How does my body fuel exercise?
It depends on the exercise!
The body has 3 main fuel tanks it uses - glucose stored as muscle glycogen, fatty acids stored as triglycerides, and muscle protein (amino acids).
(For this topic we are not discussing blood glucose as a fuel source because - while it is a fuel source - its supply is extremely limited and is does not provide very much fuel for an activity. See ==> how-far-you-can-go-on-blood-glucose-alone.)
During low intensity exercise, fat metabolism (fatty acids) is the main fuel source. A long steady state aerobic endurance run or swim (one which is relatively easy and does not cause shortness of breath) is sustainable and lets us use a large amount of fat as fuel. Fat is the most concentrated source of energy we have (over twice as much potential energy as muscle glycogen or protein). It is an almost unlimited source of energy for low aerobic activities.
In order for your body to be able to use fat metabolism to fuel exercise, a small background of carbohydrate metabolism is required. Think of it as a candle. The fat is the candle wax, and the carbs are the wick. Most of what fuels the candle is the wax (fat metabolism) but it still requires a wick (carbohydrate metabolism) to burn.
But as the exercise intensity increases, a larger proportion of the fuel your body uses will come from muscle glycogen instead of fat metabolism. Muscle glycogen is the storage form of carbohydrates. It is the fuel tank the muscles use to be able to perform more intense activity.
The amount of time that your muscle glycogen stores can last depend on a variety of factors - your fitness level, the intensity of the activity, and even your blood sugar.
Carbohydrates that are consumed during exercise can be metabolized and used to keep blood sugar from dropping. Your muscles can also use some of the carbohydrates that have been consumed and released as glucose into the blood, but most of what your muscles use in more moderate to intense exercise is the glycogen that is already stored there. Your liver will also release its limited glycogen stores to fuel the exercise.
After extended periods of exercise, the available muscle glycogen stores become low. As this fuel tank becomes empty, the body will start to rely more on blood glucose to provide energy. This can lead to a low blood sugar! More discussion about that later.
Muscle protein can also be used to fuel activity, but its contribution is rather small, usually less than 5% for most activities.
As shown in the graph below, as the intensity of exercise increases, the dependence on muscle glycogen - the stored carbohydrate - goes up, and the dependence on fatty acids goes down.
The way the body uses fat metabolism or muscle glycogen is the same for diabetics and non-diabetics. However, it is important to understand how it can affect what happens to our blood sugar depending on the type of activity being performed. This will be covered in a different thread…
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