows of 6 to 12 hours. The most popular pattern is 16:8 (16 hours fasting, 8 hours eating). Variations include 12:12 (entry level, just an overnight fast), 14:10 (moderate, often the easiest sustainable middle ground) and 18:6 (more aggressive). Time-restricted eating is the easiest to incorporate into daily life because the schedule repeats. The 2020 Wilkinson Cell Metabolism trial documented metabolic benefits with this approach. Best for: most beginners and most sustainable long term.

2. The 5:2 diet

Five days of normal eating per week and two non-consecutive days of very low calorie intake (typically 500 to 600 kcal). Popularised by Michael Mosley. The low calorie days produce a roughly 30 percent weekly caloric deficit on average. Trials suggest similar effectiveness to daily 16:8 for weight loss. Best for: people who prefer flexibility on most days and can tolerate two challenging days per week. Less effective if low-calorie days produce binge eating compensation on normal days.

3. Alternate day fasting

Strict alternation between full eating days (normal calories) and fasting days (zero or up to 500 kcal). The 2020 Trepanowski JAMA Internal Medicine trial compared 1 year of alternate day fasting to calorie restriction at matched deficits. Both produced approximately 6 percent weight loss with similar dropout rates around 30 to 40 percent. Best for: people who tolerate a more demanding protocol and prefer all-or-nothing structure rather than daily restriction.

4. One meal a day (OMAD)

A single meal per day producing roughly 23 hours fasting and 1 hour eating. The most aggressive sustainable intermittent fasting pattern. Produces substantial caloric deficit by default because eating a full days food in one meal is difficult. Risks include nutritional inadequacy, social inflexibility and disordered eating patterns for some. Best for: experienced fasters who tolerate it well, not recommended as a starting protocol.

Choosing your protocol

How to pick the right protocol for you

Five factors that determine which protocol fits your life.

Your social and family schedule

Daily time-restricted eating means consistent skipping of one meal. If breakfast with family matters skip dinner. If dinner socialising matters skip breakfast. 5:2 means two demanding days where social eating is limited. Alternate day fasting affects half your social meals. Pick the protocol that fits your existing routines rather than fighting them.

Your relationship with food

Some people find structural eating windows freeing because they remove constant food decisions. Others find them restrictive and anxiety-provoking. People with binge eating tendencies should approach restrictive protocols cautiously: aggressive restriction can trigger binge cycles. If you have any eating disorder history, intermittent fasting is contraindicated regardless of protocol.

Your activity level

Athletes and people with high physical demands often do best with time-restricted eating that includes their training periods within the eating window. 5:2 and alternate day fasting can compromise training quality on low calorie days. Adjust protocol to support not undermine your physical activity.

Your sex and life stage

Women may need to start more conservatively (12:12 or 14:10 rather than 16:8) and watch for cycle disruption. Postmenopausal women often tolerate longer fasting windows comfortably. Men typically tolerate any protocol although individual variation is high. Anyone over 65 should discuss with GP first.

Your primary goal

Weight loss responds similarly to all protocols at matched deficits. Insulin sensitivity may benefit slightly more from early time-restricted eating. Cardiovascular markers improve modestly with all. Match protocol intensity to your goals and to what you can sustain. The protocol you can sustain for years beats the perfect protocol you abandon in 6 weeks.

Safety

Who should not do intermittent fasting

Standard contraindications apply across all intermittent fasting protocols.

History of eating disorders. Intermittent fasting is contraindicated.
Pregnancy or breastfeeding. Not appropriate without specific GP or midwife approval.
Type 1 diabetes or insulin dependent type 2 diabetes. Specialist supervision required.
BMI under 18.5. Contraindicated.
Children, adolescents or adults under 18. Contraindicated.

Anyone on regular medications, with significant medical conditions or over 65 should discuss any intermittent fasting plan with their GP first. Stop the protocol if cycle disruption, sleep disturbance, mood changes, fatigue or any concerning symptoms appear and persist beyond the first 2 weeks of adaptation.

For the wider picture on fasting from the gentlest protocols to extended fasts plus the science behind hunger, metabolism and refeeding, our Understanding Fasting hub brings every guide together in one place.

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This article sits inside our complete knowledge base on fasting covering protocols, physiology, safety and practical guidance. Head back to the hub for the full index.

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Keep reading

Drill into specific protocols

Several pages cover individual protocols in detail. Our piece on 16:8 fasting explained covers the most popular protocol. Alternate day fasting explained covers the more aggressive option. And time-restricted eating explained covers the family of daily window protocols.

Frequently asked

Intermittent fasting questions

What is intermittent fasting?

Intermittent fasting describes any eating pattern where periods of normal eating alternate with periods of zero or very low calorie intake on a regular schedule. The most common patterns are time-restricted eating (daily fasting windows of 12 to 18 hours), the 5:2 diet (two low-calorie days per week), alternate day fasting (every other day) and one meal a day (OMAD). All are forms of intermittent fasting differing in window length and frequency.

Which type of intermittent fasting is best?

There is no single best type. The best protocol is the one you can sustain comfortably. For most people 16:8 daily time-restricted eating is the gentlest sustainable protocol. People who prefer flexibility on most days may find 5:2 easier to maintain. Trial periods of 4 to 12 weeks help identify your best fit. Choosing based on lifestyle and personal preference matters more than choosing based on protocol-specific theoretical advantages.

How much weight will I lose with intermittent fasting?

Modest amounts. Trials of 16:8 produce 1 to 5 kg loss over 12 to 16 weeks on average. 5:2 produces similar amounts. Alternate day fasting may produce slightly more (3 to 8 kg) over 6 to 12 months. Individual results vary widely. The 2020 Lowe JAMA Internal Medicine trial reported less than 1 kg average loss with 16:8 over 12 weeks. Expectations of dramatic rapid weight loss are usually disappointed.

Is intermittent fasting safe long term?

Yes for healthy adults who tolerate the protocol well. Trials up to 12 months show good safety profile for time-restricted eating and alternate day fasting in healthy participants. Long term effects beyond 1 to 2 years are less well studied but no major safety signals have emerged from observational data. The standard contraindications apply. Some people experience adverse effects (cycle disruption, sleep issues, mood changes) and should stop if these appear.

Can I drink coffee during intermittent fasting?

Yes black coffee, plain tea and water are all fine during fasting windows. These have negligible calories and do not break the fast in any meaningful way. Adding milk, sugar or sweeteners technically breaks the fast although the effect of artificial sweeteners is debated. Limit caffeine after 2pm to protect sleep. Some people find black coffee helps reduce hunger during fasting windows.

Will intermittent fasting slow my metabolism?

Short term intermittent fasting does not slow metabolism. The 2000 Zauner study documented a slight 3.6 percent increase in resting metabolic rate at 84 hours of fasting. Sustained caloric deficit from any approach can cause modest metabolic adaptation (5 to 15 percent) which is similar between fasting and calorie restriction. The popular claim that intermittent fasting protects metabolism better than calorie restriction is not strongly supported.

Should women do intermittent fasting differently?

Possibly yes. Some women experience cycle disruption, increased anxiety or sleep issues with aggressive fasting protocols. Conservative recommendations include starting with 12:12 or 14:10 rather than 16:8, considering cycle-aware fasting (lighter in the luteal phase), avoiding fasting during preconception attempts and stopping if cycle changes appear. Women who tolerate fasting well can continue normally. The British Menopause Society and Stacey Sims have raised these considerations.