Keto Diet 101: Bad Breath

When you begin making ketones, bad breath happens.
Yes. This is true. Fumes of acetone pollute your breath when you first switch to the chemistry of ketones.

Keto- bad breath goes away. When?

For most people, bad breath lasts for about a month as they adapt to ketosis. The odor usually improves over time as blood ketone levels stabilize in the second or third week.

When your body uses ketones, the surplus ketones escape out the air you breathe. Unused ketones flush out of the body as acetoacetate in the urine or acetone through your breath.

Acetone breath is a dead giveaway you’re burning fat for fuel. Acetone smells weird causing metallic- or fruity-scented breath. Surprisingly, many don’t experience this problem at all. Others only have acetone breath for a few days while others can smell the ketones coming out of their sweat and their breath for months on end.

For most people, bad breath lasts for about a month as they adapt to ketosis. The odor usually improves over time as blood ketone levels stabilize in the second or third week.

Remember: acetone ends up in your breath when lots of extra ketones circulate in your blood. When first transitioning to ketosis, your blood can fill with a whole bunch of ketones waiting their turn to be used as fuel. As your cells ‘remember’ how to use this kind of fuel, more and more cells turn on their ketone-burning parts. At first, only a few cells burned ketones, but by the end of the third or fourth week, many cells throughout the body can burn either ketones or glucose.

With that improved army of ketone-furnaces inside your mitochondria, less excess compounds float around and in turn fewer acetone fumes out of your mouth. Gradually, your body burns ketones as fast as you make them. With less excess ketones, your breath won’t stink of acetone.

To Deal With Acetone Breath, Try The Following:

#1: GET YOUR TEETH CHECKED. I found it helpful to remind patients that we evolved using ketones. The human race did not evolve eating starches and sugars. Today’s diet fills your mouth with a sugar-bath. It leaves your teeth with tiny pockets where sugar feeds bacteria and other microbes. These hidden cavities allow smelly bacteria to flourish. Cavities and their bacteria are the sources of the long-standing bad breath associated with ketosis.

Please note that the longer you bathe your teeth in ketones, the stronger and healthier your teeth get. Your ancestors kept their teeth for a lifetime by constantly circulating ketones in their blood and spit. Thanks to their high-fat diet, this continued exposure to ketones strengthened their teeth while fighting the bacteria living in their mouths.

#2: DRINK ENOUGH WATER: Dehydration leaves your mouth dry. Mouth dryness concentrates the power of the bacteria living in your oral cavity. Proper hydration flushes and continually washes your mouth while also diluting the places where bacteria live.

Also, when you drink enough water, you flush ketones from your blood into your urine. You only breathe out acetone when your blood is overflowing with extra ketones. Staying properly hydrated allows your body to get rid of excess ketones through your kidneys. It takes the pressure off your lungs to process ketones and breathes them off as acetone.

#3: STAY THE COURSE. The best way to keep that acetone fuming out of your mouth is to go in and out of ketosis. Steady ketone production allows the system to adapt and stabilize fully. Stabilize in this context means that the production of ketones matches the burning of ketones. There will never be a perfect match between these two parts of the equation, but they should get rather close once you have hit your keto-rhythm. The way to mess up this equation is to go in and out of ketosis. That leaves the extra acetone molecules stinking up your breath.

Hungry for more information on the ketogenic diet? Check out the book ANYWAY YOU CAN on Amazon or Audible by Annette Bosworth, MD.


  1. Anderson JC, Measuring breath acetone for monitoring fat loss: Review. Obesity (Silver Spring). 2015 Dec;23(12):2327-34. doi: 10.1002/oby.21242.PMID: 26524104
  2. Toth, Csaba, and Maria Schimmer Zsofia Clemens. “Complete Cessation of Recurrent Cervical Intraepithelial Neoplasia (CIN) by the Paleolithic Ketogenic Diet: A Case Report.” Journal of Cancer Research and Treatment, vol. 6, no. 1, Apr. 2018, pp. 1–5., doi:10.12691/jcrt-6-1-1

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