Body odour during intermittent fasting is a common but often unexpected concern for people adopting this eating pattern. When the body shifts from burning glucose to metabolising fat for fuel, it produces ketone bodies and other volatile compounds — including acetone and ammonia — that can alter the smell of breath, sweat, and urine. These changes reflect genuine biochemical shifts rather than poor hygiene, and for most healthy adults they are temporary and manageable. This article explains why fasting affects body odour, when changes may signal a medical condition, and how to manage symptoms safely in line with NHS and UK clinical guidance.

Why Intermittent Fasting Can Change Your Body Odour

Intermittent fasting depletes glycogen stores, prompting fat metabolism that increases excretion of acetone, ammonia, and volatile organic compounds through sweat and breath, altering body odour.

Intermittent fasting (IF) involves cycling between periods of eating and fasting, and whilst it is widely practised for weight management and metabolic health, many people notice unexpected changes in their body odour during fasting windows. These changes reflect genuine shifts in the body's biochemistry and metabolism rather than being merely cosmetic.

During a fasting period, the body depletes its glycogen (stored glucose) reserves. The timing of this varies considerably between individuals depending on prior diet, activity levels, and the specific IF pattern followed — it may occur anywhere from roughly 12 hours onwards. Once glycogen stores are substantially reduced, the body begins to mobilise fat as its primary fuel source. This metabolic transition may alter the chemical composition of sweat, breath, and urine.

Body odour is primarily determined by the interaction between sweat and skin bacteria, but the compounds present in sweat may change during fasting. Substances such as ammonia, acetone, and various volatile organic compounds (VOCs) may be excreted in greater quantities, potentially altering the characteristic smell of perspiration. These changes are generally harmless and temporary for most healthy people, resolving once normal eating patterns resume. However, people with diabetes should be aware of specific warning signs (see below). Understanding why these changes occur can help individuals manage them more effectively and recognise when a change in odour might warrant medical attention.

For general information on body odour and self-care, see the NHS Body Odour page. For dietary guidance on IF, see the British Dietetic Association (BDA) Food Fact Sheet: Intermittent Fasting.

The Role of Ketosis in Fasting-Related Odour Changes

Ketosis during fasting produces acetone, a volatile ketone exhaled via the lungs and excreted in sweat, causing a fruity or nail-varnish-remover smell distinct from the dangerous odour of diabetic ketoacidosis.

One of the most significant metabolic shifts associated with intermittent fasting is the induction of ketosis — a state in which the liver converts fatty acids into ketone bodies (acetoacetate, beta-hydroxybutyrate, and acetone) to serve as an alternative energy source for the brain and body. This is a normal physiological response to prolonged carbohydrate restriction or fasting.

Acetone, one of the three primary ketone bodies, is volatile and is exhaled through the lungs as well as excreted through sweat and urine. This is the primary reason many people on intermittent fasting or ketogenic diets may notice a fruity or slightly sweet-smelling breath, sometimes described as similar to nail varnish remover. The same compound can contribute to a subtly altered body odour, particularly noticeable in sweat.

It is important to distinguish between the mild, physiological ketosis associated with fasting and the dangerous condition of diabetic ketoacidosis (DKA), which occurs in people with poorly controlled diabetes (particularly Type 1 diabetes). In DKA, ketone levels rise to dangerously high concentrations and are accompanied by symptoms such as nausea, vomiting, abdominal pain, confusion, and rapid breathing. The odour in DKA is typically much more pronounced than in dietary ketosis.

If you have diabetes and notice a strong fruity or acetone-like smell alongside any of these symptoms, check your blood glucose and ketone levels if you are able to do so, and seek urgent medical attention immediately — call 999, go to your nearest A&E, or call NHS 111 for same-day advice. Do not wait to see if symptoms improve on their own.

For otherwise healthy individuals practising intermittent fasting, mild ketone-related odour changes are not a cause for concern.

See the NHS Diabetic Ketoacidosis (DKA) page and the Joint British Diabetes Societies (JBDS) guideline on the Management of DKA in Adults for further information.

Other Metabolic and Hormonal Factors to Consider

Ammonia from protein catabolism, potential cortisol-driven sweating, and sulphur-rich foods consumed during eating windows can all contribute to altered body odour during intermittent fasting.

