Carbohydrates vs Fats
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Carbohydrates vs Fats

Now let's make a brief comparison of carbohydrates and their use in energy generation versus fats for energy generation.

Let's superimpose the diagrams for both the oxidation of glucose and the oxidation of fats. Most of the process is the same, but notice that glucose gets into the process more quickly than do the fats. The fats go through several more steps than do carbohydrates to become acetyl CoA and enter the citric acid cycle.

Diagram showing overall process for oxidation of carbohydrates and fats. [67078.jpg]

Time

Consequently, one of the advantages of glucose and other carbohydrates is that they can enter into the oxidation process much more quickly and provide energy more rapidly.

Polysaccharides, also known as complex carbohydrates, are one step further removed from the citric acid cycle than is glucose. As a result, polysaccharides or complex carbohydrates provide glucose more steadily and more slowly (kind of a time-released glucose) than simpler sugars such as glucose and fructose. If we eat sugar directly, either in the form of glucose or the disaccharide sucrose, it's available almost immediately.

Fats make energy available at a slower pace than carbohydrates.

Energy

On the other hand, gram for gram, fats provide more energy than carbohydrates.

The reason for this is the amount of oxidation that takes place as these compounds are converted to carbon dioxide and water. Carbon for carbon, fats require more oxidation to become CO2 and H2O than do carbohydrates. Roughly speaking, carbohydrates already have one oxygen for every carbon atom, thus, each carbon atom needs only one more oxygen and each pair of hydrogen atoms needs one more oxygen. However, almost every carbon atom in a fat molecule needs two oxygens instead of just one additional one, and each pair of hydrogen atoms still needs one more oxygen. So, just from counting the number of oxygens needed to be added, fats require about half again as much oxygen for the same number of carbon atoms. Because of this, the oxidation of fats takes longer, but it also gives off more energy.

When comparing gram to gram, instead of carbon to carbon, the effect is exaggerated. When you weigh a carbohydrate, more oxygen is included in that weight. When you weigh a fat, you get more carbon atoms per gram and therefore, gram for gram, the fats will give even more energy (over twice as much) than will the carbohydrates. Generally, fats provide about 9 kilocalories per gram and carbohydrates provide about 4 kilocalories per gram. (Using nutritional units, that is 9 Calories/gram for fats and 4 Calories/gram for carbohydrates.)

 

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E-mail instructor: Sue Eggling

Clackamas Community College
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