Not So Fast: Common Symptoms of Adapting to Intermittent Fasting

DietThe body is an amazingly efficient machine utilizing multiple forms of energy to fuel itself.  The body’s most accessible form of fuel, is glucose – simple sugar molecules.  For most people who consume a diet high in carbohydrates and eat these several times a day, their machine is running mostly on glucose that is constantly being replenished through breakfast, lunch and dinner.  During fasting (at around 12 hours), when the body has depleted its glucose stores, it turns to utilizing stored fat for fuel.  When the body becomes adept at making the switch from mostly burning sugar (during the most common eating schedule of three meals a day) to mostly burning fat and sometimes burning sugar (during a fasting schedule) we have successfully transitioned from being a “sugar burner” to being “fat-adapted.”

Of course a lifetime of being sugar burners means that the transition to primarily using fat for fuel usually brings with it unpleasant symptoms.  Being aware of these common symptoms, assessing their severity and frequency and determining our tolerance for fasting, helps us make an informed decision on which fasting protocol is right for us.  Here are a few symptoms you may experience in the process of becoming fat-adapted:

  • Light-headedness or dizziness
  • “Brain fog” or inability to concentrate
  • Changes in vision such as blurry vision
  • Insomnia
  • Headaches
  • Nausea
  • “Hanger” – irritability due to hunger

Unfortunately this is not an exhaustive list, and you may experience other symptoms as each person’s body is different.  Nevertheless, it’s important to understand that, as with any lifestyle change, some discomfort is a normal part of transition and adaptation.  The bottom line: listen to your body and trust your instinct.  You may need to slow down, drink some water, and yes, even eat something then re-evaluate your protocol.  Most people who have successfully transitioned to a lifestyle of daily fasting have done so after some trial and error with at least a few fasting schedules.  With the right information and a commitment to listening to and understanding your body, you, like many before you, will successfully live the IF lifestyle and thrive in it.



Hi, I'm Mimi and I've lost 70 pounds (so far) through daily intermittent fasting or "scheduled eating." I'm committed to the fasting lifestyle and to making it as easy and fun as possible. (Yes, I said fun!) If you're on the fasting journey with me or if you're contemplating it, you've found the right place for information, tips, and support. You may also want to follow me on Twitter @foodcanwait or join us on Facebook at

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Do you have to be fat adapted to intermittent fast? I don’t enjoy eating a high fat diet and prefer to eat whole food plant based, but want to intermitten fast.


Thanks for the information!