Foods to Avoid While Intermittent Fasting

Foods to avoid while intermittent fasting can make the difference between a sustainable routine and one that leaves you hungry, fatigued, or nutritionally depleted. Intermittent fasting cycles between defined eating and fasting periods, but what you eat during your eating window matters enormously. Refined carbohydrates, sugary drinks, ultra-processed snacks, and alcohol can undermine blood glucose stability, increase hunger, and make fasting periods harder to maintain. This article explains which foods to limit, what you can safely consume during a fast, and when to seek advice from a GP or registered dietitian.

What is intermittent fasting and how does it work?

Intermittent fasting cycles between defined eating and fasting periods, with common approaches including 16:8 and 5:2 methods. It is not suitable for everyone, and people with diabetes, eating disorders, or certain medical conditions should consult their GP before starting.

Intermittent fasting is an eating pattern that cycles between defined periods of eating and fasting, rather than a diet that prescribes specific foods. The most widely practised approaches include the 16:8 method (fasting for 16 hours and eating within an 8-hour window), the 5:2 method (eating normally for five days and restricting calories significantly on two non-consecutive days), and alternate-day fasting.^[13][5] These patterns have been studied for their potential effects on weight management, metabolic health, and insulin sensitivity, though the evidence base continues to evolve and individual responses vary considerably.

During the fasting period, the body gradually depletes its glycogen stores and begins to draw on fat as an alternative fuel source, a process that may contribute to changes in body composition over time.^[6][13] Insulin levels fall during fasting, which some researchers suggest may support fat metabolism, though findings are not consistent across all studies.

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

• People who are pregnant or breastfeeding.

• People with a history of eating disorders.

• People with type 1 diabetes.

• People with type 2 diabetes who are taking insulin or sulfonylureas, as fasting can increase the risk of hypoglycaemia and medication doses may need adjustment.^[5][6]

• People prescribed SGLT2 inhibitors (such as dapagliflozin, empagliflozin, or canagliflozin), as fasting or very low calorie intake in this context may increase the risk of diabetic ketoacidosis, including at near-normal blood glucose levels.

• Children and adolescents.

• Underweight individuals or frail older adults.

• People with active gallbladder disease, a history of gout, or other unstable medical conditions.

People with diabetes considering fasting should also consult Diabetes UK's guidance on fasting and diabetes, which covers medication safety and blood glucose monitoring during fasting periods.

NICE guidance on weight management does not currently identify intermittent fasting as a standalone recommended intervention; any approach to weight management should be discussed with a healthcare professional.^[7] The British Dietetic Association (BDA) provides patient-facing information on intermittent fasting that may be a helpful starting point.

The quality of food consumed during the eating window matters as much as the timing. Choosing nutrient-dense foods and avoiding those that undermine the goals of fasting are central to making this approach effective and sustainable.

Which foods should you avoid during your eating window?

Ultra-processed foods, refined carbohydrates, sugary drinks, alcohol, and foods high in trans fats should be avoided during the eating window, as they impair blood glucose stability and make fasting periods harder to sustain.

During your eating window, ultra-processed foods, refined carbohydrates, and foods high in added sugars are best avoided, as these can undermine the nutritional quality of your diet and make the subsequent fasting period harder to sustain. The eating window is where nutritional quality becomes critical. Foods that cause rapid rises and falls in blood glucose can increase hunger, reduce satiety, and make it more difficult to maintain the fasting period that follows.

Specific categories to limit or avoid include:

• Sugary drinks and fruit juices: These provide calories with little nutritional value and can drive rapid rises in blood glucose. The NHS recommends limiting free sugars, including those in fruit juices, to no more than 30 g per day for adults.^[8]

• White bread, white rice, and pastries: Refined grains are digested quickly, offering little sustained energy or dietary fibre.

• Highly processed snack foods: Crisps, biscuits, and similar products tend to be calorie-dense, low in nutrients, and easy to overconsume.

• Alcohol: Alcoholic drinks add calories with little nutritional value, can impair judgement around food choices, and may disrupt sleep, which in turn can affect appetite regulation.

• Foods high in trans fats: Industrial trans fats are restricted by law in the UK, so they are less prevalent than in some other countries, but some imported processed baked goods and fried foods may still contain partially hydrogenated oils.^[15] These are associated with adverse cardiovascular outcomes and offer no nutritional benefit.^[9]

Instead, prioritising whole foods, lean proteins, healthy fats, vegetables, legumes, and wholegrains during the eating window supports satiety and helps maintain more stable blood glucose levels. The NHS Eatwell Guide provides a practical framework for balanced eating that translates well to structuring an intermittent fasting eating window. If you are unsure how to plan your meals, a registered dietitian can provide personalised guidance.

What can you eat or drink during the fasting period?