Beyond ketosis, several other metabolic and hormonal changes associated with intermittent fasting may influence body odour. It is important to note that robust clinical evidence specifically linking IF to body odour changes remains limited, and a number of the mechanisms described below are theoretical or inferred from broader nutritional and metabolic research rather than established by direct clinical trials.

Ammonia excretion is one such factor. During fasting, the body may increase protein catabolism — breaking down amino acids for energy — which produces ammonia as a by-product. This ammonia can be excreted through sweat, potentially contributing to a sharper, more pungent odour. The extent to which this occurs varies considerably between individuals and depends on factors such as the duration of the fast, overall protein intake, and activity levels; it is not a predictable consequence of all IF patterns.

Hormonal fluctuations during fasting may also play a role, though the evidence is limited. Fasting is known to influence cortisol levels and insulin sensitivity. There is some suggestion that elevated cortisol could increase sweating, including via the apocrine glands most associated with body odour, but a direct, well-evidenced link between fasting-induced cortisol changes and body odour has not been firmly established. Similarly, changes in gut microbiome composition during dietary shifts have been proposed as a potential contributor to altered odour, but this remains an area of ongoing research without definitive clinical data in the context of IF.

Dietary choices during eating windows can also influence odour in a more straightforward way. Foods rich in sulphur compounds — such as garlic, onions, and cruciferous vegetables (broccoli, cabbage, Brussels sprouts) — contain volatile molecules that may be excreted through sweat and breath. These effects are well recognised and are not specific to fasting.

See the BDA Food Fact Sheet: Intermittent Fasting for further dietary guidance.

When to Seek Medical Advice About Unusual Body Odour

Consult your GP if you notice a persistent fruity, fishy, or ammonia-like odour, especially alongside systemic symptoms such as fatigue, excessive thirst, or reduced urine output, as these may indicate an underlying condition.

Whilst changes in body odour during intermittent fasting are usually benign and self-limiting, there are circumstances in which an unusual or persistent odour may signal an underlying medical condition that requires professional evaluation. It is important not to dismiss significant changes as simply diet-related without appropriate assessment.

Contact your GP if you notice any of the following:

• A strong, persistent fruity or acetone-like odour, particularly if you have diabetes or are at risk of it — this may suggest diabetic ketoacidosis (DKA), which requires urgent medical attention (see above)

• A fishy body odour that does not resolve with hygiene measures, which may suggest trimethylaminuria (fish odour syndrome), a rare metabolic disorder; your GP can refer you to a specialist in metabolic medicine or clinical genetics for investigation

• A sweet, maple-syrup-like odour in an infant or young child, which may be associated with maple syrup urine disease (MSUD) — a rare inherited metabolic disorder that typically presents in newborns or infants; in adults, a new sweet or fruity odour with systemic symptoms is more likely to suggest DKA than MSUD

• An ammonia-like odour on the breath, accompanied by fatigue, swelling, reduced urine output, or other symptoms of kidney disease, which may warrant assessment for chronic kidney disease (CKD) — note that this is a non-specific sign and should be considered alongside other symptoms and risk factors

• Any new or unexplained odour change accompanied by systemic symptoms such as unintentional weight loss, excessive thirst, frequent urination, or persistent fatigue

Some medicines can affect the smell of sweat or breath as a side effect. If you notice a change in body odour after starting a new medicine or after beginning intermittent fasting whilst on regular medication, discuss this with your GP or pharmacist. Suspected side effects from medicines — including changes in body odour — can be reported to the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk.

See the NHS pages on Trimethylaminuria (fish odour syndrome) and Diabetic Ketoacidosis (DKA), and NICE NG203: Chronic Kidney Disease for further information.

Practical Steps to Manage Body Odour During Fasting

Adequate hydration, reducing sulphur-rich foods, daily washing with soap, and using antiperspirant deodorants are the most effective NHS-aligned measures for managing fasting-related body odour.

For most people, fasting-related changes in body odour are manageable with straightforward lifestyle and hygiene measures. The following strategies, aligned with NHS guidance, can help minimise odour whilst maintaining the benefits of an intermittent fasting regimen.

