1.4 Major types of foodstuffs

Catabolism starts with foodstuffs. The three major categories of foodstuffs relevant to human metabolism are named on every box of cereal or cup of yoghurt (Figure 1.4-1a). If you look at the top of Figure 1.3-1, you will see lots of names ending in '-ose'. These are sugars:

Glucose, fructose, and galactose are single sugar molecules (monosaccharides).
Sucrose, lactose, and amylose are oligomeric (oligosaccharides) or polymeric sugars (polysaccharides) that have to be broken down to simple ones before they can be utilized.

The sum formulas of sugars can approximately be written as (CH₂O)_n, i.e. they formally are multiples of carbon (C) plus water (H₂O). They are therefore collectively referred to as carbohydrates. These form an important part of our diet. In the diet of many countries, the most calories by far are derived from carbohydrates, contained for example in rice, wheat, or potatoes.

At the center, right, of Figure 1.3-1 you will find 'triacylglycerol'. This is another major player in our diet: Fat. The decomposition of fat yields fatty acids as the main component; these are further degraded to acetyl-CoA. Acetyl-CoA is, at the same time, an intermediate in the complete degradation of carbohydrates and of proteins. Therefore, acetyl-CoA is a central metabolic 'hub' through which all substrates destined for complete oxidative degradation must pass. Acetyl-CoA is also a precursor in several biosynthetic pathways (Figure 1.4-1b).

The third major type of foodstuff are proteins. Of the twenty standard amino acids found in proteins, we can synthesize only ten ourselves; the other ones are called 'essential' amino acids. Therefore, while a low amount of dietary carbohydrates or fat can be compensated by our own biosynthesis,¹ lack of dietary protein cannot; in third world countries, lack of dietary protein is the most common form of malnutrition.

One class of macromolecules that is not covered here are the nucleic acids. Nevertheless, nucleic acids do make up a sizeable fraction of our food. Only the sugar (ribose or deoxyribose) part of each nucleotide can be utilized to gain energy; the bases (A,C,G,T) can be 'salvaged' for use as building blocks for the synthesis of new nucleotides and nucleic acids, or they are modified and excreted.

1: The traditional diet of Eskimos, for example, is extremely low in carbohydrates, since it consists mostly of meat and fish.