Cycling Nutrition: The Value of the Glycemic Index for Cyclists

idiots guide

Information about the glycemic index and many other topics can be found in Nutrition for Cyclists: Eating and Drinking Before, During and After the Ride which can be purchased on  For information about the book and how it relates to what I’ve posted to Tuned In To Cycling, please check out this post.

The diet industry is big business; in the US it is estimated that people spend in the neighborhood of $35 billion dollars a year on diet-related products.  (To make that number more comprehensible, if you make $50,000 a year Americans spend your yearly salary on diet products approximately every 80 seconds.  24, 7, 365.)  A large part of this industry relies on consumer ignorance and gullibility to enable the promotion and sale of one fad diet idea after another.  One of the ideas heavily promoted in the diet industry is the glycemic index.

Is the glycemic index useful? Yes.  Is it all the diet industry makes it out to be?  Not even close.  For our purposes here, is it useful to cyclists?  I think the best answer here is – marginally.

First of all, what is it?  The glycemic index is an indicator of how long it takes food to be converted to glucose in the blood.  Glucose is the fuel that muscles burn when they are working and on-the-bike cycling nutrition and performance is largely about glucose production and consumption.  Having an idea of how long it will take to get the food you eat while you ride converted into blood glucose has the potential to be very useful.

Will the glycemic index tell you how long it takes for the Powerbar or raisins you ate to be converted to blood glucose in seconds, minutes and hours?  No.  The glycemic index compares the time it takes for a particular food to be converted to glucose in the blood to the time it takes for pure glucose ingested orally to appear as glucose in the blood.  Glucose is arbitrarily assigned a glycemic index value of 100 to serve as a basis for comparison.  Particular foods are then given a glycemic index value, typically less than 100, based on how long they take to be converted to blood glucose in comparison to pure glucose.  A high glycemic index indicates a food is rapidly converted to blood glucose; a low glycemic index value means it takes a relatively longer time for that food to be converted to blood glucose.  The glycemic index doesn’t tell you how long it takes to get blood glucose out of food, it gives you a rough idea which foods are converted to blood glucose more quickly than others.

eating on bikeThis sounds like it should be of great use to the cyclist.  You’re burning glucose constantly on the bike, you need more, you’re eating to get more, the glycemic index will tell you what to eat to get that glucose as quickly as possible.

It’s all good, right? Not really. Why not?

The glycemic index of most food varies with so many factors that the rough idea the glycemic index gives you of which foods are converted into blood glucose faster can be very rough indeed.  Here are some examples.

  1. For many foods, glycemic index varies as a function of how the food was prepared (pasta boiled for 10 minutes has a different glycemic index than pasta boiled for 15 minutes), when it is eaten relative to when it was cooked (potatoes often have a higher glycemic index when eaten after cooking than they do if refrigerated  and then reheated and eaten the next day), how ripe the food is when eaten (generally, the glycemic index of fruit increases as the fruit ripens), or which variant of the food type you are eating (different types of raisins have different glycemic indices).
  2. Glycemic index for a particular food varies depending on what is eaten along with that food.  Fat, protein or fiber eaten along with a particular food usually results in a lower overall glycemic index.  For example, you’ll usually get faster blood glucose from raisins eaten alone than the raisins in a cookie.
  3. Different people will have a different glycemic index for the same food because individuals differ in how efficiently they digest carbohydrates.  In addition, the same person may have a different glycemic index for the same food when that food is eaten at different times of the day.
rough categories

The glycemic index values that are used to define the categories in this image are rough guides.

Considering all of these factors, I think the glycemic index can be modestly useful to the cyclist planning what to eat during a ride.  There are many glycemic index charts for different foods that can be found on the internet.  The specific numbers given in these charts are best thought of as rough estimates.  For the cyclist it’s probably most useful to consider glycemic index in terms of three rough categories: High, medium and low glycemic index foods.  High glycemic index foods will probably provide needed glucose throughout the ride.  On long rides of two hours or more, low glycemic index foods can produce needed glucose later in the ride if the food is eaten early in the ride.  As you get closer to the end of the ride, higher glycemic index foods are more likely to be beneficial while you’re still on the bike.

