Transport and storage of fuels

Now that it is clear how energy carrying fuels arrive in our body, it is time to look at how they are stored and transported to the places actually expending energy. Different kinds of fuels are stored in different forms and in different amounts. To reiterate, once digested, the following fuels are available:

Fatty acids
Glucose
Amino acids

Fatty acids

Almost all cells can store fatty acids. They can also be transported directly in the bloodstream - no conversion is needed. If it turns out that you do not have enough cells to store all fatty acids, your body can easily generate special 'adipocytes', which put together form the so called 'adipose tissue'. This adipose tissue lives around your thighs and stomach. So from now on, you aren't 'fat', you are 'adiposely well disposed'. Sounds a lot better. These adipocytes store fatty acids in the form of triacylglycerol ^2b.

As can easily be observed, the body easily stores tens of kilos of fat. As each kilo of fat can power an adult for many days, an average person will carry enough energy to survive for a month. An overweight person often carries enough fuel to survive for months on end. ³

Glucose

Glucose is a very small molecule and easily travels from cell to cell. In this way it is easily transported. This motility is not very well suited for actually storing sugars, so for storage sugars are converted to glycogen. Glycogen is a molecule which consists of lots of smaller glucoses. This size makes it easier to store.

As glycogen, sugar is stored in the liver and in muscles. Both liver and muscles can convert glucose to glycogen. The liver can convert glycogen back to glucose but muscles can't. Muscles can however use glycogen directly if needed, or release it into the bloodstream.

Very notable is the limited amount of sugars which can be stored. Ingested glucose and small carbohydrates like table sugar travel nearly directly to the bloodstream. While this allows the body to rapidly utilize ingested sugar, the amount of glucose allowable in blood is easily exceeded.

People of average weight will generally have in the order of 5 grams of glucose in their blood at any one time. Levels above 10 grams are considered too high. This means that a regular candy bar, which contains in the order of 30 grams of sugar, poses a great challenge to the body.

When glucose arrives in excess of the aforementioned 10 grams the body releases insulin which instructs the liver and muscles to absorb glucose from the blood. Furthermore all parts of your body which can run on glucose start doing so. The burning of fatty acids is reduced.

Beyond the bloodstream, the body can store a few hundred grams of glucose. Amounts differ with body mass and bodily condition but is generally in the order of 150 grams. The glucose storage can generally be depleted in a single day, making it a very short-term fuel.

Longer carbohydrates cannot transfer to the bloodstream and must be converted first, which can take quite some time. The delay thus incurred is actually a good thing because it makes sure that the blood isn't flooded with glucose, obviating the need for insulin release and other measures.

Proteins & Amino Acids

These abound throughout the body, either bunched up as proteins or freely available. Can be converted into muscles or cells or lots of other things, which can also be broken down again into proteins or even amino acids.

Proteins are broken down to amino acids in the intestine and then brought to the liver, where they are partly reassembled and partly released into the bloodstream.

Compared to glucose, a lot of proteine is available at any one time. The blood alone will contain in the order of 100 grams.

Compared to either glucose or fatty acids, amino acids also have *huge* uses. It might be stated that you ARE amino acids. They make up your DNA and mostly everything else that is interesting. I find it somewhat revolting that the body actually burns amino acids!

Continue reading about the conversion of energy.