Welcome to the finale, the third of three articles that have addressed several aspects of nutrition and, in particular, of carbohydrates. This article will address the timing of carbohydrate consumption and the importance of carbs to endurance athletes. And it will explain why: sometimes athletes can choose to eat anything from cake and cookies to beer and bread, why sometimes they hold back and eat sanely, and why sometimes they absolutely must eat.
First to quickly reiterate the takeaway points from the first two articles I wrote:
1. Weight Management = Energy Input - Energy Output
2. Your carbohydrate intake should be varied each day to meet your energy output for the day.
3. Generally avoid high glycemic index foods, and favor low glycemic index foods, which are generally more nutrient dense and help keep energy levels stable, help keep you satiated longer and help avoid overconsuming calories.
So how much should you eat? Well, in order to gain, lose or maintain weight, the first item to address is #1, your energy balance. And what should you eat? Per #2, more carbohydrates if you are particularly active on a given day, and less carbohydrates when you are less active. And per #3, generally more low-GI and less high-GI foods.
So these three points take care of how much and what to eat. Take a look at the articles again if you need a reminder or more detail.
Nutrition/Carbohydrate Article #1 and Article #2
Timing and Glycemic Index
The final point I am going to address is when food should be eaten, and this timing aspect is particularly relevant for endurance athletes.
You may have noticed that I keep saying that you should generally avoid high-GI foods and favor low-GI foods. I say generally, because it applies to most people, most of the time. However, low-GI foods are not always going to address your needs, especially if you are an endurance athlete. Sometimes, high-GI foods are not only acceptable, but they are preferable and necessary to fuel the endurance athlete. That’s because carbohydrates and their glycemic-index are tricky two-facers.
The body usually responds to high-GI foods with a rapid increase in blood sugar, an insulin response, and tendency to store fat. This response, however, is not seen during or immediately after aerobic exercise. In fact, the high-GI foods seem to help an endurance athlete to go harder for longer and to recover better for the next exercise session.
Let’s say, for simplicity and practicality, that your body has two states.
State #1 - Rest
State #2 - Exercising (aerobically)
At rest, your body has a set of responses to high-GI foods that is not conducive to good health.
While in an exercising state (during or immediately after moderate to high intesnsity aerobic exercise), your body has a different set of responses to high-GI foods, and the high-GI foods actually become beneficial.
Check out this article for additional discussion of glycemic index, carbohydrates and endurance athletes.
At rest, insulin is released in order to lower your blood sugar by taking glucose molecules out of your bloodstream and storing them as fat. While exercising, however, this insulin response is blunted. Instead of glucose adding to fat stores, the glucose molecules are used by muscles as fuel to continue your exercise.
Here is a visual for that idea - this is probably going to get me in trouble with any biochemists, doctors or physiology people out there, but the general effect is correct and it might serve as a useful working concept.
Sugars are the preferred muscle fuel for mid- to high-intensity aerobic exercise, but your body only has a finite supply (appx. 2 hours at high intensity) of such sugars (primarily muscle and liver glycogen). Once those sugars (glycogen) are depleted, you are forced to slow down (this is the ‘hitting the wall’ or ‘bonking’ effect often seen in marathons and long triathlons). High-GI foods consumed during exercise have a ‘glycogen sparing’ effect. Instead of burning the glycogen stored in your muscles and liver, your muscles will use the sugars from the high-GI food you are eating as their fuel source. During an Ironman, an athlete can easily consume in excess 3000 calories from high-GI sources such as sports drinks and bars. These foods are quickly digested and used by muscles as fuel. Even table sugar mixed with water would be an acceptable and reasonably effective food choice for prolonged exercise. Low-GI foods would not be an effective choice, however. One, they are digested much too slowly to achieve a glycogen sparing effect (you are burning calories waay faster than you can digest low-GI foods). Two, low-GI foods would cause massive gastrointestinal distress. Eating 3000 calories of a high fiber food such as spinach, a high fat food such as olive oil or a high protein food such as eggs would all clearly cause some stomach issues during intense exercise.
In addition to during exercise, immediately after exercise your body is still in an ‘active’ state and high-GI foods can be beneficial. If you did some duration of aerobic exercise, your muscle and liver glycogen stores may be partially depleted. These glycogen stores are particularly sensitive to glucose immediately after you finish exercising – that is, you have an opportunity to replenish your glycogen stores by eating high-GI foods for a period of time after exercising. This period of time depends on how long you exercised (and on who you ask), but is somewhere around 30 minutes or an hour. If you miss this window of opportunity, glycogen stores may take one to several days to be completely replenished. Continually failing to replenish glycogen stores has a cumulative detrimental effect on training and racing, causing you to fatigue and ‘bonk’ sooner than you would with full glycogen stores.
That’s it, that’s all! Everything you needed (or never wanted) to know about carbohydrates! Don’t be scared of them – carbohydrates (sugars) are your brain’s and muscle’s preferred fuel source. Try not to overdo them on less active days, and increase your carb intake on highly active days. Try to minimize high-GI foods, except during or after moderate to intense aerobic exercise. And, at rest, if you’re looking for more sustained and even energy levels, don’t just eat plain high-GI foods (toast or rice), add some of the stuff we all love anyway – olive oil, eggs, butter, nuts, seeds, and peanut butter.
As always, thank you for reading, and best luck to you in your nutritional endeavors and experiments.