Sunday, October 19, 2008

Metabolism Part III: How and Why Our Bodies Use Fat

Our bodies use fat for energy for two main reasons.

The first reason is cyclic. We use fat for energy because when we have an abundance of energy, such as after we have had a meal, we store the excess carbs and proteins as fat. We convert the excess carbs and proteins into fat for storage because fat is a very efficient storage medium. One hundred calories of fat has less mass and volume than one hundred calories of carbohydrate.

The second reason is that the brain uses primarily glucose as its fuel source. The brain does not use fat as an energy source because fat cannot cross the blood brain barrier. Since the brain is kind of important, when blood glucose levels are not high, other cells in the body switch fuel sources from blood glucose to fat. Thus, the blood glucose is saved for the brain.

As a side note, during starvation, when blood glucose levels drop, the brain can use ketone bodies as a secondary fuel source. Ketone bodies are derived from fatty acids. However, even during starvation, the brain gets at least 25% of its energy from glucose.

In my next post, I will discuss the liver's role in maintaining blood glucose levels.

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