16:8 Intermittent Fasting: Benefits and Risks

16:8 intermittent fasting is one of the most popular approaches to time-restricted eating, involving an 8-hour window for all food intake followed by a 16-hour fast. Rather than prescribing specific foods or calorie targets, it works by extending the natural overnight fast, encouraging the body to shift towards fat oxidation as glycogen stores deplete. This article explains how 16:8 fasting works, who it is and is not suitable for, what to eat and drink, what the current evidence shows about its benefits and risks, and practical tips for getting started safely.

What is 16:8 intermittent fasting and how does it work?

16:8 intermittent fasting limits eating to an 8-hour window and fasts for 16 hours, extending the overnight fast to lower insulin and increase fat oxidation. Food quality within the eating window remains central to any health benefit.

16:8 intermittent fasting is an eating pattern in which a person restricts all food intake to an 8-hour window each day and fasts for the remaining 16 hours. It is one of the most widely practised forms of time-restricted eating, and it works primarily by extending the natural overnight fast rather than requiring calorie counting or food elimination. The approach is sometimes called the Leangains protocol, after the fitness researcher who popularised it.

During the fasting period, insulin levels fall and the body gradually shifts from using glucose as its primary fuel source towards mobilising stored fat. After a prolonged period without food (the precise timing varies between individuals depending on prior intake, activity level, and metabolic state), glycogen stores in the liver begin to deplete and fatty acid oxidation increases. This metabolic shift is thought to underlie many of the proposed benefits of time-restricted eating, though the mechanisms in humans are still being studied and the evidence base continues to evolve.

The 8-hour eating window can be positioned at any point in the day, though many people choose a midday to early evening window (for example, noon to 8 pm) to align with social mealtimes and to allow a normal morning routine. Some research suggests that earlier eating windows, better aligned with circadian rhythms, may offer additional metabolic advantages in certain populations, though this remains an area of active investigation and individual preference and sustainability matter considerably in practice.^[9][10]

Importantly, 16:8 fasting does not prescribe what to eat, only when. This means the quality of food choices within the eating window remains central to any health outcomes. Eating a nutrient-poor diet within an 8-hour window is unlikely to produce meaningful health benefits.

Who is 16:8 fasting suitable for, and who should avoid it?

16:8 fasting suits healthy adults seeking weight or metabolic management, but should be avoided by people with diabetes on insulin or sulfonylureas, pregnant women, children, and those with eating disorders. Anyone on regular prescription medication should consult their GP first.

16:8 intermittent fasting is generally considered suitable for healthy adults who wish to manage their weight, improve metabolic health, or simplify their eating habits, provided they can maintain adequate nutrition within the eating window. It may also be explored as a complementary strategy alongside medical weight management, though any significant dietary change should be discussed with a GP or registered dietitian first.

Several groups should avoid 16:8 fasting or approach it only under medical supervision:

• People with type 1 or type 2 diabetes, particularly those taking insulin or sulfonylureas, as fasting can increase the risk of hypoglycaemia and may require medication adjustment.^[11][12] People with diabetes who take SGLT2 inhibitors (such as canagliflozin, dapagliflozin, empagliflozin, or ertugliflozin) face an additional recognised risk of euglycaemic diabetic ketoacidosis during periods of markedly reduced food intake or prolonged fasting; anyone in this group should seek clinician review before attempting 16:8 fasting. If a person with diabetes experiences severe hypoglycaemia, confusion, or loss of consciousness, this is a medical emergency and 999 should be called immediately.

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

• Children and adolescents, whose nutritional needs during growth make prolonged fasting inappropriate.

• People with a history of eating disorders, as structured fasting patterns can reinforce disordered thinking around food.

• Those who are underweight or have significant nutritional deficiencies.

• People on certain medications that must be taken with food at regular intervals, such as NSAIDs or bisphosphonates. A pharmacist or prescriber can advise on whether any regular medicine has specific food-timing requirements.

For individuals considering medically supervised weight management, a clinician can assess whether dietary approaches such as time-restricted eating are appropriate alongside other interventions. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where a range of approaches may be discussed.

Anyone with an existing health condition, or who takes regular prescription medication, should speak to their GP before starting 16:8 fasting. NHS 111 can provide guidance if a concern arises outside of normal GP hours.

What can you eat and drink during the 8-hour eating window?

No foods are prohibited by the 16:8 protocol, but meals should follow NHS Eatwell Guide principles, prioritising protein, fibre, and minimally processed foods. Plain water, black coffee, and unsweetened tea are acceptable during the fasting hours.

During the 8-hour eating window, there are no specific foods that are prohibited by the 16:8 protocol itself, but the quality and balance of meals consumed within that window will largely determine whether the approach supports good health. A diet rich in vegetables, wholegrains, lean proteins, legumes, and healthy fats is consistent with NHS Eatwell Guide recommendations and is advisable regardless of the eating pattern followed.^[13]

Practical guidance for the eating window includes:

• Prioritise protein at each meal to support satiety and preserve muscle mass, particularly important if the fasting period is combined with physical activity.

• Include fibre-rich foods such as oats, lentils, fruit, and vegetables to support digestive health and help maintain stable blood glucose levels.

• Limit ultra-processed foods, refined sugars, and high-saturated-fat foods, which can undermine metabolic benefits even within a restricted window.

• Stay well hydrated throughout the day, including during the fasting period.

