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Fitness for Body, Mind and Spirit

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Fitness for the body, mind & spirit

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Thoughtful Thursday…Is intermittent fasting leaving you in a brain fog? Learn why and what you can

Welcome to Thoughtful Thursday. This GreenNote Fitness newsletter mindfully gathers and distills useful information that is supportive to our journey. It is my mission to educate, inspire, and propel you into action that moves you towards your goals and life of purpose. Take control of your journey today.

If you have been reading the GreenNote Fitness newsletters for a while then you know that I’m a huge fan of intermittent fasting for its many benefits; for fat loss and longevity; and for getting results.

One of the many benefits of intermittent fasting is mental clarity and brain function. As Dr. David Perlmutter explains:

Another benefit of fasting is the stimulation of BDNF, a protein that plays an integral role in stimulating the growth of new brain cells and the performance of existing neurons. BDNF is best described as the brain’s “growth hormone,” and stimulating its production is easily one of the best things you can do for your body and your brain.

For some individuals just starting out with intermittent fasting leaves them feeling lethargic and with brain fog.

Today we will look at what may be causing brain fog and the feeling of low energy. Next we will look at ways to get your brain and body operating optimally. We begin with an understanding of what’s happening in the body when we fast.

Why you may experience brain fog when intermittent fasting

If intermittent fasting has so many benefits, including mental clarity and reducing brain fog, then why would one be experiencing it while intermittent fasting? It may be that the body hasn’t yet adapted to switching from burning glucose (sugar) for fuel to burning fat instead.

A common goal of intermittent fasting is to make your metabolism more flexible so that you can burn more fat. This “flexibility” means that your body gets used to switching from burning glucose (sugar) for fuel to burning fat to provide your body with energy. When we are eating every three or four hours, snacking throughout the day, the body looses the practice of utilizing the fat for fuel.

So if your metabolism is still working on being “flexible”, you may feel like you don’t have energy or may not have the mental clarity that you are familiar with. Given time, your body will adapt to using fat for fuel instead of sugar. Utilizing intermittent fasting will actually help the brain.

When we fast, it gives the body a break from digesting and allows it to focus on other things - like the brain. As Dr. Gundry explains, “The brain needs huge amounts of blood flow. Digestion is incredibly energy-expensive and we divert all of our blood flow to our digestive system.”

Now we know that by switching to different fueling systems, moving from the body relying on glucose (sugar) to fat for fuel may take some getting use to. Let’s look at what can help the brain function optimally in this transition as well as a for every day operation.

What to eat for a healthy brain and reduce brain fog

For overall healthy brains and reducing brain fog, we want to be sure when we do fuel our bodies it’s with something that will be beneficial. As best-selling author of Head Strong: The Bulletproof Way to Activate Untapped Brain Energy in Just Two Weeks, Dave Asprey explains what to eat to fuel the brain.

Low-carb, High-fat Diet

"When you eat a diet high in healthy fats (think grass-fed meat, wild fish, pastured eggs, avocados, and grass-fed butter), and few carbs, you teach your body to burn fat instead of glucose (aka sugar) for energy. Instead of using glucose for fuel your body switches to burning fat for energy. This puts you into a metabolic state known as ketosis. Not only does ketosis accelerate weight loss and reduce inflammation in the brain, it also helps the brain produce more mitochondria, the power generators within cells.[1] More energy in your cells equals more brain power.” ~

Now we know optimally fueling our body gives our brain more power, let’s look at some other tips to help you with your intermittent fasting.

Fasting expert and author of The Complete Guide to Fasting, Dr. Jason Fung suggests the following for fasting:

  • Drink water

  • Drink coffee or tea

  • Stay busy

  • Follow a low-carb diet between fasting periods. This reduces hunger and makes fasting easier. It may also increase the effect on weight loss and type II diabetes reversal

  • Don’t binge after fasting

  • Give yourself one month to see if intermittent fasting (such as 16:8) is a good fit for you

Lastly, I will leave you with some suggestions for fasting from my mentor and best-selling author, Ben Greenfield. You will notice again how important proper eating of healthy fats to get the body ready to switch from glucose to using fats for fuel. Snacking on processed foods full of carbohydrates and sugar impair our ability to burn fat. Let’s take a look at Ben’s recommendations.

Preparing for your fast - you want to enhance your body’s ability to burn fat and go for longer stretches without food

Ease into mild ketosis: you’ll want to improve the body’s fat-burning capability and shift away from relying on glucose for fuel. Focus on limiting your carbohydrate intake and increasing your intake of healthy fats. Your meals should be primarily composed of vegetables and protein, supplemented with healthy fats like avocado and coconut. Your only carbohydrates should be incidental carbs from non-starchy vegetables and plant fats, ideally eaten at dinner or post-workout.

Restrict your feeding window: If you are preparing for a moderate to advanced fast {multiple-days}, during your preparation week try to restrict your feeding window to 12 hours per day. For example, if you have dinner at 7:00 PM, eat breakfast at 7:00 AM the next day (or later). This will help ease the body into mild ketosis and get it accustomed to time without food.

Hydration: staying hydrated is very important during your fast. You may feel hungry or slightly lethargic. Pay extra attention to hydration and make sure you are consuming plenty of electrolytes to stay energized and ward off hunger pangs.

If you are experiencing low energy or brain fog while fasting, do give your body time to adapt to it. We want to make sure we have plenty of water and eating foods that will support us. A quick way to derail your efforts is to consume copious amounts of carbohydrates / sugar. Try fasting on non-consecutive days. Follow these suggestions and do give it time. The benefits are very rewarding for the brain and the body.


  • Understand that our body utilizes different forms of fuel: glucose (sugar) and fat. It may take time for your body to switch to using fat for fuel; give it time

  • For proper brain function, we need to consume healthy fats (fish, eggs, avocado, grass-fed butter)

  • If new to fasting, ease into mild ketosis

  • Hydration is very important during your fast

Enjoy your journey,

Lisa Schaffer

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Did you miss last week’s newsletter on 3 benefits of combining weight training and cardio...?

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