Fasting Hypoglycemia?

Read time: 4.5 minutes

Last week I made myself a guinea pig. But I’ll tell about that toward the end of this post.

As many of you know, I use intermittent fasting (IF) as part of a healthy lifestyle because there are way too many health benefits independent of fat loss and weight management. (You can click on those links, if you wish, but don’t forget to come back to this post!)

From past emails I know many of you also use IF. But a few of you also expressed concerns about fasting hypoglycemia — or more to the point, you don’t want to pass out at the copy machine at work. Which is fair, because I suppose it can be almost as embarrassing as the boss seeing you drunkenly flirt with his wife at last year’s holiday party.

So let’s look into hypoglycemia.

A quick review: Hypoglycemia occurs when blood glucose (blood sugar) drops below normal. Typical symptoms are:

  • Confusion
  • inability to complete routine tasks
  • Visual disturbances, such as double vision and blurred vision
  • Loss of consciousness (though uncommon)
  • Heart palpitations
  • Tremor
  • Anxiety
  • Sweating
  • Hunger

These symptoms seem as much fun as sticking your head into an active beehive. But fear no beehive, or copy machine — our bodies have systems in place to prevent hypoglycemia; therefore, in normally healthy individuals, hypoglycemia is actually very rare.

Your Body Prevents Hypoglycemia, even During Fasting

When blood glucose begins to fall below normal, or during a fast, your body releases glucagon, a hormone that signals the liver to convert stored glycogen, plasma protein and muscle protein to glucose. Glucagon also regulates blood glucose through lipolysis, the breakdown of stored fat. This conversion is called gluconeogenesis (“glucose creation”). Your body also releases hormones like epinephrine, catecholamines, and growth hormone, which help deliver glucose to the brain and muscles for energy.

Wait, Muscle Protein Converts to Glucose? That Can’t be Good, Can it?

Don’t fret — you won’t become girly men or skinny fatties. Muscle protein turnover occurs regularly to meet the body’s immediate needs, and your overall and long-term muscle mass has more to do with complex signaling from exercise stressors and your genome expression. In fact, fasting improves muscle protein uptake, and some studies show this to be especially enhanced after exercise in a fasted state. (Of course, increased protein uptake in muscle doesn’t mean synthesis of actual muscle, but it demonstrates the natural and cyclical occurrence of protein influx and outflow.)

What About Hypoglycemia During Fasting?

During fasting, glucose can be depleted by the body in normal activities and, without food intake for its replenishment, blood glucose can fall. But glucagon, epineprhine, catecholamines, and growth hormone signal for gluconeogenesis, producing more blood glucose. These hormones are especially active about 12 hours into the fast, peaking somewhere around the 16th hour. It is a tightly regulated system that keeps your blood glucose stable.

The release of these hormones may also enhance the mobilization and utilization of stored fat. It would seem this is desirable in those seeking to lose some body fat.

So under a fasted state, these hormones can stabilize your blood glucose and keep you from falling on your face, and help you look good.

What About Hypoglycemia Induced by Exercising in a Fasted State?

The research in exercise hypoglycemia seems to be based mostly on studies of diabetics, and some information exists on exercise hypoglycemia in individuals with glucose metabolism impairment and insulin regulation problems; however, there doesn’t seem to be much research on exercise hypoglycemia during fasting in healthy individuals. And any information found seems to be based on anecdotal evidence, which I’ll sum up here:

It’s assumed that high-intensity exercise might deplete blood glucose faster than that produced through gluconeogenesis. People who experience dizziness and lightheadedness during exercise generally give this explanation.

Intense exercise can quickly deplete glycogen from muscle, which demands replacement from blood glucose. And since there’s only about 5 grams of glucose in the blood (or about 1 teaspoon), you can imagine how this produces hypoglycemia.


Here’s something we need to keep in mind: experienced exercisers can store more glycogen in their muscles, enough to fuel an intense workout at a reasonable length, even in a fasted state. Newbies, however, don’t have the same glycogen storage capacity, which questions their ability to even push themselves to the level of complete glycogen depletion and disrupt blood glucose to the point of hypoglycemia, as energy production for intense (fast-glycolitic) exercise is rate-limited. Additionally, some studies (animal and human) show that intense exercise stimulates gluconeogenesis, providing increased blood glucose to compensate any loss, even in untrained subjects.

Alternative Explanations to Consider

Exercise-induced dizziness and lightheadedness are reported mostly by inexperienced exercisers, often while exercising first thing in the morning without breakfast. This may have 3 potential explanations:

  1. Exercise in a fasted state may require a period of adaptation, by both the experienced and the inexperienced
  2. dizziness and lightheadedness may be symptoms of lactic acid intolerance by the inexperienced
  3. autonomic imbalances in venous and arterial blood flow (blood dumping back into the heart at a faster rate than ejected, suddenly emptying blood in the brain).

I don’t argue against exercise hypoglycemia during fasting, but this is an area that could use more research.

