What happens to the fat and protein in the food?

The physical act of chewing breaks down the fat (triglycerides- three molecules of fatty acids ) into smaller droplets. In the stomach, gastric lipase starts to break down triglycerides into diglycerides and fatty acids. Within two to four hours after eating a meal, roughly 30 percent of the triglycerides are converted to diglycerides and fatty acids.

The stomach’s churning and contractions help to disperse the fat molecules, while the diglycerides derived in this process act as further emulsifiers. Once it reaches the small intestine it gets mixed with the bile juice produced by the liver. Bile contains bile salts, lecithin, and substances derived from cholesterol and acts as an emulsifier. It attracts and holds onto fat. Emulsification increases the surface area of lipids over a thousand-fold, making them more accessible to the digestive enzymes. Once they are emulsified the pancreatic lipase breaks down the triglycerides to free fatty acids and glycerol. Bile salts envelop these fatty acids to form micelles which makes them water-soluble and helps in efficient transportation inside the intestinal cells. Here, the fat components are released and disseminated into the cells of the digestive tract lining.

As fats are not water-soluble, they need to be coated by a protein in order for them to travel in the blood to be transported to various cells. Inside the intestinal cells, the monoglycerides and fatty acids reassemble themselves into triglycerides. Triglycerides, cholesterol, and phospholipids together form lipoproteins when joined with a protein carrier. Lipoproteins have an inner core that is primarily made up of triglycerides and cholesterol esters and the outer envelope is made of phospholipids interspersed with proteins and cholesterol. Together they form a chylomicron, which is a large lipoprotein that now enters the lymphatic system and will soon be released into the bloodstream via the jugular vein in the neck. Chylomicrons thus transport food fats( triglycerides) perfectly through the body’s water-based environment to specific destinations such as the liver, muscles, breasts, external layers under the skin, and internal fat layers of the abdomen, thighs, and buttocks. Capillary walls contain an enzyme called lipoprotein-lipase that dismantles the triglycerides in the lipoproteins into fatty acids and glycerol, thus enabling these to enter into the adipose cells. Once inside the adipose cells, the fatty acids and glycerol are reassembled into triglycerides and stored for later use. Muscle cells may also take up the fatty acids and use them for muscular work and generating energy. When a person’s energy requirements exceed the amount of available fuel presented from a recent meal or extended physical activity has exhausted glycogen energy reserves, fat reserves are retrieved for energy utilization. When the body needs additional energy, the adipose tissue responds by breaking down its triglycerides into glycerol and fatty acids directly into the blood. Upon receipt of these substances, the energy-hungry cells break them down further into tiny fragments. These fragments go through a series of chemical reactions that yield energy, carbon dioxide, and water.

Chylomicrons are transporters of fats throughout the watery environment within the body. After about ten hours of circulating throughout the body, chylomicrons gradually release their triglycerides until all that is left of their composition is cholesterol-rich remnants. These remnants are used as raw materials by the liver to formulate specific lipoproteins.

The liver makes VLDL (Very low-density lipoproteins) from remnants of chylomicrons and transport triglycerides from the liver to various tissues in the body. As the VLDLs travel through the circulatory system, the lipoprotein lipase acts on VLDL and removes the triglyceride component and delivers it to the cells. Once the VLDL has lost half of its triglyceride it becomes IDL ( (intermediate density lipoprotein). IDL transports cholesterol and phospholipids in the blood. While travelling in the bloodstream, it gains cholesterol from other lipoproteins and loses its phospholipid component. When IDLs return to the liver, they are transformed into low-density lipoprotein.

LDLs carry cholesterol and other lipids from the liver to tissue throughout the body. LDLs are composed of very small amounts of triglycerides, and house over 50 percent cholesterol and cholesterol esters. Each cell in our body has a receptor for the LDL to bind. Circulating LDL binds with these receptors and once bound they are consumed by the cell and taken apart inside the cell to release the cholesterol which is used up by the cell.  Sometimes the receptors in the cell may not bind well or the mechanism may not be working well too and the LDL may be left in the blood vessel leading to atherosclerosis. Diets rich in saturated fat may lead to the malfunctioning of these receptors. On the other hand, High-density lipoproteins are responsible for carrying cholesterol out of the bloodstream and into the liver, where it is either reused or removed from the body with bile. Hence, these high-density lipoproteins are commonly called “good cholesterol.”

Protein Digestion

Protein from the food you eat is broken down into amino acids, which are absorbed by the cells and used to make several other proteins, and other molecules like DNA.

Although the mechanical breakdown of the protein happens in the mouth most of the digestion of proteins begins in the stomach. The stomach releases gastric juices containing hydrochloric acid and the enzyme, pepsin which initiate the breakdown of the protein. The acidity of the stomach facilitates the unfolding of the proteins that still retain part of their three-dimensional structure even after cooking. Pepsin, breaks down the protein chains into smaller and smaller fragments.

Protein digestion in the stomach takes a longer time than carbohydrate digestion, but a shorter time than fat digestion. Eating a high-protein meal increases the amount of time required to sufficiently break down the meal in the stomach. Food remains in the stomach longer, making you feel full longer.

The pancreas and the cells of the small intestine release additional enzymes that finally break apart the smaller protein fragments into the individual amino acids. In the lower parts of the small intestine, the amino acids are transported from the intestinal lumen through the intestinal cells to the blood.

Once the amino acids are in the blood, they are transported to the liver. Amino acids contain nitrogen, so the further breakdown of amino acids will release nitrogen-containing ammonia. Because ammonia is toxic, the liver transforms it into urea, which is then transported to the kidney and excreted in the urine. Urea is a molecule that contains two nitrogens and is highly soluble in water. This makes it a good choice for transporting excess nitrogen out of the body. Because amino acids are building blocks that the body reserves in order to synthesize other proteins, more than 90 per cent of the protein ingested does not get broken down further than the amino acid monomers.

The cells in our body are constantly breaking down protein (wear and tear) but simultaneously new proteins are also being built. This is referred to as protein turnover. Every day over 250 grams of protein in your body are dismantled and 250 grams of new protein are built. To form these new proteins, amino acids from food and those from protein destruction are used. So amino acids are used to build not only other proteins but to build RNA, DNA and are also used to some extent for energy production( although most of the energy production is taken care of by carbohydrates and fats). It is critical to maintaining amino acid levels by consuming high-quality proteins in the diet, or the amino acids needed for building new proteins will be obtained by increasing protein destruction from other tissues within the body, especially muscle. Protein is not stored a whole lot, unlike carbohydrates and fats and needs to be consumed through diet regularly. This is true especially when you are engaging in Resistance and strength training exercises.