Fasting Odors: Why do people smell when fasting?

October 2, 2025 •

5 min read

During fasting, your body enters a metabolic state called ketosis, which can cause a distinct, temporary change in body and breath odor. This shift explains why do people smell when fasting, addressing the metabolic processes and physiological responses that cause the phenomenon.

Quick Summary

Fasting can cause bad breath and body odor due to metabolic changes like ketosis and reduced saliva production. The body produces ketones, particularly acetone, which are released through breath and sweat. Proper hydration and hygiene are key management strategies.

Key Points

Ketosis causes odor: Fasting puts the body into ketosis, where it burns fat for fuel, producing ketones like acetone that are expelled through breath and sweat.
Dehydration reduces saliva: Fasting can lead to dry mouth from decreased saliva, which allows odor-causing bacteria to multiply unchecked.
Acetone breath and sweat: The ketone acetone creates a distinct fruity or nail-polish-remover smell in both the breath and sweat, known as 'keto breath' and 'keto body odor'.
Hygiene and hydration are key: Regular and thorough oral hygiene, including brushing and tongue scraping, combined with ample water intake during non-fasting periods, can manage and reduce odor.
Consider excess protein: A diet too high in protein can lead to ammonia-smelling sweat, a byproduct of protein metabolism, which can be mitigated by adjusting macronutrient ratios.
DKA is a medical risk for diabetics: While nutritional ketosis is normal during fasting, a fruity odor can indicate diabetic ketoacidosis (DKA) in diabetics, which requires immediate medical care.

The Metabolic Shift: Ketosis and Odor

One of the primary reasons for changes in body odor and bad breath during fasting is the metabolic state of ketosis. When food, particularly carbohydrates, is restricted, the body's primary energy source (glucose) becomes depleted. As a result, the body begins to burn stored fat for fuel. This process, known as ketosis, produces chemicals called ketones as a byproduct.

There are three main types of ketone bodies: acetoacetate, beta-hydroxybutyrate, and acetone. Acetone, being volatile, is expelled from the body through various routes, including the breath, sweat, and urine. It is this acetone release that is responsible for the characteristic fruity or nail-polish-remover smell associated with fasting or low-carb diets. The intensity of this odor can vary from person to person, and it is a clear sign that the body is effectively using fat for energy.

How Ketones Cause Smell

Breath: Acetone is exhaled through the lungs, causing a distinct, sometimes sweet or metallic, breath odor, often referred to as 'keto breath'.
Sweat: Ketones are also released through sweat glands in the skin, contributing to a change in overall body odor. This is particularly noticeable during the initial adaptation phase to fasting.
Urine: Excess ketones are eliminated through urine, which can give it a strong, unusual smell.

The Role of Dehydration and Dry Mouth

Beyond ketosis, dehydration is another major factor contributing to bad breath while fasting. When you are fasting, especially from both food and water, your body produces significantly less saliva. Saliva is crucial for oral health as it helps wash away food particles and bacteria, neutralize acids, and fight off infection.

Impact of Decreased Saliva

Bacterial Growth: With reduced saliva flow, odor-causing bacteria can multiply more rapidly in the mouth, leading to the buildup of foul-smelling volatile sulfur compounds (VSCs).
Exacerbating Ketone Odor: The dry environment in the mouth can make the acetone from ketosis more concentrated and noticeable.
Risk of Oral Issues: A consistently dry mouth, or xerostomia, can increase the risk of cavities and gum disease if oral hygiene is not carefully maintained.

Other Factors Influencing Fasting Odor

While ketosis and dehydration are the primary culprits, other elements can play a role in how a person's odor changes while fasting.

Digestive Changes

The shift in your eating pattern during fasting can impact your digestive system. Increased stomach acid due to long periods without food can contribute to bad breath, especially in individuals prone to acid reflux. Additionally, changes in the balance of gut bacteria may also contribute to altered body odor.

Excess Protein Intake

For some individuals, especially those combining fasting with a low-carb diet, a diet that is too high in protein can cause an ammonia-like smell in their sweat and breath. This occurs because the body breaks down protein into amino acids, and the metabolism of excess protein produces ammonia as a byproduct. The liver converts ammonia into urea for excretion, but an excess can be released through sweat.

Strategies to Manage Fasting-Related Odor

Fortunately, there are several simple strategies to minimize or manage the odor changes that come with fasting, especially during the adaptation period.

