Continuous Glucose Monitoring During Exercise

Part 1 in this series

Part 2 in this series

When exercising, your body will draw on two types of fuel, glycogen (from carbohydrate) and/or fat.  Ketones can supply some substrate for work, but that is a topic for another day.  In general, easier efforts draw primarily on fat for fuel.  These are known as “aerobic” efforts.  More intense “anaerobic” training burns glycogen.  There is a point at which a person switches from primarily burning fat to primarily burning glycogen.  This is known as a “crossover point”, and it occurs at different efforts for different athletes.  We all have fat stores which can provide many, many hours of fuel for aerobic efforts, but glycogen stores are limited.  The more judiciously those glycogen stores are used, the more you’ll have left for efforts later in training. When you deplete those stores, you will “bonk”.  But sometimes an athlete “bonks”, only to find that glycogen stores are not empty.  There are many factors that influence this.  It’s hard to say you outstripped your glycogen stores if your blood sugar is 140, and only with something like a CGM can you know this data.


Let’s look at two different training sessions during which I wore the CGM.  The first is a 3+ hour bike ride with some intense efforts and a “race” up a 4 mile climb.  The other is a long run (for me…12 miles) where I started the run having fasted for more than 14 hours, then maintained an easy pace without fueling for over 90 minutes.

I knew the ride would be a “spirited” group ride, so I fueled ahead of time for an effort which I presumed would be highly glycogen-dependent.  (Remember, more intense efforts use more glycogen and therefore require more carbohydrate intake.)  I started the day with a carbohydrate-rich breakfast (oatmeal, fruit, maple syrup) so that my stores were topped off.  This morning saw one of my highest average blood sugars on my CGM, but that was expected given my nutrition.  You can see that during my ride, my blood sugar remained in the 120 - 150 range, which is appropriate for the effort.  This was fueled by both intake of simple carbohydrates and release of stored glycogen while riding.  Given the intensity and duration of this ride, I would say that this was appropriate fueling.  I was able to keep those glucose levels at a reasonably high level through refueling on the bike, though we’re not talking about a gel every 20 minutes.  During the ride, I had 2 bottles of Skratch Labs hydration mix, a Clif Bar Banana Mango Coconut pouch, and a homemade oat and nut bar.

Interestingly, you will notice that my blood sugar is quite low for the rest of the day.  This is likely due to increased insulin sensitivity (consumed carbohydrate is quickly taken from the bloodstream and stored after exercise) and plenty of room in the tank to store that glycogen.  What you can’t tell from this graph is that I ate fairly indiscriminately after this ride.  We were at the USA Cycling National Championships and there was plenty of barbecue, cornbread, beer, and desserts.  My blood sugar stayed low for the next 12 hours, despite stuffing my face!  While this was an abnormally intense day on the bike for me, it shows how beneficial such exercise can be, especially when I don’t splurge like this very often.

Overview of the data from my ride, as well as my 24hr glucose monitoring.  The time of the ride is indicated by the blue bar.

Overview of the data from my ride, as well as my 24hr glucose monitoring.  The time of the ride is indicated by the blue bar.


The fasted run was a very different effort.  For this workout, it had been 14+ hours since I’d last eaten.  I woke up, had a big glass of water, did some mobility exercises, and headed out for a long, easy run.  The majority of this run was in Zones 1 & 2, so the goal was to draw primarily on fat stores for fuel.  Even after an overnight fast, I knew I’d have 1000-1500 calories of glycogen stores I could use when needed.  Having the CGM attached let me see that, indeed, my blood sugar levels remained stable through the run.  To be clear though, this was not the first time I’d done this type of fasted effort.  I do similar training weekly, but this was different due to the CGM and the data gathered.

Overview of the data from my run, as well as my 24hr glucose monitoring.  The time of the run is indicated by the blue bar.

Overview of the data from my run, as well as my 24hr glucose monitoring.  The time of the run is indicated by the blue bar.


The trick here is that every athlete is different, and every athlete’s physiology is trainable.  Your diet and training may be causing beneficial adaptations, or it may be detrimental for your health and performance.  You may think you are doing the right thing in your training and racing, but it could be that you are constantly digging a hole for yourself.  Unless you check, you can’t be certain.

While I was compiling the information for this post, the Training Peaks Blog published a great 2-part series on the same topic.  For more information on CGM, I recommend reading it.  HERE's a link to the first article in that series.

