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 Amazon.com.  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.

This 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.

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.