Hydration is an important factor. Drinking adequate fluid throughout both fasting and eating windows supports kidney function and may help reduce the concentration of ketones, ammonia, and other volatile compounds in urine and sweat. The NHS recommends aiming for around 6–8 cups or glasses of fluid a day — this includes water, lower-fat milk, sugar-free drinks, and tea or coffee. You may need more in hot weather or during physical activity. Note that whilst hydration supports overall metabolic function, it has a limited direct effect on breath acetone levels.

Dietary choices during eating windows can significantly influence odour. Reducing intake of sulphur-rich foods (such as garlic, onions, and cruciferous vegetables like broccoli and cabbage) during periods when odour is a concern may help. Ensuring adequate protein intake can reduce the need for amino acid catabolism and thereby limit ammonia production.

Hygiene measures aligned with NHS body odour guidance include:

• Washing the affected areas (particularly underarms and groin) with soap and water at least once daily, and after exercise or prolonged fasting periods

• Applying an antiperspirant deodorant (which reduces sweat production) rather than a deodorant-only product; antiperspirants are more effective at reducing odour at source

• Wearing natural, breathable fabrics such as cotton or linen to reduce sweat accumulation

• Laundering clothes regularly, particularly items worn close to the skin

• Keeping underarm hair trimmed or removed if this is comfortable for you, as hair can trap sweat and bacteria

• Oral hygiene — brushing teeth twice daily and using sugar-free mints or mouthwash can help manage ketone-related breath odour

These measures are generally sufficient for managing mild, fasting-related odour changes and do not require any modification to the fasting protocol itself.

See the NHS Body Odour page and NHS guidance on water, drinks and your health for further self-care advice.

NHS and NICE Guidance on Safe Fasting Practices

NICE and the NHS do not formally endorse intermittent fasting as a first-line intervention, and it requires medical supervision or is contraindicated in people with diabetes, eating disorders, pregnancy, or those on certain medications.

Intermittent fasting is not formally endorsed as a first-line dietary intervention by NICE or the NHS for the general population, though it is recognised as one of several dietary approaches that may support weight management. NICE guidance on obesity (CG189) and weight management lifestyle services (PH53) emphasises individualised, sustainable dietary strategies rather than prescribing specific eating patterns. Clinicians are encouraged to support patients in choosing approaches they can maintain long-term, and IF may be appropriate for some individuals when properly supervised. The BDA Food Fact Sheet on Intermittent Fasting provides a useful UK dietetic perspective on suitability and safe practice.

The NHS advises that intermittent fasting is not suitable for everyone. It is generally contraindicated or requires medical supervision in the following groups:

• Pregnant or breastfeeding women

• Children and adolescents

• People with Type 1 diabetes or insulin-dependent Type 2 diabetes

• Individuals with a history of eating disorders

• Those who are underweight or malnourished

• People on certain medications, including those requiring food for absorption or those affecting blood glucose

In particular, people taking insulin or sulfonylureas are at risk of hypoglycaemia (low blood sugar) during fasting periods and should seek medical supervision before attempting any fasting regimen. Any change to eating patterns in this group should be discussed with a GP or diabetes care team.

If you are considering intermittent fasting and have any pre-existing health conditions or take regular medication, it is strongly advisable to consult your GP before starting. For most healthy adults, routine blood monitoring is not necessary before beginning IF; however, your GP may recommend monitoring of relevant parameters — such as fasting glucose, HbA1c, or kidney function — if you have diabetes, chronic kidney disease, or take medicines that affect blood glucose.

It is also worth noting that rapid or significant weight loss of any kind can increase the risk of gallstones. Gradual, sustainable dietary changes are preferable to very rapid weight loss.

Persistent or distressing symptoms — including significant changes in body odour, dizziness, fatigue, or mood disturbance — should prompt a review with a healthcare professional. Your GP can also refer you to a registered dietitian if you need personalised dietary support. The goal of any dietary intervention should be to support overall health, not to compromise it.

See NICE CG189: Obesity: identification, assessment and management; NICE PH53: Weight management: lifestyle services for overweight or obese adults; BDA Food Fact Sheet: Intermittent Fasting; and NHS Gallstones for further information.

Frequently Asked Questions