At Podium Sports Medicine, we’ve put together a package to help you assess your individual glucose metabolism and make personalized recommendations on improving your health and performance.  This is a great tool for both diabetic and non-diabetic patients, regardless of your athletic or wellness goals.

Individualization of Glucose Metabolism (CGM)

In the last post, I discussed the fact that carbohydrate intake and glucose metabolism are both crucial for optimal health and performance.  Diet and exercise both impact this interplay of sugar and insulin, but there is individual variation that dictates what may be best for you.  A lifetime of diet and exercise habits, coupled with genetic and epigenetic influences create a unique environment in your body.  Generic recommendations can be fine, but if you want better than that, it is important to discover your own personal physiology.  This used to be a difficult and expensive task, but new technology has made this process much more accessible.

Continuous Glucose Monitoring (CGM) has become readily available in forms which do not impede your day to day activity.  At Podium Sports Medicine, we offer the Abbott Freestyle Libre system to our patients.  This allow you to wear a small sensor on the back of your arm while you work, swim, bike, run, eat, and sleep.  You get data 24/7, which we can then correlate with your training and diet to give you personalized recommendations.

To give you an idea of how such information can be gathered and utilized, I’ll tell you a bit about my personal experience using CGM.

The Freestyle Libre sensor is about the size of a quarter.  I placed one on the back of my arm and wore it for 10 days.  It has a maximum utility of 14 days, but you can get reasonable data in as few as 3-4 days.  The sensor has a small catheter that stays just below the skin, constantly measuring and recording blood sugar levels.  The data cannot be immediately viewed by the patient, but once downloaded, it is easy to review and analyze glucose levels and trends.  Surprisingly, it was entirely painless to place the sensor.  I thought there’d be a small sting or pinch, but you really can’t feel it at all.

When I downloaded my data and compared it to my diet, training, and sleep logs, some interesting things presented themselves.  Some of it I would not have expected!

My diet directly impacted my blood sugar levels in many situations, but not all.  I generally eat a moderate amount of carbohydrate (125 - 300 grams per day, depending on activity level).  I have, in the past, measured my blood sugar and ketone levels upon waking.  I have developed a diet that keeps my blood sugar appropriately low through the morning hours.  Interestingly (to me), there were some afternoons where my levels were much higher than I’d desire or suspect.  This could be traced to both dietary choices and stress.  Also of interest, there were times when I would splurge dietarily but see little to no increase in my blood sugar.  This relative immunity was again due to stress (or the lack thereof) and exercise.

Stress leads to secretion of cortisol, a hormone which signals the liver to release stored glycogen and cause an increase in blood sugar.  From an evolutionary standpoint, this helps fuel your escape from the wooly mammoth that is threatening to eat you.  (Unless wooly mammoth were vegetarian…I have no idea.)  When stressed at your desk though, the sudden rise in blood sugar (and subsequently, insulin) are more likely to be harmful than life-saving.  Seeing this process objectively with CGM made me realize how necessary it is to mediate such stress.  As with most such stressors, it also made me realize how trivial most of these issues were…certainly not worthy of risking my health!

In addition to the impact of stress on blood sugar, I learned that lack of sleep can be a major health detriment in this regard.  I remember years of medical training when I averaged 4-5 hours of sleep per night, if I slept at all.  Since that time, I’ve seen the studies that show that a single night of sleeplessness can render a non-diabetic person as insulin resistant as a Type II diabetic.  Intellectually, I understood the link, but it’s hard to appreciate damage that you can’t see.  Using CGM though, I saw near-diabetic blood sugar levels in myself on days after I slept only 5 or 6 hours.  It was shocking to me!  After sleeping 5 hours one night, my sugar jumped to inappropriately high and remained that way for too long.  And I’ve seen this repeat itself in numerous patients as we’ve been offering this service.  While you may argue that you can function on 5 or 6 hours of sleep, it’s hard to say that’s “healthy” when your glucose metabolism is so dysregulated.

CGM readings after too little sleep.

CGM readings after too little sleep.

CGM monitoring can offer you an objective and individualized insight into how your body reacts to your diet, stress, and sleep patterns.  Without gathering such information, you can make educated guesses, but I’ve found that those can be rather inaccurate.  Sure, they can be useful, but it is nearly impossible to optimize your health and performance on conjecture alone.  As a corollary, you could go out on the bike and train “by feel” for 3-4 hours every day, but you will get much fitter and faster if you use a power meter and target your training.

