Bulletproof coffee and intermittent fasting have become increasingly intertwined trends in the UK, with many people blending butter and MCT oil into their morning coffee during fasting windows. Proponents claim this high-fat drink supports ketone production, curbs hunger, and preserves the metabolic benefits of fasting — but does the evidence support these claims? This article examines what bulletproof coffee actually is, how it interacts with intermittent fasting, what UK dietary guidelines say, who should avoid it, and what healthier alternatives exist for those following a fasting protocol.

What Is Butter and MCT Coffee and How Is It Used in Intermittent Fasting?

Bulletproof coffee is a high-fat drink made from coffee, butter, and MCT oil, consumed during fasting windows on the basis that its negligible carbohydrate content minimises insulin response, though it does end a strict caloric fast.

Butter and MCT coffee — sometimes called 'bulletproof coffee' after the brand that popularised it — is a high-fat beverage typically made by blending brewed coffee with unsalted grass-fed butter and medium-chain triglyceride (MCT) oil. Originally popularised by American entrepreneur Dave Asprey in the early 2010s, it has since gained considerable traction in the UK, particularly among those following low-carbohydrate or ketogenic dietary patterns.

In the context of intermittent fasting — a dietary approach that cycles between defined periods of eating and fasting — this drink is often consumed during the fasting window. Proponents argue that because it contains negligible carbohydrates and protein, it does not trigger a significant insulin response and therefore does not 'break' the fast in a metabolically meaningful way. However, it is important to be clear: because butter and MCT coffee contains calories (typically 200–500 kcal per serving), it does end a strict caloric fast. Whether this matters depends on the individual's specific fasting goals. For those following time-restricted eating with a fat allowance, the low glycaemic impact may be acceptable; for those pursuing strict caloric fasting, it is not equivalent to a true fasted state.

Claims that this drink supports ketone production or cellular autophagy are based largely on animal studies and small human trials. Evidence from robust human studies is limited, and these potential effects should not be overstated. Common intermittent fasting protocols in which it is used include:

• 16:8 fasting (16 hours fasting, 8-hour eating window)

• 5:2 fasting (normal eating five days per week, restricted calories on two days)

• Extended fasting protocols lasting 24 hours or more

The NHS acknowledges intermittent fasting, including the 5:2 approach, as one of several dietary strategies some people find helpful for weight management, but emphasises that any fasting regimen should be nutritionally balanced overall.

Potential Benefits and Risks of Butter and MCT Coffee During Fasting

MCT oil may modestly support satiety and ketone production, but each serving delivers 20–30 g of saturated fat and 200–500 kcal, posing cardiovascular and caloric risks that outweigh anecdotal benefits for many people.

Advocates of butter and MCT coffee during intermittent fasting point to several potential benefits, though it is important to distinguish between anecdotal claims and evidence-based findings. Some research suggests that MCT oil may modestly support ketone production and satiety compared with long-chain fatty acids, which could be helpful during a fasting period. A 2018 review in the Journal of the Academy of Nutrition and Dietetics (Maher et al.) noted that MCTs may promote feelings of fullness, potentially aiding adherence to fasting protocols, though the evidence is mixed and effect sizes are modest. The SACN 2019 report on saturated fats and health provides the UK scientific basis for understanding the dietary risks associated with high saturated fat intake.

Additionally, caffeine has well-documented effects on alertness, cognitive performance, and fat oxidation. For individuals who struggle with hunger or low energy during fasting windows, this drink may offer a practical coping strategy. Claims of improved mental clarity from the specific combination of butter and MCT oil are largely anecdotal; robust clinical evidence is limited.

The risks deserve equal attention:

• High saturated fat content: A single serving can contain 20–30 g of saturated fat. Both butter and MCT oil contribute substantially to this — a tablespoon of butter contains approximately 7 g of saturated fat, whilst a tablespoon of MCT oil contains approximately 13 g. Consistently high saturated fat intake is associated with raised LDL cholesterol, a recognised cardiovascular risk factor (SACN, 2019; NICE NG238).

• Caloric density: At 200–500 kcal per serving, this drink may undermine weight loss goals if not accounted for within total daily energy intake.

• Gastrointestinal discomfort: MCT oil is known to cause nausea, loose stools, or cramping, particularly when introduced rapidly or in large quantities. It is advisable to start with a small amount (for example, one teaspoon) and increase gradually to tolerance. If significant gastrointestinal symptoms occur, stop use and consult your GP.

• Nutrient displacement: Replacing a balanced breakfast with this drink may reduce intake of fibre, vitamins, and minerals.

Individuals with pre-existing hyperlipidaemia or cardiovascular conditions should exercise particular caution, discuss the practice with their GP or a registered dietitian, and consider lipid monitoring if adopting a consistently high saturated fat intake.

What UK Dietary Guidelines Say About High-Fat Coffee Drinks

UK guidelines recommend saturated fat comprise no more than 10% of daily energy intake; a single serving of bulletproof coffee can meet or exceed this limit, and no official UK body has endorsed its use during intermittent fasting.

Current UK dietary guidelines, as outlined by the NHS and the Scientific Advisory Committee on Nutrition (SACN), recommend that saturated fat should account for no more than 10% of total daily energy intake. For the average adult, this equates to approximately 20 g per day for women and 30 g per day for men. These are population-level targets; individual recommendations may vary depending on clinical circumstances and should be discussed with a healthcare professional. A single serving of butter and MCT coffee can approach or exceed this daily limit in one drink alone.