Plain water, black coffee, and unsweetened herbal or green tea are the safest choices during a fasting period, as they contain no or negligible calories and do not break the fast.

During the fasting period, the aim is to consume only zero or near-zero calorie drinks, as any meaningful calorie intake will break the fast. Plain water is the safest and most universally accepted option, and staying well hydrated during fasting periods is important for general wellbeing and can help manage feelings of hunger.

The following are generally considered compatible with most intermittent fasting approaches:

• Plain water: Still or sparkling, without added flavourings or sweeteners.

• Black coffee: Without milk, cream, or sugar. Caffeine may have a mild appetite-suppressing effect for some people, though tolerance varies.^[14]

• Plain herbal or green tea: Without milk or sweeteners.

• Plain sparkling water: A useful alternative for those who find still water monotonous.

Items that are commonly debated include:

• Diet drinks and artificially sweetened beverages: The UK's Scientific Advisory Committee on Nutrition (SACN) has concluded that low and no calorie sweeteners are safe and can help reduce free sugar intake.^[10][11] Whether they affect fasting outcomes is less clear; evidence on any metabolic effect during fasting is limited and mixed. If you are following a strict fasting protocol, plain water and unsweetened tea or coffee are the most straightforward choices.

• Bone broth: Contains calories and protein, so it technically breaks a fast, though some practitioners allow it in modified fasting protocols.

• Milk or cream in coffee: Even small amounts add calories and may interrupt the fasting state.

If you experience dizziness, significant fatigue, or difficulty concentrating during fasting periods, contact your GP or call NHS 111 for advice. If you experience severe dizziness, fainting, or confusion, seek urgent medical attention. These symptoms may indicate that the approach is not appropriate for your individual circumstances, or that adjustments are needed.

How do certain foods affect hunger and break your fast early?

High-glycaemic foods such as white bread, sugary cereals, and sweets cause rapid blood glucose rises and falls that trigger early hunger; even small caloric additions like milk in coffee can interrupt a strict fast.

Certain foods consumed during the eating window can significantly increase hunger during the subsequent fasting period, making it harder to sustain the fast. Foods that cause rapid rises and falls in blood glucose are a key factor. When blood glucose drops after a high-sugar or high-refined-carbohydrate meal, the body signals hunger through hormonal pathways. Ghrelin, often described as a hunger hormone, rises in anticipation of meals and in response to fasting; its regulation is influenced by meal timing, circadian patterns, and overall dietary composition, rather than by any single factor alone.^[13][6]

High-glycaemic foods such as white bread, sugary cereals, sweets, and sweetened yoghurts are digested rapidly, providing a short burst of energy followed by a pronounced dip that can trigger cravings and early hunger. In contrast, meals built around protein, healthy fats, and high-fibre carbohydrates digest more slowly, promoting a more gradual and sustained release of energy and supporting satiety for longer. The British Nutrition Foundation provides accessible information on glycaemic index and glycaemic load that may help with meal planning.

Some foods and drinks can also break the fast unintentionally. Even small amounts of milk in coffee, flavoured waters that contain added sugars (check the ingredients label), or chewing gum containing sugar alcohols add small amounts of calories that can interrupt a strict calorie fast. It is worth checking ingredient labels on flavoured waters and similar products carefully.

Prioritising foods with a lower glycaemic index and higher fibre content during the eating window is a practical strategy for making fasting periods more comfortable. The NHS Eatwell Guide provides a useful framework for balanced eating that translates well to intermittent fasting eating windows. If hunger during fasting feels unmanageable or is affecting daily functioning, speaking with your GP or a registered dietitian is advisable.

Scientific References

1. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
2. Life-threatening dapagliflozin-associated euglycaemic diabetic ketoacidosis in a postoperative patient.
3. The peri-operative implications of sodium-glucose co-transporter 2 inhibitors: a narrative review.
4. Predictors of asymptomatic ketonemia in outpatients with diabetic kidney disease receiving SGLT-2 inhibitors: a cross-sectional study.
5. Is Time-Restricted Eating Safe in the Treatment of Type 2 Diabetes?—A Review of Intervention Studies.
6. Intermittent fasting and 'metabolic switch': Effects on metabolic syndrome, prediabetes and type 2 diabetes.
7. Overweight and obesity management (NG246).
8. Sugar: the facts.
9. Trans Fatty Acids and Cardiovascular Disease.
10. SACN statement on the WHO guideline on non-sugar sweeteners.
11. Joint position statement on the use of low or no calorie sweeteners (LNCS).
12. The truth about sweeteners.
13. Intermittent fasting: Impact, mechanisms and cautions across medical and lifestyle domains.
14. Caffeine, coffee, and appetite control: a review.
15. Prevention of cardiovascular disease at population level – evidence paper: trans fats.

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