Speaking of training, the final post in this CGM series will look at how glucose metabolism is impacted by exercise.  This is the good stuff!  Stay tuned…

At Podium Sports Medicine, we’ve put together a package to help you assess your individual glucose metabolism and make personalized recommendations on improving your health and performance.  This is a great tool for both diabetic and non-diabetic patients, regardless of your athletic or wellness goals.

Glucose Metabolism and Why It’s Important

Carbohydrates and their effect on blood sugar are among the latest “hot topics” in health, wellness, and even sports performance.  Every week there is a new article extolling the benefits of a high fat diet or bashing those experts that still have the gaul to recommend sugars to aid in performance.  Surely a low carbohydrate, high fat diet (dubbed LCHF) is now the only way to go!  Well…not so fast.

In medicine and performance, new trends come along all the time.  A promising theory and a few early studies can lead everyone to jump on the latest fad, regardless of whether they understand the subtleties.  Let’s take a step back and examine this one, and we’re going to use a new tool to do so.

Unfortunately, nutrition tends to fall into the same category as politics and religion.  It generally should not be the topic of conversation among people with whom you hope to remain friends.  However, rather than looking for a “right” or “wrong” on this topic, I want to examine some specifics that tend to be dependent on situational and individual variation.  In other words, let’s look at the nutritional shades of gray.  Nutrition is not black and white, and if anyone tells you that you “must” adhere to any one diet, you should probably look elsewhere for advice.

In this article, I’m going to take a look at glucose metabolism with regard to both overall health and athletic performance.  Some will say that optimal health and maximal performance cannot coexist, but I disagree.  The goal of this post is not to delve deeply into the science.  We’ll touch on big concepts, but there are many other resources available if you want to examine physiologic processes and mechanisms.

As a quick and basic primer, here is a brief overview of what occurs in your body when you eat carbohydrates.  As the food is absorbed, the glucose (or “sugar”) levels in the bloodstream increase.  Your pancreas then secretes a hormone called insulin, which is necessary to move that glucose into the cells of the muscle and liver, where it is stored as glycogen.  When excessive carbohydrates are consumed, insulin levels must remain high.  If this occurs for long periods of time, the body becomes less responsive to insulin, requiring more of this hormone to do its intended job.  It’s a vicious dietary cycle.  To add to this, insulin can have a very inflammatory effect, especially if it hangs around in large quantities for extended periods of time.  To be clear, insulin is not bad!  It serves a very important purpose as an anabolic hormone, but anything in excess can be problematic.

With regard to overall health, it is now well-established that excessive sugar and carbohydrate intake can cause significant problems.  Diabetes, neurologic diseases, dementia, and even some cancers can be caused or hastened by dietary carbohydrate intake.  But this is not a binary issue.  A person with Type II diabetes (characterized by severe insulin resistance) is not “healthy” up until they are diagnosed as diabetic.  Often this diagnosis is preceded by years of insulin resistance and other symptoms of “metabolic syndrome”.  As with many such chronic conditions, this is one which brews for years until the body is no longer able to compensate.  Excessive carbohydrate intake leads to excessive insulin excretion.  As more insulin circulates at chronically elevated levels, your body develops a resistance to this important hormone.  This leads to impaired ability to metabolize carbohydrates, as well as a concurrent increase in inflammation.  While physical activity helps to slow and off-set this process, exercise is not entirely protective.  Don’t get me wrong, exercise is requisite for health!  But as they say, you can’t outrun a poor diet.

Glucose metabolism is a key component of athletic performance as well.  Carbohydrates are an important source of fuel when exercising or competing.  If your diet is inappropriate for your activity level, your performance will suffer.  If you have a diet which is extremely high in carbohydrate, you may have difficulty efficiently accessing fat stores for fuel when undertaking easier, endurance-type efforts.  Conversely, if you excessively limit your carbohydrate intake, you run the risk of exhausting your glycogen stores.  This could lead to poor performance at higher intensities, and chronic depletion may lead to hormonal dysregulation and symptoms of Overtraining.  Like most things in medicine and science, the answer does not lie in the extremes.  Balance is key, and finding your own, individual balance is crucial to optimal health and performance.

