Intermittent Fasting Part 1: The Shakes

Read time: 2 minutes

Most of us are probably familiar with “the shakes.”  They happen when we go without food for longer than we’re used to. When the shakes set in, we get irritable, lose patience, and even make careless decisions.

We’ve seen those funny Snickers commercials in which someone inadvertently says or does something utterly stupid because of hunger. Maybe we’ve done the same in our own state of hunger.

The shakes, the mindless dribble, the impatient trembling. They’re the result of low fuel in the brain. “I’ve got to get some food…”

This physical response to diminishing fuel for the nervous system and the brain — blood glucose — is real. Although the body has mechanisms to prevent blood glucose from dropping below the physiologic safe point, glucose does fluctuate and this drop can affect the brain. (And the words that come out of the mouth.)

In the absence of food, glucagon, epinephrine and other catecholamines trigger the release of fatty acids from fat cells. While peripheral tissues like those of the muscles and the heart can use fatty acids, the brain cannot.

But there’s another source of fuel for the brain — ketone bodies. Ketone bodies are the byproduct of fatty acid metabolism.

But even with the small amount of ketone bodies produced naturally from fatty acid metabolism (e.g. from a missed meal, or during sleep), the typical brain remains inefficient at using ketone bodies for fuel. The frequent-meal pattern typical in North America acclimates the brain to using the available glucose as the primary source of fuel, instead of ketones.

So thus the shakes, the stupid slip of the tongue, the left turn instead of the right, when blood glucose begins to diminish.

However, given the chance, the brain undergoes metabolic changes to adapt to this decreased availability of glucose. A major change in the brain is the upregulation of enzymes that help to metabolize ketones. The brain can learn to use ketone bodies for fuel.

Many of you are probably thinking that a very-low-carb diet can produce this adaptation to a fat-based fuel source. It takes a couple of weeks, yadda yadda. You’re right. But a very-low-carb diet sucks, and my wife and I love our sweet potato fries.

The other option may be the regular use of intermittent fasting (IF). A period of fasting decreases blood glucose and causes the body to mobilize and metabolize fatty acids to meet energy demands, thus producing ketone bodies.

Given a regular exposure to intermittent fasting, the brain may adapt to metabolizing ketone bodies for fuel. So even if you eat carbohydrates during your feeding windows, the upregulation of ketone-metabolizing enzymes may help to continue fueling the brain even when food is not available — such as when you miss a meal or during intermittent fasting.

As discussed frequently here and on other blogs, intermittent fasting has its many benefits. Now you can add sustained brain function even in the prolonged absence of food intake.

With regular use of intermittent fasting, there may no longer be a need to eat a Snickers bar, or to inadvertently say something utterly stupid.

What has been your own experience, before the regular practice of IF and after?

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22 Responses to Intermittent Fasting Part 1: The Shakes

  1. Trinity Selle says:

    Great blog and great post!

    I started intermittent fasting around this past Christmas, mostly because of your blog site. (I wanted to enjoy the holiday celebrations without gaining weight) I remember the first few fastings requiring more effort to get through. Now? I don’t even think about it. Until this post, I hadn’t thought about the stark difference between how I used to feel before adopting IF and how I feel now doing it. I literally don’t even think about it, it’s so easy.

    The result wasn’t just in not gaining weight over the holidays but also that I haven’t weighed this little since my early college years. I now consistently feel so much lighter and healthier. If I could only tell you how many diets I’ve started and abandoned in less than a month! I’ve been doing IF now for 5 months, without even realizing it.

    I don’t think I have ever posted here before, much less tell you how much I appreciate your blog. So… THANK YOU so much for being so influential on my eating habit. I’m more healthy and look better for it!


    • Johnny says:

      Congratulations, Trinity. I can only say that IF gets easier and easier… as you have personally discovered. It becomes just an easy habit, slotted quietly into the background while you move forward living your life.


  2. Chris says:


    During your daily non-fasting eating window, how many calories on average would you say you consume?


    • Johnny says:

      Hi Chris,

      It’s hard to say. But I’d guess my calorie intake during any particular feeding window can fluctuate between 1,000 all the way up to 3,000, depending on circumstances like being too busy or celebrating something, respectively.

      But mostly I stay comfortably toward the low end, probably averaging around 1,600 to 1,900. I weigh 150 pounds these days, engage in 2 to 3 formal exercise sessions a week, and stay active through most days.


