To Eat or Not to Eat

By David Warden

Have you ever found yourself nearing the middle of a workout, and suddenly you just feel flat? Your energy drops, you feel fatigued, and start to count the minutes until the workout is over? Maybe the same workout the previous week, you felt great, full of energy and able to meet the intensities you had planned for the workout. And we’re not talking about a 2-hour workout where it is normal to start to feel fatigue at the end, but a workout where you began to feel crappy just 15-20 minutes in.

While there are many factors that could have created that fatigued state so early in a workout, including sleep and recovery, there is another potential overlooked cause. How soon before your workout you last ate. I’m guessing that many of you are thinking that particular type of fatigue would be caused by eating too far in advance of a workout, and that eating close to the workout would ensure you had enough energy. The following results might surprise you.

Dave Costill, one of the leading sports physiologists, published a study in the Journal of Applied Physiology that looked at the effects of a feeding 45 minutes prior to exercise. This study took 6 trained males, with a VO2max ranging from 53-65, indicating a good level of fitness for all the test subjects. On two separate occasions, the men performed a 90-minute running trial.

On the first run trial, the men took in 75 grams of glucose, or about 300 calories, 45 minutes prior to exercise. What happened in that 45 minutes is what you might have expected to happen. Blood glucose spiked by 38% in that 48 minutes. But, just 15 minutes into the workout, the athletes blood glucose dropped by 300% to just a fraction of what it was even before the athletes took in the glucose 45-minutes before exercise. Over the next 60 minutes, the athletes’ blood glucose did begin to climb again, but it never even reached the level it was at prior to the workout. In fact, several of the test subjects couldn’t even finish the 90-minute time trial because they felt exhausted at 75 minutes.

The same group of athletes did the same test over a week later. This time their last meal was several hours prior to the run test. Their blood glucose levels were the same 45 minutes before the trial, and those levels stayed the same until they began to exercise. To compare from the previous week, the blood glucose levels of the athletes in the second week were 40% lower than they were the previous week at the moment they began to exercise. They did not experience the spike in blood glucose that had had the week before. As they began to exercise at the same intensity as the previous week, the subjects blood glucose began to rise steadily through the first 75 minutes by 26%, while the week before with the pre-exercise meal the athletes had had their blood glucose cut in half at 75 minutes.

You may be thinking that this doesn’t make sense. The athletes who ate 45-minutes before had a decrease in blood glucose after 15 minutes of exercise, and the athletes who did not eat had an increase through the first 75 minutes. How can that be?

The answer is insulin. Just like a diabetic who has to understand the impact of their diet and the timing of their diet, the same thing applies to an athlete. Carbohydrates digested in that 45-minute window, when the body is at rest, stimulates insulin secretion, which causes the muscles to use an enormous amount of glycogen when exercise starts, leading to a dramatic decrease in available blood glucose leading to hypoglycemia and early fatigue.

Does this mean that you should not take in glucose or carbs during exercise? Not at all. This same study showed that insulin levels did not increase when the glucose was taken in during exercise, and in fact it steadily increased the glucose in the blood when taken in during exercise.

Does this does mean that these effects are universal. There are some athletes who genuinely seem to have a high tolerance both at a hormonal and digestive level, and who eat whatever they want, whenever they want, and it seems to have no effect. But if you have the choice, it seems the best thing to do is not try and eliminate eating about an hour before exercise.

This is easier said then done. The reality is that we are triatheltes as well as husbands or wives, parents, and we have to make a living. It is not always possible to plan our meals and eating so perfectly around training. So let offer a couple of alternatives to meticulous meal planning and timing.

First, if you can, eat something during your exercise, even right at the beginning. This also depends on your tolerance for certain foods, but if you are accustomed to grabbing a bagel when you first wake up, and then getting ready for your run, consider eating the bagel over your first half mile.

Second, although not explicit in the study we jut reviewed, it is implied that eating foods with a low glycemic index would not result in the same type of insulin spike that something like glucose would. There are lots of foods, even carbs, that have a much lower impact on blood glucose and insulin levels. These low glycemic index foods include apple sauce, barley, peaches and most nuts. A list of these foods can be found on page 244 of the Triathlete’s Training Bible. Proteins of course have very little effect on insulin. These are the types of foods that you can take in within that 45-minute window that might have less of an effect on your insulin, and therefore blood glucose depletion.

My title for the article “To eat or not to eat” is really a misleading title. Of course you have to eat. But it was catchy. The key is to understand the timing of when, and what you eat. So, if you have been feeling curiously fatigued in your workouts, consider this small change to your diet and training regimen.