Metabolism of Carbohydrates in Nutrition

Metabolism of Carbohydrates:

The metabolism of carbohydrates, proteins, and fats is very complicated involving many biochemical reactions. To understand how these nutrients break down in our body, at first, you have to understand many procedures occurring especially in the liver and mitochondria of all cells of our body.

Glucose is quantitatively the most important carbohydrate available to the body whether it be by absorption from the diet or by synthesis within the body. Galactose from the diet or from endogenous sources is rapidly converted to glucose in the liver. Therefore, any discussion of carbohydrate metabolism is essentially that of glucose. The following figure shows the pathway of glucose from the digestive tract to the liver to blood and cells.

Glucose Metabolism:

It consists of an interrelated series of biochemical reactions facilitated by enzymatic activity. Glocose metabolism can’t be completely separated from the metabolism of fats and proteins. On the other hand, proteins are potential sources of glucose. It can converted to fatty acids, glycerol, and certain amino acids. A number of points in the sequence of glucose metabolism are also the cross roads for amino acids and fatty acid metabolism.

Function of Liver in Carbohydrate Metabolism:

The following absorption from the small intestine, the monosaccharide are carried by the portal vein to the liver. Just as the control tower in an airport regulates the flow of traffic in the air, so the liver exercises the principal control of the pathways that glucose shall take. The liver converts galactose and fructose to intermediates in glucose metabolism.

It synthesizes glycogen from glucose, stores it and reconverts it to glucose according to need. It de-aminates amino acid so that the carbon skeletons can be used for the synthesis of glucose if the glycogen stores are depleted. It can transform excess glucose into fatty acids and can also use the glycerol fraction of lipids to form glucose. From carbon skeletons donated by carbohydrate, the liver can also synthesize non-essential amino acids.

Blood Glucose:

By means of the blood circulation glucose is made continuously available to each and every cell of the body as a source of energy and for the synthesis of a variety of substances. The glucose taken from the circulation by the cells is constantly replaced by the liver so that the blood glucose level is maintained within relatively narrow limits.

In the fasting state the blood glucose concentration is normally 80-110 mg per deci-liter (d). Shortly after a meal it rises to about 140-150 mg/dl, but within a few hours the concentration will have returned to the fasting level. Should the blood sugar level reach 160-180 mg/dl. Some glucose will be excreted in the urine. This level, varying somewhat from one individual to another, it is know as the Renal Threshold for Glucose. The regulation of the blood sugar level the liver. It is so efficient that glycosuria doesn’t normally occur. Occasionally, an individual who has a lower renal threshold for glucose but who has no other abnormalities will excrete some glucose after meals that are especially rich in carbohydrate.

Regulation of the Blood Sugar Level:

The liver is the only organ able to supply glucose to the circulation and it also participates in the removal of glucose if not immediately needed. The sources of glucose to the blood and the avenues of its removal. In fig, it shows that glucose is made available to the circulation by –

    i. The absorbed sugars from the diet
    ii. Glycogenolysis
    iii. Gluconeogenesis
    iv. To a lesser extent, re-conversion of pyruvic and lactic acids formed in the glycolytic pathway.

Removal of Glucose from the Blood:

Six pathways are available for the removal of glucose from the blood:
i. The continuous uptake of glucose by every cell in the body and its oxidation for energy.

ii. The conversion of glucose to glycogen by the liver (Glycogenesis).

iii. The synthesis of fats from glucose (Lipogenesis).

iv. The synthesis of numerous carbohydrate derivatives.

v. Glycolysis in the cells.

vi. Elimination of glucose in the urine when the renal threshold is exceeded.

Oxidation of Glucose:

There are two kinds of oxidations. Anaerobic respiration is the process of producing cellular energy without involving oxygen. Aerobic respiration is the process of producing cellular energy involving oxygen.

Within each of the billions of cells of the body, the oxidation of glucose is continuously taking place. The end products of this oxidation are carbon dioxide, water and energy. If the potential energy of glucose, fatty acids and amino acids were released in an explosive reaction, much of it would be lost and wasted as heat. The body cells utilize energy efficiently by releasing a small amount of it at a time in a series of steps that occur in the mitochondria of the cell. The energy liberated in these steps is trapped in the form of Adenosine Tri Phosphate (ATP). This is the compound consisting of adenine, ribose and three phosphate groups, two of which are high-energy phosphate bonds. One of these bonds is broken to release energy whenever required for the innumerable transactions of the cell and in doing so ATP is converted to ADP (Adenine Di-phosphate).


The chemical reactions that constitute glycolysis takes place in the cytoplasmic matrix of the cell. These reactions degrade glucose is to pyruvic acid in preparation for entrance into the mitochondria. They are catalyzed by a specific enzyme in each case, some of which require the presence of inorganic phosphate, inorganic ions, nicotinamide adenine dinucleotide (NAD), and nicotinamide adenine dinucleotide phosphate (NADP).

The reactions don’t require oxygen. Most of the glucose catabolised in the body undergoes breakdown through these steps. The entrance of glucose into the cell is facilitated by insulin. Within the cell the first step in glycolysis is the phosphorylation of glucose with ATP in the presence of glucokinase and magnesium to form glucose 6-phosphate and ADP. The phosphorylated glucose then proceeds through the glycolytic pathway to pyruvic acid and lactic acid.

Lactic Acid:

Pyruvic acid can proceed anaerobically to form lactic acid, which is utilized for muscle contraction under conditions when the energy need exceeds the supply of oxygen. Thus, the runner in a race can continue beyond his or her capacity to supply oxygen to muscles. Under normal conditions only a small amount of lactic acid is formed.