  3. Kevin says:


    always hungry, thinking about food, stomach problems because it never got a break, nearly pass out if I didn’t eat something quick enough of heavy workout days (almost passed out driving home from work one day!), weight loss stalled, ate too much, etc


    feels completely natural, don’t even think about it, never feel hungry, feel light, can’t even “feel” my stomach anymore, tons of energy that’s balanced all day, eat a lot less and tuned in to “real” hunger which is less frequent as it should be …. it’s like everything has dropped into place and running like a quiet machine (a quiet fat burning machine!)

  4. Rami says:

    How many days per week are you all fasting? I am just starting this, and while I am not convinced the research is very strong to support it, I am willing to give it a try, mostly from a spiritual standpoint…….

    • Kevin says:

      Just about every day up until the “workday” is over…sometimes on the weekend I’ll eat lunch with my wife if that’s how things work out.

      For me it was easy to start by skipping breakfast. It felt completely natural & I felt better throughout the day.

      Then I started going longer and it’s been no big deal at all. The increase in alertness, energy, & cut-back stress is awesome!

      The “hardest” part has been getting used to never really being hungry. A bit strange after years of eating frequently throughout the day… but when the stomach says NO WAY, it means it! haha

    • Johnny says:


      I agree that more research is needed on IF, but there are quite a few studies already done on fasting at varying length and at different intermittent periods. It would help to be more specific with what it is you’re looking at in the research.

      Anecdotal experiences by thousands of people through history and religions tell us that IF can generally be safe as part of a lifestyle. And for those using IF to get lean, the evidence is plenty. You can check out some real-world examples from reading about Martin Berkhan’s clients over at Lean Gains (just look under the “Physique” tab).

      As for my own IF style: I typically eat only after 4 PM, and often my last meal is right before bedtime. I call this particular eating regimen “Eat After 4″… or EA4. It’s similar in concept to Fast-5 or Warrior Diet, and the fast/feed windows to be similar to Lean Gains.


  5. Jessica says:

    The only problem for me is…I can’t survive I.F. without coffee and heavy cream. I don’t know how to NOT eat without a little heavy cream in me.

    • Johnny says:

      Hi Jessica,

      Heavy cream (or fat) is satiating. Perhaps this helps with your IF. But one of the reasons many people do IF is to reduce overall calorie intake — keep this in mind as you put cream into your coffee. As for me, I still use a little cream in my coffee during the fasting period, and I feel that I still enjoy most of the benefits of IF while still enjoying my coffee.


  6. Kat Eden says:

    I lost focus in the article after ‘sweet potato fries’ …. mmm ….

  7. Sarah says:

    Hi! I am so glad I found your website! A few questions. The shakes you referred to: I also viewed those as a touch of hypoglycemia. Is this so and if so, is it “fixable” by eating mainly whole foods and less processed foods during the feeding period? With the exception of an occasional treat. 🙂 I had my blood sugar tested and fasted it was about 69 or 68, which I was told was on the low side, but not too low. I would be like Jessica and need some coffee in the am, but take it with 1 tbsp creamer, some sweetner and I’m done. I HATE breakfast and have been trying low carb or basically a modified primal/paleo diet. But I do not care for it on a regular basis. I do not like breakfast foods except on a special occasion so I don’t see me keeping this up as a lifestyle. Plus, I do love sweet potato fries, too!! And another question: I lift weights 4 days a week and hit up other classes, step, yoga, and I want to eventually do a half-marathon. You have said before about not being a strict counter of cals, fat, protein, and this is seriously the lifestyle for me! I always quit when I have to count stuff. It’s miserable and hurts my social life! 🙂 I want to lose weight but don’t want to lose lean muscle mass. I don’t really care a ton about building up more, at least not right now. I just want to lean up as my first goal. How concerned should I be with the 1g protein per lb? That’s 163 gms and a LOT to fit into a short eating period, especially when I get sick of meat easy. Thanks! I so love your website and sorry I wrote so much!

    • Johnny says:

      Hi Sarah,

      Intermittent fasting takes getting used to with some people. From observation and communication with those who adopted IF, the time through which the metabolism efficiently switches from glucose metabolism to fat metabolism can vary depending on the individual, and may have a lot to do with how they’ve been eating for a while, their metabolic differences, and maybe even genetics (but I’m not sure on the last one). In any case, with patience and time, everyone loses the shakes and begin to feel much better than they ever felt before, now with a constant supply of a fuel source (fat).

      You don’t have to worry about losing muscle on fast, as far as a minimum amount of protein intake is met for muscle, organ, and hormonal support… and this amount is a lot lower than the mythical 1g per pound of body weight.

