Fat for better endurance performance
The other day I got a question about fat as fuel during moderate exercise from an elite cyclist. So, as a little continuation on my blog article “Carbs, Glycogen and Performance”, I will now go into the different muscle fuel sources and how fat can aid you.
When you’re at rest, about 60 percent of the energy needed to fuel your body is coming from fat, or more correctly, free fatty acids (FFA) that are circulating in the bloodstream. Now, these FFA comes from fat stored in cells in areas where you accumulate body fat. When you exercise at low levels (about 25 percent of your aerobic capacity), fat provides about 80 percent of the muscle’s fuel, with the remaining fuel coming from carbohydrates (and less than 5 percent from protein).
Now, of these 80 percent, most of the energy comes from the FFA in the blood. But a small amount is derived from stored fat inside muscle cells; the intramuscular triglycerides (IMT).
Usually, most people focus on glycogen, (stored carbohydrates inside muscle cells), when it comes to endurance performance. That’s okay if you were to live in the 70’s, but today that’s a terrible close-minded approach.
If we start at a low level of exercise and as exercise intensity increases, so does the usage of IMT to fuel the muscles involved. At about 65 percent of max VO2, IMT supplies about 25 percent of the energy needed for sustained muscle contraction. And then, as you finally approach 100 percent of your aerobic capacity, glycogen becomes the necessary and preferred fuel.
The problem with fat at high intensities is that it takes considerably more oxygen for muscle cells to burn fat than carbohydrate. Fat yields 4.69 calories per liter of oxygen, while carbs yields 5.05 calories. Even though the difference is only 7 percent, this 7 percent caloric advantage of carbs turns into a three fold faster energy production during aerobic metabolism in the muscles.
Therefore, we come to the conclusion that glycogen is absolutely needed once you precede 85 percent of your aerobic capacity. If your glycogen stores are empty, or if you run out of glycogen, then you will hit the well-know wall.
Now, we know that you can refuel with carbs during exercise. But you can only metabolize about one gram per minute. That is not fast enough to replace what is being lost during hard exercise. That is why athletes need to pace themselves.
So, therefore you’re forced to rely on fat as fuel, and thus you need to slow down since fat needs more oxygen to be burned.
Now, there is a way to slow down muscle glycogen loss and better your performance. You can do that by increasing your body’s efficiency at burning fat as energy and by increasing your IMT stores. This can be of great benefit to those of you that perform in extreme endurance events such as really long marathons and bicycle road races. In events like these you will deplete your glycogen reserves because you can not refuel as quickly as needed from energy- drinks, bars or gels.
A study performed in Switzerland by Michael Vogt showed that a diet of 53 percent fat for five weeks doubled the IMT stores without compromising muscle glycogen stores. So, a higher fat diet (from mostly poly- and monounsaturated fats) can definitely give you an edge in these scenarios.
It’s also known among coaches and athletes that a diet higher in fat and lower in carbohydrates promotes a metabolic shift towards fat for fuel (and thus sparing carbohydrates during exercise).
I’ve used this “higher fat” approach on several athletes with great success. I usually recommend a year-round diet of about 25 to 35 percent fat because of all the health and performance benefits. And if you’re a really hardcore endurance fan, you might give an ever higher fat diet a try for five to six weeks prior to a race and see how it works out.