WARNING:  The nutritional needs of a person engaged in 60 to 90 minutes of moderate to intense exercise or a long ride lasting more than 2 hours are very different from the nutritional needs of that same person going about their daily activities.  High glycemic foods can be very useful while you’re on the bike.  A steady diet of high glycemic foods when you’re not engaged in endurance exercise has been shown to be related to various health problems such as obesity (and all of its related problems), diabetes and, at least in animal studies, a shortened life span.  High glycemic foods eaten while you’re riding will generally help you.  A day-in, day-out diet of high glycemic foods when you’re not exercising will generally hurt you.

Cycling Nutrition: The Bonk

The information in this post has been moderately revised and combined with information about cramping, dehydration, and electrolyte loss in Nutrition for Cyclists: Eating and Drinking Before, During and After the Ride which can be purchased on  For information about Nutrition for Cyclists and how it relates to what I’ve posted to Tuned In To Cycling, please check out this post.

So . . . you know that muscles burn glucose for fuel, your body stores enough glucose in the form of glycogen to fuel about 90 minutes of moderate exercise, and you need to ingest carbohydrates to provide more glucose for longer or more strenuous rides.  You know about what to eat and when to eat it to get the carbs you need while you’re on the bike.  But you keep hearing about this “bonk” thing.  You keep hearing you’ll bonk if you don’t get the carbs you need.  What’s this bonk everyone keeps talking about?  Believe me, you don’t want to know.  At least you don’t want to know on the basis of firsthand experience.  Reading about the bonk so you know what it is and how to avoid it is good.  Having a bonk is not good.

Eat before you're hungry

“Bonking” is what cyclists call hypoglycemia which is the medical term for abnormally low levels of blood glucose.  You bonk when you have exhausted your glycogen stores, haven’t ingested enough carbs to produce more blood glucose, and are still riding the bike.  Anyone can bonk if they don’t eat properly on the bike.  Lance Armstrong, who probably knows as much about cycling as anyone on the planet, got wrapped up in the race on a stage in the 2000 Tour de France, forgot to eat, and bonked on the climb up the Col de Joux Plane in the French Alps.  The only reason he didn’t lose the Tour de France that day is because he had an iron will and an inhuman capacity to suffer.  Afterwards he called it the worst day on the bike he’d ever had.

Bonking can be especially deadly for cyclists because your muscles aren’t the only things in your body that burn glucose for fuel.  Your brain burns glucose too.  That means that not having enough glucose in your blood to fuel the system has mental and emotional effects in addition to physical effects.  If blood glucose levels drop too low, the body will act to protect the brain and will begin to shut down the muscles first.  However, the muscles won’t completely shut down before the brain begins to be affected and the mental and emotional consequences of bonking can be more dangerous to the cyclist than the physical consequences.

When you bonk, physical exertion becomes extraordinarily difficult.  Your muscles don’t have the fuel they need to operate effectively and forcing them to work becomes more and more difficult.  You feel extremely weak and lethargic.  You may tremble and shake uncontrollably and sweat profusely.  You feel dizzy and light headed.  Your sense of balance is upset.  You may have heart palpitations.  You will probably feel ravenously hungry.  On the mental and emotional side you will probably feel nervous and anxious.  You may become confused and disoriented.  You will have low emotional control and will become hostile, belligerant and easily irritated.  You may experience overwhelming feelings of being defeated, hopeless and unable to go on.  Your awareness of what’s going on around you will shrink and can arrive at an extreme form of tunnel vision in which the only thing you’re aware of is the spot on the road ahead that you’re staring at.  You may have difficulty speaking.  At the extreme, hypoglycemia can produce seizures and coma.  In a word, it sucks.

