Protein absorption and digestion

Let's talk about the digestion of protein, through polypeptides, all the way to the final product, the aminoacids.

The digestion of the protein starts in the stomach, where the enzyme called pepsin will break the polypeptides into smaller parts. The highly acid medium will help this to happen. The digestion continue in the duodenum and upper small intestine, where the pancreas will secrete another enzyme called chymotripsin, with the role of breaking the small polypeptides into even smaller polypeptides. The final work is done in the lower small intestine, where carboxypeptidase and aminopeptidase enzymes will break everything into aminoacids, and they will be absorbed into the blood stream. About 5% of the proteins will leave the body undigested, through feces.

The single molecules of aminoacids are taken up into the blood stream and distributed across the human body, used to synthesize the so much needed proteins (like albumin in liver or muscle proteins in muscle) and they can have extracellular functions, such as hormones and cell adhesion, or intracellular, such as generating fuel, cell structure, signal transduction. You can find more than 100.000 proteins in our body.

The protein digestion starts in the stomach, when the acid environment causes proteins to unfold, allowing pepsin to access the dietary protein easier. The pepsin enzyme is produced by the Chief cells lining the stomach, and it is present as an inactive pro-enzyme called pepsinogen, being activated by the high acidity. The pepsin will cleave the peptide bonds, creating small polypeptide from the protein. The process is further going into the small intestine (upstream), with the polypeptides being cleaved into even smaller polypeptides under the influence of trying and chymotripsin (which are also created by pancreas as inactive pro-enzymes, being activated when they reach the intestine). In the downstream on the small intestine, the polypeptides are finally transformed into individual aminoacids, under the action of aminopepsidase and carboxypepsidase (both of them produced by the intestinal cells). Those last mentioned enzymes will remove single aminoacids from each end of the peptide. The single aminoacids are taken to the liver, after they were being absorbed via portal circulation, and to the rest of the body, used as building block for the synthesis of the body proteins. In the liver, they are mainly used to synthesize the main serum protein called albumin.

In the next post I will talk about the protein functions (enzymes, structural, hormones, transporter, antibodies and so on).

Dietary fats - Part two (Triglycerides digestion)

Many foods contains substantial amounts of dietary fats. Cooking oils (Sunflower, peanut, olive) got 100% fats as triglycerides. Butter and mayo - got more than 80% fat, bacon and sausages - around 40% fats, as opposed to fruits and veg - little to no fat.

We can have cholesterol only in foods of animal origin (highest amount in egg yolk and shrimps). Saturated fatty acids can be found in lard, butter, coconut oil, and unsaturated fatty acids in vegetable oils. Soy bean oil - has linolenic acids (C18:3), coconut oil - has medium chain fatty acids (8-12 chains, fish oil - has fatty acids with 20-22 chain length. Flax-seed oil is a good source of essential fatty acids (50% linolenic acids).

Fat digestion - we would talk especially about triglycerides digestion. The breakdown of a triglycerides molecule is done under the influence of an enzyme called lipase. It is broken in 2 fatty acids and one monoglyceride attached to a fatty acids. We got lingual lipase, in the mouth, with a role in fat taste detection more than digestion. The process starts in the stomach under the gastric lipase, and it is continued in the intestines, with the pancreatic lipase doing most of the job. When dietary fat enters in the intestines, a hormone called cholecystokinin (CCK) is released, to activate the gall bladder contractions, and the secretion of bile acids. Bile acids are made from cholesterol and are stored in the gall bladder with cholesterol and phospholipids, and has the role of an emulsifier, changing fat in little droplets, for pancreatic lipase easy access. The main bile acid is the cholic acid, can be attached easily to amino-acids like taurine and glycine (making taurocholate and glycocholate - conjugated acids). The broken down fatty acids and the monoglycerides resulted from the digestion of triglycerides form a special structure called mycelle and the bile acids play an important role in it. Then they are taken up by the enterocyte (intestinal cell) and reconverted into triglycerides via a process called re-esterification. Then they are packaged into special particles called chylomicrons - which carry the dietary fats through the body, using the lymphatic circulation (small vessels called lacteals) to reach the subclavian vein.

How to avoid getting fat in few easy steps (part 1)

This is a story of love, a Valentine's Day special, the story of interdependence and attachment between two main macro-nutrients, let's call them carbohydrates and fats.

Carbohydrates can be simple and complex. The simple ones can be classified in monosaccharides (glucose, fructose and galactose) and disaccharides (maltose, sucrose and lactose). The complex ones are glycogen, starchi and fibers.

During digestion, the starch is broken to same degree, under the action of the amylase enzyme, secreted by salivary glands. When the carbohydrates reaches the stomach, the amylase is inactivated by the acidic medium. The digestion continues in the duodenum, under the action of the pancreatic amylase (an enzyme secreted by the pancreas). The starch is broken in maltose, sucrose and lactose. They are transformed in fructose, glucose and galactose under the action of enzymes such as maltase, sucrase and lactase).

Absorbed in the blood stream, the monosaccharides (glucose, fructose and galactose) are then processed in the liver, where almost all the fructose and galactose is used, and a big part of the glucose goes back in the blood stream, in direct relation to the blood sugar level (glucose level).

When glucose level goes up, the pancreas is secreting the insulin hormone, the role of insulin being to transform the glucose from the blood stream into glycogen, and to deposit the glycogen into the muscle and in the liver. After this process the blood sugar level is decreasing. Because of this action, the pancreas is secreting another hormone called glucagon, to break the glycogen from the liver into glucose, and when the liver deposit is finished (for example during a period of fasting), the glycogen from the muscle is used. The glycogen deposit from the muscle it is also used to generate energy when we are exercising. If we finish both of them (the deposits from muscle and from liver), the muscle is used for glycogen and we lose weight.

If both the deposits are up to 100%, the glucose is transformed into fat. But the fat is never transformed back into glucose.

Conclusions:

Fasting and exercise are helping us to deplete the glycogen deposits, so the glucose will not be transformed in fat. (Search on Google about intermittent fasting)

If we fast and exercise too much, we will lose weight (but this will be muscle weight and not fat weight).

It is possible to eat plenty of sweets, as long as you are having a fasting period right before/after, or you exercise, or both. (This is by far my favourite conclusion)


(More to come soon - this is an excerpt from the future second edition of the book The Macronutrients Pocket Guide, written by me, of course )

Take care of your glycogen deposits,
G.