Question

Answer each of the following briefly:

a) Compare the digestion and absorption of carbohydrates, proteins and fats.

b) Describe the three phases of digestion - cephalic, gastric and intestinal.

thank you!

Answer 1

a).

The three primary macronutrients required for the body include, carbohydrates, proteins, and lipids. As per the National Academy of Sciences, the recommended Acceptable Macronutrient Distribution Range (AMDR) for proteins, carbohydrates, and fats as a percent of daily intake in calories is,

· Proteins – 10% to 35%

· Carbohydrates – 45% - 65%

· Fats – 20% to 35% (for all ages, the saturated fats must be less than 10%)

Digestion and absorption of carbohydrates:

The carbohydrate digestion begins in the mouth. Salivary amylase is a digestive enzyme present in the saliva, which catalyses the hydrolysis of complex carbohydrates into simple trisaccharides or disaccharides. It is synthesised by salivary glands present in the mouth and pancreas also synthesise amylase (it is pancreatic amylase). The disaccharides in the small intestine trigger the release of maltase, sucrase, and lactase, which metabolise the disaccharides into monosaccharides such as glucose, fructose, etc. These monosaccharides are absorbed into the circulation.

The carbohydrates are stored in the form of liver glycogen and muscle glycogen, which releases glucose into the circulation whenever needed by the body.

Digestion and absorption of proteins:

Pepsin is a proteolytic enzyme released by chief cells present in the stomach. The digestion of proteins by pepsin best occur at the pH of 2. Now it enters into small intestine. The enzymes trypsin and peptidase are released by the small intestine, where they hydrolyze the peptide bonds and the released amino acids are absorbed into the systemic circulation.

Digestion and absorption of lipids:

Lipase is a digestive enzyme that is involved in the digestion of lipids. The lipase released by pancreas becomes active in the small intestine, where it digests the triglycerides by breaking them into monoglycerides and fatty acids.

The products of lipid digestion are fatty acids and glycerol. The fatty acids are initially dissolved in the epithelial membranes of the villi, from where they diffuse into the villi. The smooth endoplasmic reticulum present in these cells again resynthesize the fats and are encased in a protein. This is known as “lipoproteins.”

Now, the lipoproteins (also known as chylomicrons) are transported to the lacteal (lymphatic capillary) of villi. The villi empty the lacteals into the cysterna chyli (an expansion of thoracic duct). From the thoracic duct, the chylomicrons are empty into the systemic circulation.

b).

The secretion of gastric juices are controlled by the inbuilt enteric nervous system of the gastrointestinal tract, the peripheral nervous system parasympathetic), the hormonal secretions of the gastro intestinal tract and the central nervous system. The secretion of gastric juice occurs in three stages, namely,

· Cephalic phase

· Gastric phase

· Intestinal phase

The effects of ANS (autonomic nervous system), and local reflexes on gastric secretion are given below:

The cephalic phase begins before eating the food, stimulated by the smell, sight, taste or thought of food. The parasympathetic reflex causes stimulates the release of gastric juices (including the hydrochloric acid and pepsin) by means of vagus nerve (Xth cranial nerve).

The gastric phase starts when food enter into the stomach. The presence of food in the stomach causes extension of stomach, it stimulate the vagus nerve. Vagal stimulation causes the release of gastrin, acetylcholine and histamine.

Gastrin and acetylcholine stimulate the release of pepsinogen from the chief cells, and hydrochloric acid from the parietal cells. These gastric juices and digestive enzymes process the food. It is the positive feedback loop occurring during the gastric phase.

Once the pH falls below 2.0 (due to acid secretion), the secretion of gastrin is inhibited, thus, inhibiting the further release of gastric juices. It is the negative regulation occurring during the gastric phase.

The intestinal phase begins when the food starts leaving into the intestine. As more food enter into the intestine, the small intestine inhibits the release of gastric juices from the stomach through the enteric nervous system signalling.

Also, the signals from the duodenum to the medulla oblongata activates the sympathetic neurons, inhibits Vagus activity, which suppress the gastric activity. The cells duodenal endothelium releases hormonal secretions, namely secretin and cholecystokinin, which also reduces the gastric activity.