Buttermilk During Intermittent Fasting: Key Facts

Buttermilk during intermittent fasting is a common source of confusion: it is nutritious, relatively low in calories and contains beneficial cultures, yet it still breaks a fast. Understanding why this matters, what buttermilk actually contains and how to incorporate it sensibly can help you get the most from your fasting protocol. This article explains the science behind buttermilk and fasting, covers when it is safe to drink it, highlights who should seek medical advice before fasting and outlines any health risks to be aware of.

Does buttermilk break a fast?

Yes, buttermilk breaks a fast. A 200 ml serving provides 80 to 100 kilocalories, along with protein and carbohydrate that trigger insulin release and end the fasted state.

Yes, buttermilk breaks a fast because it contains calories, protein and carbohydrates, all of which trigger metabolic and hormonal responses that end the fasted state. Intermittent fasting works partly by keeping insulin levels low and allowing the body to shift towards fat oxidation.^[5][8] In some protocols and with longer fasting durations, ketosis may also occur, though this depends on the length of the fast, individual carbohydrate intake and personal metabolic variation rather than time restriction alone.

The threshold at which a drink is considered to break a fast is not universally agreed upon and is not defined by any clinical guideline. Some practitioners apply a rough practice preference of fewer than 50 kilocalories per serving as acceptable during a fasting window, but this is not evidence-based and should not be taken as a clinical recommendation. Stricter approaches permit only water, black coffee or plain tea. A standard 200 ml serving of traditional cultured buttermilk provides roughly 80 to 100 kilocalories, along with around 8 grams of protein and 10 grams of carbohydrate, based on UK food composition data (Public Health England, McCance and Widdowson's Composition of Foods Integrated Dataset).^[1] These values vary by brand and preparation, so checking the product label is advisable. Even the more lenient informal thresholds are comfortably exceeded by a typical serving.

It is worth distinguishing between the goals of a particular fasting protocol. Someone fasting primarily for weight management may tolerate a small caloric intake differently from someone fasting for metabolic health, gut rest or blood glucose control. Regardless of the goal, buttermilk consumed during the fasting window should be considered a break in the fast rather than a neutral addition. If you are following a medically supervised fasting plan, speak to your GP or dietitian before making changes.

What is in buttermilk and how does it affect the body?

Buttermilk contains protein, lactose, fat, calcium and B vitamins, and often live bacterial cultures. Its protein and lactose both stimulate insulin release, signalling a fed state and suppressing fat breakdown.

Traditional cultured buttermilk is a fermented dairy drink that contains protein, carbohydrates, fat, calcium, B vitamins and, in many products, live bacterial cultures. It is produced by fermenting low-fat milk with lactic acid bacteria, which gives it a tangy flavour and a thicker consistency than plain milk. A typical 200 ml serving provides approximately 80 to 100 kilocalories, 8 grams of protein, 10 grams of carbohydrate (predominantly lactose) and around 2 grams of fat, based on UK food composition data; exact values vary by brand and preparation, so checking the product label is recommended.

The protein content, primarily casein and whey, stimulates insulin release and is associated with anabolic signalling. Some research suggests that dairy protein intake is associated with raised insulin-like growth factor 1 (IGF-1) levels, though the clinical significance of this in the context of intermittent fasting is not well established.^[3] These effects are relevant to fasting because they signal a fed state to the body. The lactose content similarly raises blood glucose and prompts an insulin response, which suppresses lipolysis (the breakdown of stored fat for energy).^[5]

Not all buttermilk products contain live cultures; some are heat-treated or acidified after fermentation, which destroys the bacteria. If you are seeking probiotic benefit, check the label for a statement confirming live or active cultures. Where live cultures are present, they are typically Lactobacillus species.^[10] Some systematic reviews and meta-analyses suggest that fermented dairy consumption is associated with modest benefits for blood pressure and bone health in the general population, though evidence is not uniform across products, strains or doses, and these associations do not establish causation. Evidence specifically in the context of intermittent fasting is limited. Buttermilk is also a useful source of calcium and phosphorus, which support bone mineralisation.^[1][13] These nutritional qualities make it a worthwhile addition to a balanced diet, but they do not change its caloric status during a fasting window.

Can you drink buttermilk in your eating window without losing results?

