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Inline Skating Newsletter Article

Fuel and Athletic Performance

By Dan Kibler

If you've been a regular reader of the Orbit, you know that Liz and I have embraced The Zone as a dietary lifestyle. The Zone's alternate, and somewhat controversial, approach to eating has had a very positive effect on our athletic performance. However, Barry Sears, the author, is somewhat vague on how the Zone fits into an athletic lifestyle, especially when it comes to actual training and competition. His answer to everything is a Zone snack. (I suspect he is not much of an athlete himself.) Thinking there must be another approach, I investigated further into the effects of diet on performance. I learned some very interesting things.

Liz and I are both basically weekend warriors. But even if we are not officially competing, we like to put in a good showing on weekend long-distance skates, bike rides and skiing. Being squarely (double-entendre alert!) in middle-age, we often have to keep up with folks half our age. So I set myself up for a little experiment. For the past 3 years I've competed in the Sea Otter Classic cross-country mountain bicycle race. The Sea Otter is a challenging 16-mile race that takes me just under two hours to complete. Being held in March, it provides a good incentive for winter training and is a good test of my early season fitness. I'll probably never be at the front of the pack, but I like to hold my own. Last year, on The Zone, I was able to halve my finish placement from the previous year. I hoped to improve again this year. Later in the article I'll explain my strategy for the race and the results. (Oh, the suspense!)

Food, the Fuel that Drives Your Body

Brocolli Guy Fuel, in the form of food, is one of the three essentials that your body needs to stay alive. Along with oxygen and water, deprive your body of fuel and it will eventually stop working. On the other end of the spectrum, athletic performance is also dependent on the availability of these three essentials. In this article, I'm going to concentrate on fuel, how the human body uses it and how it affects performance.

Your body is very flexible in its utilization of the fuels available to it. It can effectively use any of the three macro-nutrients--carbohydrate, fat and protein--as fuel. Our Western diet provides a pretty good mix of all three. However, Eskimos survive quite nicely on a very high fat diet (comprising 80% or more of total calories). American natives and the explorers who emulated their ways, such as the members of the Lewis and Clark "Corps of Discovery," went for long periods eating mainly meat. On the other hand, many Asian peoples, with a diet that strongly relies on rice, are eating a higher percentage of carbohydrates.

Although carbohydrates are a great source of quick energy, surprisingly, they provide no essential nutrients. The body can meet all of its energy and structural needs from the other two macro-nutrients. (Don't confuse carbohydrates with the essential micro-nutrients, i.e., vitamins and minerals, that often accompany them.). However, your body needs certain fatty acids (called essential fatty acids - EFA) from dietary fat and certain amino acids (called, you guessed it, essential amino acids - EAA) from protein to continue to function properly. This means you need a minimum amount of both fat and protein in your diet to maintain a healthy body.


Whole books have been written about digestion, but here's the very short course. In the digestive track carbohydrates are converted to monosaccharides, mostly glucose and fructose. Proteins are broken down into amino acids and fats are broken down into fatty acids. Glucose and amino acids are absorbed directly into the blood from the intestines. The fatty acids take a more circuitous route via the thoracic duct, part of the lymphatic system, from which they are "metered" into the blood stream at the junction of the left jugular and subclavian veins.

Once the fuel is in your body it has to be used immediately or stored. You are constantly using your fuel to support the functions of the body's various organs. Different organs have different fuel preferences. Your brain accounts for a large percentage of your resting energy consumption. In a well fed body, it uses glucose as fuel almost exclusively. The heart is a scavenger. Although it prefers fatty acids, it will use any fuel available to it. This makes sense, since it is one of the most ancient organs from an evolutionary standpoint and if it stops pumping, everything else stops soon after.

Your skeletal muscles are somewhere in between in flexibility. They can effectively use energy from two sources, fatty acids and glucose. These muscles use fatty acids when available, but switch to glucose when the fatty acids cannot meet the demand.

Fuel is stored in two ways, as glycogen, the storage form of glucose, and as triglyceride, the storage form of fat. Glycogen is stored mainly in the liver and in the muscles, while fat is stored in the adipose tissue (body fat) and intramuscularly. There is no storage form of protein. However, when you need to use protein as fuel (predominately during long periods of fasting) muscle tissue is broken down and converted to glucose. This is called ketosis. A small percentage of our regular energy also comes from protein.

Converting Fuel to Energy

Energy is actually produced in the muscles from only one substance, ATP. As it is used up, complicated chemical reactions replace it by converting the available fuels and oxygen into more ATP.

The chemical processes that result in ATP in the muscles can be divided roughly into three cycles. Which cycle predominates depends on the intensity of the demand - how hard you are working and the availability of the fuels and other metabolites such as oxygen.

At rest and during moderate-intensity exercise, muscular energy is supplied by aerobic conversion of fatty acids to ATP. The fatty acids are supplied by the muscles themselves or are transported from the liver and adipose tissue by the blood stream. Since most bodies have a huge supply of stored fat, this pace can be maintained indefinitely. The intensity of exercise that can be supported by this "fat burning" cycle is limited by the rate at which fatty acid can be mobilized from storage and delivered by the blood stream.

At some point, increasing demand will exceed the supply of fatty acid and the muscle will be forced to switch to using glucose. Interestingly, use of fatty acids will drop at this point. The glucose comes from the glycogen stored in the muscle itself, and as glucose via the blood stream from the glycogen stored in the liver. Glycogen can't be moved from muscle to muscle, so the glycogen in a lightly used muscle isn't available to a heavily stressed muscle. At this point, the process is still aerobic. Enough oxygen is available to completely oxidize the fuel (glucose) to ATP.