My Experiment

Experiment 1

I did a 24-hour fast (Tuesday night to Wednesday night). Then around hour 21 I exercised intensely for thirty minutes using a Crossfit benchmark exercise called ” Chelsea,” which is 5 pull-ups, 10 pushups, 15 squats, on the minute, every minute, for 30 minutes. It was a tough exercise with fast onset of accumulative fatigue, but no more challenging than usual and I pushed myself as hard as always. I experienced no dizziness or lightheadedness. I waited for 3 hours before eating anything, to see if I would experience late hypoglycemia. Which I didn’t.

Then I ate a 10-ounce New York steak with cauliflower mashed in butter, drank a glass of wine, and later had 2 tablespoons of fish oil and chased it with two persimmons, and a handful of almonds before bed.

Experiment 2

I did another 24-hour fast (Thursday night to Friday night). Then around hour 21, I did heavy back squats, heavy overhead presses, and weighted pull-ups — all 5 sets of 5 reps. Weights used were typical of those from previous workouts. I rested only about a minute between sets. This took less than 1/2 an hour, and I added treadmill walking (3.8 mph) after. I walked for about 35 minutes, got bored, and went home. I waited for a couple of hours before eating.

No dizziness, no lightheadedness.

My Recommenation?

  • Just as you would start your experimentation with intermittent fasting slowly, start exercise experimentation slowly when you’re fasting. Take it easy, and allow a period of adaptation.
  • If you’re unsure whether or not you’ll suffer hypoglycemia, have within reach a banana or a baked sweet potato.
  • Your exercise should be short and intense, not long and drawn-out. Intense exercise may help stimulate gluconeogenesis, which increases blood glucose, but it may not outlast those long and drawn-out exercises.
  • There’s no need to always exercise in a fasted state (fasting is not for everyone), but do know that it is OK and beneficial.

I don’t like to give guidelines for exercise during fasting, because exercise hypoglycemia during fasting may be a rare condition, but that hypoglycemia can be dangerous and sometimes deadly, I feel a post of caution on the subject is justified — especially since I have friends, family, clients, and people I care about reading this blog!

Does anyone have additional information on exercise hypoglycemia during fasting? Please share in the comments.

This entry was posted in Dietary Habit, Exercise and Physical Activities, weight Loss and tagged , , , , . Bookmark the permalink.

9 Responses to Fasting Hypoglycemia?

  1. Torrie C says:

    Awesome post, Johnny. I can’t say I had ever experienced hypoglycemia during fasting, and I fast only on non workout days. But I might try a workout toward the end of a fast, since it sounds like it could add to fat utilization?

  2. Aaron says:

    Probably for over 10 years I train first thing in the morning and have never once felt dizzy or faint. Hypoglycemia has never happened to me, but that’s just after a normal overnight sleep fast, about 8 or 9 hours in total. I’ll have to try working out after a longer fast to see how it affects me. Thanks for the suggestion of having a banana near by.

    I’ve been thinking about intermittent fasting to drop about 5 pounds of body fat, but i want to look more into it first. Have you found that starting out is difficult?

    Thanks for a great blog!

  3. Jake says:

    Art DeVaney says to always exercise fasted. That is the way to maximize growth hormones and thus fat loss.

    And that is what I do with good results.

    • Johnny from The Lean Saloon says:

      Agreed, Jake. Over the past several months, a greater percentage of my workouts have been in a fasted state.

      I believe that exercise during fasting offers great benefits, especially to those with sufficient exercise experience. One of the possible mechanisms that prevents exercise hypoglycemia is the ability to recycle lactic acid as a source of exercise energy. This spares the limited availability of muscle glycogen and glucose in the blood.

      Something to look into.

  4. Pingback: Skip a Meal, Gorge Later? « The Lean Saloon

  5. Jonathan says:

    I began exercising in the fasted state about 3 months ago and have noticed that my performance in my routines has gone up. I subjectively feel “lighter” when playing basketball or lifting weights and (this is the best bonus) I’m not tasting any “pre-workout meal” that might get burped up during exercise. (Sounds gross, but it happens to most people who eat prior to exercise).

  6. Matt says:

    I too have tried working out fasted and find no inherent negatives to it (I sometimes am cold while fasting, but this goes away with the thermal increases from the exercise). Often times and lingering hunger pangs will actually dissipate while exercising, which usually means that I go even longer AFTER exercise before eating again (get a little more GH?). Additionally, I have begun training for a marathon and almost always perform my long runs in a fasted state – 15-18 hours upon beginning to run – with no negative effects, and like Jonathon above, feel better doing so.

    • Johnny says:

      Hi Matt,

      Probably indirectly related, but exercise stimulates gluconeogenesis, increasing blood glucose which may contribute to a temporary alleviation of hunger through the increased availability of fuel to the brain. Hypothesis, of course. But many fasting people have reported decreasing hunger after exercise.


  7. Alysia says:

    I’m not sure why, but immediately after waking up in the morning, often times even before I wake up, I am very very hungry. I’m dizzy until I eat something. This doesn’t happen every single day, but it does happen more often than not. If I ignore it it goes away after a while, as most things do. Any idea whats going on here? I always thought this was normal until a few members of my family told me this never happens to them. Maybe there is an issue with my diet? I’ve tested negative for diabetes.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s