Drinking buttermilk within your eating window is compatible with intermittent fasting when consumed as part of a calorie-appropriate diet. Its protein content may support satiety, but total energy intake across the eating window still matters.

Drinking buttermilk within your designated eating window is entirely compatible with intermittent fasting and is unlikely to undermine your results when consumed as part of a balanced, calorie-appropriate diet. The principles of intermittent fasting centre on when you eat rather than prescribing specific foods, so nutritious choices like buttermilk fit naturally into the eating window for most people.

Buttermilk can be a practical choice during the eating window for several reasons. Its protein content is associated with greater satiety, which may help prevent overeating later in the day.^[9] Where live cultures are present (check the label), it may also support digestive comfort. Its relatively modest calorie density compared with full-fat dairy alternatives makes it easier to include without significantly increasing overall energy intake.

Intermittent fasting is not suitable for everyone without medical or dietetic supervision. It is generally not recommended for people who are pregnant or breastfeeding, adolescents, those who are underweight, or people with a history of or current eating disorder.^[8][12] People with certain chronic conditions, including type 2 diabetes managed with insulin or sulfonylureas, face a specific risk of hypoglycaemia when fasting or changing meal timing, and should discuss any planned fasting regimen with their GP or diabetes care team before starting.^[4][6] The British Dietetic Association (BDA) and Diabetes UK both provide patient-facing guidance on fasting and diabetes that is worth reviewing.^[8][11]

Total calorie intake across the eating window still matters for weight management. If your goal is fat loss, consuming buttermilk alongside other high-calorie foods without accounting for the additional energy could slow progress. People following a low-carbohydrate or ketogenic variant of intermittent fasting should also note that the lactose in buttermilk contributes to carbohydrate intake and may need to be factored into daily limits.

For personalised guidance on structuring an eating window that supports your health goals, a registered dietitian or your GP can offer evidence-based advice tailored to your circumstances. Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where clinically appropriate options may be discussed.

Are there any health risks to watch out for with buttermilk?

Buttermilk is generally safe for most adults, but those with lactose intolerance, cow's milk protein allergy, hypertension or diabetes managed with insulin or sulfonylureas should take specific precautions and seek GP advice.

Buttermilk is generally safe for most adults, but there are several considerations worth being aware of, particularly in the context of intermittent fasting.

The most common concern is lactose intolerance. Although the fermentation process reduces the lactose content of buttermilk compared with regular milk, it is not lactose-free.^[2] People with lactose intolerance may experience bloating, flatulence, abdominal cramping or diarrhoea after consuming it. Lactose-free dairy alternatives or calcium-fortified plant-based fermented drinks may be more suitable for this group; check labels to confirm calcium fortification. The NHS provides patient-facing guidance on lactose intolerance, including dietary management.

Those with a diagnosed cow's milk protein allergy should avoid buttermilk entirely, as it contains both casein and whey proteins. Milk allergy can cause severe allergic reactions. If you or someone with you develops symptoms such as difficulty breathing, throat tightening or swelling, dizziness, or collapse after consuming a dairy product, call 999 immediately and seek emergency care. For non-emergency allergy concerns, speak to your GP or an allergy specialist, and refer to NHS or Allergy UK guidance on managing milk allergy.

Buttermilk contains sodium, and some commercially prepared varieties have added salt. People managing hypertension or following a low-sodium diet should check the product label, as sodium levels vary considerably between brands.

For people with type 2 diabetes or impaired glucose tolerance, the lactose in buttermilk will contribute to postprandial blood glucose rises. This is not unique to buttermilk among dairy products, but it is worth monitoring, particularly if buttermilk is consumed in larger quantities. People managing blood glucose with insulin or sulfonylureas face an additional risk of hypoglycaemia when fasting or changing meal timing.^[4][6] If you are in this group, discuss any dietary changes or fasting plans with your GP or diabetes care team before making adjustments. Diabetes UK provides specific patient guidance on fasting with diabetes.

During intermittent fasting, some people experience digestive sensitivity, particularly when breaking a fast. Introducing a fermented drink like buttermilk as a first food after a prolonged fast may cause temporary discomfort in some individuals. Starting with a small amount and observing your response is a sensible approach. If you experience persistent digestive symptoms, unexplained weight changes or any other concerns, contact your GP for assessment.

If you experience a suspected side effect from a medicine you are taking, you can report it to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk.

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