The Lactic Acid Connection

As exercise intensity increases, the demand for oxygen starts to exceed the cardiovascular system's ability to deliver. The muscles continue to use glucose, but only partially convert it into ATP leaving behind lactic acid. Lactic acid is removed from the muscles by the bloodstream and carried to the liver and other organs. The liver, one of the more versatile organs, recycles the lactic acid by converting it back to glucose for reuse by the muscles and other organs. If lactic acid can't be removed from the blood as fast as it is created, it starts to build up. This point is called the lactic acid threshold.

Lactic acid buildup causes the blood pH to decrease. If the pH gets too low, the muscles can no longer function. This is a natural governor on how hard you can push yourself. High intensity effort is also limited by the glycogen stored in the muscle. The glycogen acts as a reserve that lets the muscle use more glucose than the blood stream can supply. When the glycogen is used up, the muscle can only sustain a limited effort. If you've ever done a long distance event and found that you could keep up a moderate pace on the flats but died on the hills, you know what this feels like.

One more interesting note: there is enough ATP stored in your muscle tissue to sustain a short burst of speed, a sprint, for about 8 to 20 seconds.

You can actually train your muscles to use fat more efficiently and thereby raise the level of effort where the transition from fat burning to glucose burning takes place. This is important for endurance events, because it spares your glycogen stores, an important source of energy on that upcoming hill, or for your finish surge.

The Role of Hormones - Insulin and Glucagon

Ok, one last lecture to sit through before I let you in on how to apply this knowledge to your workouts. Hormones have a powerful role to play in all this. We tend to ignore hormones when it comes to training and athletic performance, but this is where the rubber meets the road when it comes to controlling how your body uses fuels during exercise. There are two hormones that control the use and storage of glucose and fat: insulin and glucagon (an over-simplification, but workable for the purpose of this article). To understand the role of insulin, you first need to know that in a healthy body, blood sugar level i.e., the concentration of glucose in the blood, is controlled in a narrow band - around 80 mg per 100 ml. After a meal containing a lot of carbohydrates, blood sugar level rises rapidly above this band. Insulin is released in response to the rise. Insulin signals several systems to remove glucose from the blood. If there is room available in the liver and muscles, glucose is taken up and stored as glycogen. Glucose is also converted to fatty acids and stored in the adipose tissue as triglyceride. Finally, the skeletal muscles are signaled to burn more glucose, in preference to fat. Remember this last item, because it's important. These reactions rapidly lower the blood sugar level back into the control band, and the insulin level drops to normal.

Glucagon has a counter affect. Glucagon promotes the release of fat from the adipose tissue and the use of fat in the skeletal muscles as fuel.

Both of these powerful hormones are always present in varying amounts. Their relative concentrations determine the overall use and storage of fuels.

So what does all this mean for you?

Well, if you've made it this far, you've learned a lot about how your body stores and burns fuel. I took my newfound understanding and came up with a training and competition fuel strategy. The most important thing here is to recognize that the fuel requirements for training and competition are different. In training, you want to teach your body to use the available fuels most efficiently. In competition, you want to do what is necessary to achieve highest performance. Of course, it's sometimes hard to decide when you are training and when you are competing. On that long weekend skate with your friends, are you training or competing? I'll let you decide that one.

Remember that Liz and I are long-term Zoners. One of the major effects of the Zone is to reduce the insulin surges and allow the glucagon level to rise. This promotes the better mobilization and burning of fat as opposed to glucose. I decided to try to build on this by not using sports drinks or other carbohydrate replacement during training, and only eat a Zone-friendly snack (for me, a Balance Bar) for training sessions and weekend events longer than two hours. My theory is that this would help train my muscles to better use fat. Not having extra carbohydrates available causes the muscles to adapt to using the fuel that's available - fat.

For the competition, I decided to go against the Zone orthodoxy, and leave my Balance Bars in the car. Instead, I filled my hydration pack with a 50% diluted sports drink. I wanted to have plenty of glucose available for high-intensity effort, especially for that long climb out of the canyon in the second half of the race.

Did it work?

Well, unfortunately the results were mixed. I had some mechanical problems during the race - mostly caused by mud build up on my bike - so my finishing place was lower than last year. But, darn it, I felt great, especially on that long hill climbing out of Engineer Canyon! There I was, pushing my heart rate well above 90% and still joking with the riders I was passing. Also, I wasn't nearly as depleted as in previous years. So, even though I don't have the hard numbers to prove it, I think my approach was successful.

In summary, here's my approach. Start with The Zone diet. It will help you mobilize fat for energy like nothing else. When training, stay away from Sports drinks and other carbohydrate-loaded snacks like Power Bars. If you go long, (more than 2 hours) supplement with Balance or PR bars. For competition and other important events, supplement with diluted sports drink. Update: I've refined my approach a little. I now add carbs to my diet a day or two before the event to help ensure I have a full load of glycogen stored in the muscles.

I'd be interested in knowing if you have had any similar experiences with training and diet, especially if you are in The Zone. Send me an email and indicate if it's ok to publish it on our site.

If you would like to explore these topics in more depth, here are some of my on-line resources:

Mammalian Metabolic Homeostasis Not accessible.
Energy Substrate Utilization During Exercise
Protein Requirements In Humans
Exercise Physiology for the MAPP Not accessible.
Partial Zone Defense - Why your racing diet shouldn't be your training diet
The Official Home of the Zone