During the 16-hour fasting period, plain water, black coffee, and plain tea (without milk or sugar) are generally considered acceptable and are unlikely to break the fast in a meaningful metabolic sense. Any drink containing calories, including milk, fruit juice, or sweetened beverages, would technically end the fast. The evidence on artificially sweetened drinks during fasting is mixed, and the simplest approach is to stick to water or unsweetened hot drinks.

Alcohol should be consumed only within the eating window and in moderation, in line with UK Chief Medical Officers' low-risk drinking guidelines, which advise drinking no more than 14 units per week, spread across three or more days, with several alcohol-free days each week.^[3] Skipping meals or eating very little within the window to accelerate weight loss is not recommended, as it risks nutritional inadequacy.

Health benefits and risks supported by current evidence

Evidence links 16:8 fasting to modest reductions in body weight, fasting glucose, and blood pressure, but a 2022 RCT found no significant advantage over calorie restriction alone. Risks include hunger, potential muscle loss, and nutrient deficiencies if the eating window is poorly planned.

Current evidence suggests that 16:8 intermittent fasting can support modest weight loss and improvements in several cardiometabolic markers, though the quality and consistency of evidence varies across studies. Reviews and meta-analyses of time-restricted eating trials have reported associations with reductions in body weight, fasting glucose, and blood pressure in overweight adults, though many individual trials are short in duration and use small sample sizes.^[4][5] Importantly, a large randomised controlled trial published in the New England Journal of Medicine in 2022 found that time-restricted eating did not produce significantly greater weight loss than calorie restriction alone when overall calorie intake was similar, suggesting that the timing of eating may matter less than total energy intake for weight outcomes.^[6]

Potential benefits observed in research include:

• Weight and fat mass reduction, often comparable to continuous calorie restriction when total calorie intake is similar.

• Improvements in insulin sensitivity and fasting blood glucose in some populations.

• Reductions in LDL cholesterol and triglycerides in certain studies.

• Possible improvements in blood pressure and inflammatory markers.

However, the evidence base has important limitations. Many studies do not control for overall calorie intake, making it difficult to separate the effects of the fasting pattern itself from simple calorie reduction. Long-term data beyond 12 months remain limited, and findings may not apply equally across different populations.

Risks and considerations include:

• Hunger, irritability, and difficulty concentrating, particularly in the early weeks.

• Potential muscle loss if protein intake is insufficient or if fasting is combined with intense exercise without adequate fuelling.

• Disordered eating patterns in susceptible individuals.

• Nutrient deficiencies if the eating window does not accommodate a varied, balanced diet.

Anyone who experiences persistent symptoms such as severe dizziness, fainting, or significant gastrointestinal disturbance should stop fasting and contact their GP. Suspected adverse effects from any concurrent medication should be reported via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Practical tips for starting and sticking to a 16:8 routine

Gradually extending the overnight fast by one to two hours each week reduces early hunger and fatigue. Choosing a consistent eating window, planning nutritionally complete meals, and staying hydrated during fasting hours support long-term adherence.

Starting 16:8 intermittent fasting gradually is the most effective way to build a sustainable habit, particularly for those who are accustomed to eating breakfast early in the morning. Rather than immediately adopting a strict 16-hour fast, many people find it helpful to extend their overnight fast by one to two hours each week until they reach their target window. This reduces the likelihood of intense hunger or fatigue in the early stages.

The following practical strategies can support adherence:

• Choose a consistent eating window that fits your lifestyle and social commitments. A noon to 8 pm window is popular, but a 10 am to 6 pm window may suit those who prefer an earlier dinner.

• Plan meals in advance to ensure the eating window contains nutritionally complete, satisfying meals rather than opportunistic snacking.

• Keep busy during the fasting period, as hunger is often habitual and can be managed with activity, hydration, or distraction.

• Stay hydrated throughout the day. Drinking water, herbal teas, or black coffee during the fasting hours can help manage appetite.

• Be flexible without abandoning the pattern entirely. Occasional deviations for social events are unlikely to undermine long-term progress.

• Track progress meaningfully, focusing on energy levels, sleep quality, and wellbeing rather than weight alone.

• Seek tailored advice if you have high physical demands. Athletes, people with very active jobs, or those with high training loads may need individualised fuelling strategies from a registered dietitian to ensure performance and recovery are not compromised.

If weight management is a longer-term goal, it is worth discussing all available options with a GP or registered dietitian, who can advise on whether dietary approaches, behavioural support, or other interventions are appropriate for your individual circumstances. NICE guidance on the identification and management of overweight and obesity (NG246) provides a framework for the range of support available through the NHS.^[14]

Scientific References

1. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
2. Bisphosphonates: use and safety.
3. Alcohol consumption: advice on low risk drinking.
4. The optimal time restricted eating interventions for blood pressure, weight, fat mass, glucose, and lipids: A meta-analysis and systematic review.
5. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters.
6. Calorie Restriction with or without Time-Restricted Eating in Type 2 Diabetes.
7. Effects of 3 months of 10-h per-day time-restricted eating.
8. Intermittent fasting for adults with overweight or obesity.
9. Time-restricted eating and metabolic health: implications for nutritional strategies and weight loss.
10. Time-Restricted Eating in Metabolic and Clinical Health: Current Evidence and Mechanistic Insights.
11. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes? A Review of Intervention Studies.
12. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes.
13. The Eatwell Guide.
14. Overweight and obesity management (NG246).
15. The Yellow Card scheme: guidance for healthcare professionals, patients and the public.

Frequently Asked Questions