This is Part 1 in a 3 part series.  Stay tuned to learn the specifics of how you can determine your ideal individual diet and fueling strategy.

At Podium Sports Medicine, we’ve put together a package to help you assess your individual glucose metabolism and make personalized recommendations on improving your health and performance.  This is a great tool for both diabetic and non-diabetic patients, regardless of your athletic or wellness goals.

Ample - A Whole Food Meal Replacement

I often travel in my profession as a Sports Medicine physician.  Currently, I’m at the Tour de France in my role as Head of Medicine for the Cannondale-Drapac Pro Cycling Team.  Sometimes the food options on the road are incredible!  Many times, though, the pickings are slim.  Over the years, I’ve learned to travel with some healthy food options to which I can turn when not much else is available.

While on the road, I try to maintain a reasonable level of activity as well, but that too can be difficult.  Given this and the regular lack of healthy dietary options, there are many days where I’m stuck with a 20-30 minute run and a breakfast that consists only of various pastries and jams.  Even worse is the 8-10 hour trans-Atlantic flights with only airline food on offer.  In such situations, you can either choose to give up on your dedication to health and fitness, or you can get creative and do your best to maintain.  I prefer the latter…unless there is a regional culinary specialty that is not to be missed!

Recently, I was introduced to a meal replacement product called Ample.  In general, I’m not a fan of “meal replacements”.  They tend to be full of preservatives, sweeteners, and all sorts of things I’d rather not ingest.  Ample is different.  With my interest piqued, I took some of their product to the Tour de Suisse last month and now to the Tour de France.

In the interest of keeping this review short and pertinent, here are my bulleted thoughts:

  • Ample has taken great care in selecting their ingredients.  
  • The whey protein is from grass-fed cattle, as is the collagen protein.  
  • The fats are provided by macadamia nut oil and coconut oil.
  • They include a mix of greens and probiotics.
  • There are only 4g of sugar!
  • The macronutrient breakdown is well-balanced at 20g fat, 28g carbs (10g fiber), and 25g protein.
  • Ample is sweetened with honey, monk fruit, and stevia.  I hate the taste of stevia!  It’s normally a deal-breaker for me, but I can’t even taste it in this mixture.
  • It comes as a dry powder in its own bottle.  Just add water, or milk, or whatever.  I use water.  This packaging makes it bulky for travel, but I hear they are considering selling larger containers of powder which can be scooped and mixed in your own bottle.
  • Ample is expensive, starting around $6 per serving.  It will cost you more than most “meal replacement” bars or shakes, but you get a lot for the money.  If you took the time to source the same ingredients, dehydrate, blend, and package them, I suspect it would not save you any money.  And if you ever visit your local juice bar for a smoothie, you probably pay more.  The price seems fair, but it’s not going to be a daily drinker for most.
  • The product comes in a 400 calorie serving, a 600 calorie serving, and even in vegetarian options.

All of this is great, but if it tastes like crap, who cares?  I like to enjoy my food, not just choke it down.  Life is too short for that!  I can’t say Ample is going to win any taste tests when put up against your favorite milkshake or smoothie.  At first, I honestly didn’t care for it.  It wasn’t that I found it to be repulsively bad, but it just wasn't terribly appetizing.  The flavor has an earthen quality and is decidedly not sweet.  (If you can describe a wine as having “barnyard” qualities and it still be a compliment, why not a protein shake?!?)  The taste is imparted by macadamia, honey, cinnamon, cocoa, greens powder, and whey protein, which like all whey, adds a certain bitterness.  I can honestly say that it has grown on me though.  If you’re worried about this though, they offer a “Taste and Quality Guarantee”.  You can get your money back if you don’t like it.  I kind of like it now, though I’d still gladly opt for an omelet and salad any day.  I think Ample is a great product to keep in your luggage or at your desk, for those times when your options are either to regret your meal or go hungry.  Ample provides a healthy mix of ingredients and gives you no reason to regret what you’ve eaten.

For future travel, and to keep around the office, Ample will continue to have a place in my diet.  For me, it’s a contingency plan.  But it’s a good one.

For the sake of transparency, I have no financial relationship with Ample and was not even one of their Indiegogo backers when the company was getting started.  They have paid me nothing, other than being kind enough to send me a free case of product for my trip to the Tour de France.