      I average probably about 80 to 100 per day. Some days way more, some days way less — but over a period of a week, I get what my body needs in protein.

      Muscle mass is maintain or increased through a sufficient resistant training program. Gene expression for muscle mass is dictated by mechanical demand. Your body keeps the muscle it needs, loses what it doesn’t.


  8. Sarah says:

    Thanks so much! I appreciate it. Pardon me while I try to learn more about this! Do you have a how-to article here or recommend a book? Just wondering how often I should do this! A free e-book was recommended to me for doing a daily 19 hour fast, basically only having a feeding hour from 5 pm til 10 pm everyday. I didn’t know if that would be a little too much or not, considering the amount of exercise I sometimes do! The author of that book talked about the body using ketones, which becoming ketogenic scares me a bit. I didn’t know if it was natural for all IFing to be in a ketogenic state (or that it was possible if you were still eating). Thanks again!

    • Sarah,

      Here’s a quote from Johnny in the comments section of another post:

      “I keep my IF generally flexible, but most of the time I follow the Warrior Diet pattern, eating my first meal around 4 or 5 PM. Although the Warrior Diet includes some food during the day (light fruits, eggs, etc.), I usually don’t consume anything but a couple cups of coffee with some heavy cream.

      I also am influenced heavily by Lean Gain’s fasting window — 16 hours of fasting. Except, I sometimes fast between 16 to 18 hours.

      Another IF pattern I’m influenced by is Fast-5, in which the first meal is eaten at 5 PM and the last at 10 PM. Although my first meal is around 4 or 5 PM, I usually eat my last meal right up to before bedtime, and that could be 10, 11, or midnight.

      To give you a brief but more concrete structure of my fasting philosophy:

      -Eat later in the day, like around 4 or 5PM, when eating most often counts.
      -Later in the day is when you get to sit down with friends, family, and celebration, and enjoy really good food without rushing. Oh, and happy hour usually happens later in the day!
      -Later in the day and into the night is when I tend to “just eat.”
      -Fasting in the morning and through the day makes sense as fasting increases the sympathetic nervous system for wakefulness and energy, allowing for work, play, and mental focus.
      -Eating in the evening stimulates the parasympathetic nervous system for relaxation and higher quality sleep.
      -Fasting for 16 to 18 hours means that the digestive system is resting and the body in greater fat metabolism for more than half of the time in a 24-hour period. In simplified terms, you’re either storing consumed energy (fed state) or burning stored energy (fasted state) — I like to make sure there’s a balance.”

      If you are going to do it you should cut back on your workouts. They should be quick & infrequent. There is no need for long, frequent workouts when eating this way. Good luck.

    • Johnny says:

      Hi Sarah,

      Ketosis and keto-acidosis are two different states, the latter a complication typically limited to diabetics.

      Ketone bodies are the by-product of fatty acid metabolism, used for fuel serving the brain and nervous system. A natural process.

      The better and more efficient your body can switch between glycolysis and ketosis, the more constant your energy. Think of a hybrid car that swtches between energy states. In this case, you readily switch between burning sugar and fat.

      Very low carb diets trains your body to do this. The other option is IF, which also trains your body to do this…but without having to eliminate all carbs (I love my sweet potato fries!)


      • Sarah says:

        Thanks to both you and Chris! Is it really necessary to cut down on workouts if I do IF? Especially if I’m getting enough calories in? I really enjoy most of my workouts (I’m definitely willing to cut out some of it, i.e. the cardio on machines which I despise). But I love going to step class, which is an hour, or doing weights 4 days a week. I am almost 29 and a big goal of mine is to complete a half marathon running (I recently completed one, but was so new to running I had to walk the majority of it). Thanks so much for all the wonderful responses. I am so sick of struggling with my diet and it does seem like IF would be right for me, just breaking the traditional rules and skipping breakfast or sometimes lunch on a non workout day. I could even condense workouts on a day with less fasting time, if that would possibly work. Thanks again for all the patience!

      • Johnny says:

        Hi Sarah,

        It’s a pleasure to help.

        You don’t need to cut back on the exercises you enjoy. Just know that “more” is not needed for good health and body composition.

        Intermittent fasting really is a simple way to reduce calorie intake and teach the body to metabolize fat more efficiently. Keep IF simple and flexible to meet the demands of your life.


  9. Johnny is right.

    I’d add also that possibly Brad Pilon’s Eat Stop Eat might be the better way to go. In any case, it’s another option.

    That would be one or two 24 hour fasts a week (most do just one).

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