The Eatwell Guide, published by the Office for Health Improvements and Disparities (OHID) within the Department of Health and Social Care (DHSC), emphasises a diet rich in fruits, vegetables, wholegrains, lean proteins, and unsaturated fats — a pattern that does not naturally accommodate a daily high-fat, low-nutrient beverage. NICE guidance on cardiovascular disease risk assessment and lipid modification (NG238) similarly highlights the importance of reducing saturated fat intake as a modifiable risk factor for coronary heart disease.

It is worth noting that there is no official UK guidance specifically addressing butter and MCT coffee, and neither NICE nor the NHS has issued a formal position on its use during intermittent fasting. The absence of official endorsement does not imply prohibition, but it does mean individuals are largely navigating this practice without evidence-based institutional support.

The NHS acknowledges intermittent fasting as one of several dietary approaches that some people find helpful for weight management, but consistently advises that any fasting regimen should be nutritionally balanced overall. Healthcare professionals in the UK are generally encouraged to assess dietary patterns holistically rather than focusing on individual foods or drinks in isolation.

Who Should Avoid Butter and MCT Coffee While Fasting?

People with cardiovascular disease, hyperlipidaemia, diabetes, pregnancy, eating disorders, or gallbladder conditions should avoid bulletproof coffee during fasting and seek GP or dietitian advice before trying it.

Whilst butter and MCT coffee may be tolerated by healthy adults, certain groups should approach it with caution or avoid it altogether, particularly when combined with intermittent fasting.

People who should seek GP advice before trying this drink during fasting include:

• Those with cardiovascular disease or hyperlipidaemia: Given the high saturated fat content, individuals with raised cholesterol, a history of heart attack, or established cardiovascular disease should consult their GP or a registered dietitian before incorporating this drink regularly (NICE NG238).

• People with type 1 or type 2 diabetes: Intermittent fasting itself can affect blood glucose levels and may require adjustment of diabetes medications, including insulin. Adding a high-fat beverage introduces further metabolic complexity. NICE guidelines for diabetes management (NG17 for type 1; NG28 for type 2) emphasise the importance of individualised dietary advice. Do not adjust diabetes medications without first speaking to your diabetes care team.

• Pregnant or breastfeeding women: The NHS advises limiting caffeine to no more than 200 mg per day during pregnancy. A standard mug of filter coffee contains approximately 140 mg of caffeine, so a large serving of this drink could approach or exceed this limit. The nutritional adequacy of a fasting-based diet is also a concern during these life stages.

• Children and adolescents: Fasting regimens and high-fat dietary practices are not appropriate for children or young people unless specifically advised and supervised by a clinician.

• Individuals with a history of eating disorders: Restrictive dietary practices, including fasting combined with calorie-dense but nutritionally limited foods, may reinforce disordered eating patterns (NICE NG69).

• Those with gallbladder disease or fat malabsorption conditions: High fat intake can exacerbate symptoms in conditions such as cholecystitis or pancreatic insufficiency.

• Underweight or frail adults: Fasting and calorie restriction are generally not appropriate in these groups without clinical supervision.

When to seek urgent help: If you experience severe chest pain, symptoms that may suggest a heart attack or stroke (such as pain spreading to your arm or jaw, sudden weakness, or facial drooping), or palpitations with loss of consciousness or severe breathlessness, call 999 immediately. For urgent but non-emergency concerns, contact NHS 111. For other symptoms such as persistent dizziness or gastrointestinal problems whilst following a fasting protocol, stop and contact your GP.

For most healthy adults, caffeine intake of up to approximately 400 mg per day is generally considered acceptable, though individual sensitivity varies. Fasting is not appropriate for everyone, and a personalised assessment is always advisable.

Reporting side effects: If you believe you have experienced a side effect related to a medicine or medical device, you can report this via the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk.

Healthier Alternatives to Consider During Intermittent Fasting

Black coffee, green tea, and herbal teas are calorie-free options compatible with a strict fast, while a balanced breakfast of Greek yoghurt, eggs, or porridge better aligns with the NHS Eatwell Guide when breaking a fast.

For those who find plain water or black coffee insufficient during a fasting window, there are several alternatives that may better align with UK dietary guidelines whilst still supporting the goals of intermittent fasting.

Drinks generally considered compatible with a strict caloric fast:

• Black coffee or green tea: Both provide caffeine and antioxidants without significant calories. Green tea also contains L-theanine, which may support calm alertness. Avoid adding milk, cream, sugar, or sweeteners if maintaining a strict caloric fast, as these will add calories.

• Herbal teas: Peppermint, chamomile, and ginger teas are calorie-free and may help manage hunger or digestive discomfort during fasting periods.

• Sparkling water with lemon: Provides variety and hydration; lemon slices add negligible calories, which is acceptable for most fasting goals but not a strict caloric fast.

For those who genuinely struggle with hunger and are not following a strict caloric fast, a more nutritionally balanced approach to breaking the fast is preferable to butter and MCT coffee. For example, a breakfast containing:

• Unsweetened Greek yoghurt with berries and seeds — provides protein, fibre, and micronutrients

• Eggs with wholegrain toast and avocado — offers healthy unsaturated fats alongside protein and complex carbohydrates

• Porridge with nuts and fruit — a fibre-rich option aligned with the OHID/DHSC Eatwell Guide

If the primary goal is weight management or metabolic health, working with a registered dietitian (find one via the British Dietetic Association at bda.uk.com) can help tailor an intermittent fasting approach that is both effective and nutritionally sound. The BDA also publishes practical Food Fact Sheets on intermittent fasting and healthy eating, which are freely available on their website. Sustainable dietary change is best achieved through evidence-based, personalised guidance rather than trending food practices.

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