Stay Hydrated: Drink plenty of water during your non-fasting hours to combat dehydration and promote saliva production. This helps to flush out ketones and keep your mouth moist.
Prioritize Oral Hygiene: Brush your teeth at least twice daily and use a tongue scraper to remove bacteria and debris that cause odor. Flossing is also critical for removing trapped food particles.
Rinse Your Mouth: If your fast permits, rinsing your mouth with water frequently can help keep it fresh, even when brushing isn't possible. An alcohol-free mouthwash can also be effective.
Avoid Strong Smelling Foods: During your eating window, avoid strong-smelling foods like garlic and onions that can cause lingering odor.
Eat Fiber-Rich Foods: Incorporate plenty of high-fiber fruits and vegetables into your meals to help with digestion and support a healthy gut microbiome.
Adjust Protein Intake: If an ammonia smell is present, consider slightly moderating your protein intake and ensuring you are getting sufficient fats, as your body may be breaking down too much protein for fuel.

A Comparison of Fasting vs. Standard Diet Effects

This table contrasts the metabolic and odor-related effects of fasting versus a standard, carbohydrate-rich diet.


Feature	During Fasting	Standard Diet
Primary Energy Source	Stored fat (via ketones)	Carbohydrates (glucose)
Main Odor Cause	Ketone production (acetone)	Bacterial activity on food particles
Characteristic Smell	Fruity, sweet, or nail polish remover-like	Often sulfurous, depending on diet
Saliva Production	Decreased (prone to dry mouth)	Maintained (dependent on hydration)
Management	Hydration, oral hygiene, managing ketones	Regular brushing, flossing, balanced diet
Underlying Concerns	Nutritional ketosis is temporary; DKA is a medical emergency for diabetics	Oral diseases like gingivitis

When to Consult a Doctor

While fasting-related odor is typically temporary and a sign that the body is adapting to a new metabolic state, there are instances where medical advice is warranted. If you have diabetes and experience fruity or acetone-like breath, it could be a sign of diabetic ketoacidosis (DKA), a serious condition that requires immediate medical attention.

If you don't have diabetes but your body odor or bad breath is severe, persistent, and not improving with basic hygiene and hydration, it is wise to consult a healthcare provider. An underlying health condition such as liver or kidney disease could be the cause.

For more information on the safety and side effects of ketosis, a relevant resource is provided by Healthline: Is Ketosis Safe and Does It Have Side Effects?.

Conclusion

Understanding why people smell when fasting is key to managing this common side effect effectively. The primary drivers are the metabolic state of ketosis, which releases ketones like acetone through breath and sweat, and reduced saliva production caused by dehydration. These effects are typically temporary as the body adapts to burning fat for fuel. By focusing on consistent hydration and excellent oral hygiene, individuals can significantly mitigate the unpleasant odors and continue their fasting regimen with confidence. For those with underlying health issues like diabetes, monitoring symptoms and consulting a medical professional is crucial to rule out more serious complications.

Frequently Asked Questions

The main cause of bad breath during fasting is ketosis. When the body burns fat for energy, it produces ketones, specifically acetone, which is released through the breath and causes a distinct odor.

Yes, fasting can affect body odor. Ketones produced during ketosis are released through sweat glands, which can give sweat a different, sometimes fruity or smoky, smell.

Fasting, especially without water, leads to decreased saliva production. This dry mouth environment allows odor-causing bacteria to multiply rapidly, exacerbating bad breath.

For most people, the odor-related side effects of ketosis are temporary. As the body becomes more efficient at using ketones for fuel, the unpleasant smells typically diminish over a few weeks.

To prevent bad breath, stay well-hydrated during non-fasting periods, practice excellent oral hygiene including brushing your tongue, and use an alcohol-free mouthwash.

An acetone-like smell is a normal side effect of nutritional ketosis during fasting or on a low-carb diet. However, for individuals with diabetes, it can be a warning sign of diabetic ketoacidosis (DKA), which is a medical emergency.

If your fasting rules allow it, chewing sugar-free gum can help stimulate saliva production and temporarily mask odors. Look for gum containing xylitol, which helps combat odor-producing bacteria.

An ammonia smell in sweat during fasting is often linked to excessive protein intake. When protein is broken down for energy, it produces ammonia as a byproduct, which is then released through sweat.