What do you do if you bonk?  You need to get your blood glucose levels up and you need to do it quickly.  Ingest simple carbohydrates that can be rapidly processed into blood glucose by the digestive system.  The best source for these kinds of carbs that you’re likely to have with you on the bike is a sports drink like Gatorade.  Other sources of simple carbohydrates include energy gels (make sure you drink plenty of water with these), sugar cubes or sweet candy like gumdrops or jellybeans.  Complex carbohydrates like energy bars will take longer to process into blood glucose and will only provide relief in the longer term.  If you catch the bonk early, you can keep riding while you  refuel.  If you let the bonk go too far, get off the bike until you recover.  You don’t want to be riding when your sense of balance is bad, you’re disoriented and you’re unaware of what’s going on around you.

After you’ve bonked and begun to recover, pay careful attention to what you eat for the rest of the ride and make sure you keep your glucose level up by regularly and frequently taking in fast absorbing carbs.  Sports drinks are very good for this.  Perhaps the hardest part of reacting to a bonk is mental.  You need to try and be aware that you’re not thinking clearly and not being as aware of what’s going on around you as you need to be to ride safely.  Forcibly arouse yourself from your lethargy and pay extra attention to what’s happening around you.  Of course this is easier said than done; it’s not easy to be aware that you’re not thinking clearly when you’re not thinking clearly.

Whether you’ve started to bonk or not, try and get in the habit of monitoring yourself for the early signs of a bonk.  The ride beginning to feel like a chore?  Getting irritable and angry?  Not paying as much attention to what’s happening around you?  If you catch it early and replenish your blood glucose before it gets out of hand, you can usually keep riding safely and effectively.  If you have a regular riding partner whose riding abilities and demeanor on the bike are familiar to you, be aware of their condition as well.  A rider who isn’t thinking clearly is likely to misinterpret or misunderstand what’s happening to them in the early stage of a bonk.  Help them out.  Be aware that you may have to treat them carefully as they may be experiencing increased levels of irritability and hostility combined with decreased emotional control.  If you think you might be bonking, tell your ride partner so they can help you.

Most of the time bonking happens when you’ve ridden for a long time and haven’t been eating properly.  However, a bonk can also happen in unexpected circumstances.  Laura and I once rode in a week-long cycling tour through the Rocky Mountains in the Glacier National Park area in Montana and Canada.  We approached this tour as a cycling vacation rather than a training opportunity and rode at a much slower pace than we usually do.  We also stopped and had lunch mid-ride with other riders on the tour which was something we had never done before.  The lunches were great with good company and good food but we had no experience in how to incorporate eating a meal like this into a long ride.  The first time we had lunch mid-ride I completely screwed it up.  We continued riding after lunch and when we were about 20 miles from finishing I began to experience the early stages of a bonk.  What had happened?  I hadn’t pigged out at lunch but even a small lunch was much more than I would typically eat during a ride.  My stomach was full and the idea of eating either didn’t occur to me or, if it did, it wasn’t attractive.  Although my stomach was full, it was full of mostly protein and fat which could not be broken down fast enough to provide the energy I needed to finish the ride.  I was getting very little glucose from the food I’d eaten and my glycogen stores were exhausted so I started to bonk and didn’t recognize it for what it was.  Those last 20 miles were miserable; the temperature had dropped, we were riding directly into a strong headwind, and I was completely demoralized and shivering uncontrollably by the time we reached the lodge where we were scheduled to stay the night.  Fortunately, the lodge had an enormous fire roaring in the center of the main area with a bench-like hearth running around it on all four sides.  While we waited for the van to arrive with our luggage, I sat huddled by the fire in a private little world of misery slowly recovering and getting warm.  The lesson I learned?  You can bonk on a full stomach if your stomach’s full of the wrong things.

As unpleasant and dangerous as bonking can be, the good news is that it’s easily avoided.  Start your clock as soon as you get on the bike and eat regularly and properly throughout the ride and you’ll never have to experience a bonk.  Eat before you’re hungry, eat before you bonk.