Just as lipids require special handling in the digestive tract to move within a water-based environment, they require similar handling to travel in the bloodstream.
Inside the intestinal cells, the monoglycerides and fatty acids reassemble themselves into triglycerides. Triglycerides, cholesterol, and phospholipids form lipoproteins when joined with a protein carrier.
Lipoproteins have an inner core that is primarily made up of triglycerides and cholesterol esters a cholesterol ester is a cholesterol linked to a fatty acid.
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. Cholesterols are poorly absorbed when compared to phospholipids and triglycerides. Cholesterol absorption is aided by an increase in dietary fat components and is hindered by high fiber content.
This is the reason that a high intake of fiber is recommended to decrease blood cholesterol. Foods high in fiber such as fresh fruits, vegetables, and oats can bind bile salts and cholesterol, preventing their absorption and carrying them out of the colon.
If fat malabsorption persists the condition is known as steatorrhea. Before the prepackaged food industry, fitness centers, and weight-loss programs, our ancestors worked hard to even locate a meal. They made plans, not for losing those last ten pounds to fit into a bathing suit for vacation, but rather for finding food. Today, this is why we can go long periods without eating, whether we are sick with a vanished appetite, our physical activity level has increased, or there is simply no food available.
Our bodies reserve fuel for a rainy day. One way the body stores fat was previously touched upon in the Carbohydrates chapter. The body transforms carbohydrates into glycogen that is in turn stored in the muscles for energy.
When the muscles reach their capacity for glycogen storage, the excess is returned to the liver, where it is converted into triglycerides and then stored as fat. In a similar manner, much of the triglycerides the body receives from food is transported to fat storehouses within the body if not used for producing energy.
The chylomicrons are responsible for shuttling the triglycerides to various locations such as the muscles, breasts, external layers under the skin, and internal fat layers of the abdomen, thighs, and buttocks where they are stored by the body in adipose tissue for future use. How is this accomplished? Recall that chylomicrons are large lipoproteins that contain a triglyceride and fatty-acid core.
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. Bile salts have both a hydrophobic and a hydrophilic side, so they are attracted to both fats and water.
This makes them effective emulsifiers, meaning that they break large fat globules into smaller droplets. Emulsification makes lipids more accessible to digestive enzymes by increasing the surface area for them to act see Fig. The pancreas secretes pancreatic lipases into the small intestine to enzymatically digest triglycerides. Triglycerides are broken down to fatty acids, monoglycerides glycerol backbone with one fatty acid still attached , and some free glycerol.
Cholesterol and fat-soluble vitamins do not need to be enzymatically digested see Fig. Next, those products of fat digestion fatty acids, monoglycerides, glycerol, cholesterol, and fat-soluble vitamins need to enter into the circulation so that they can be used by cells around the body.
Again, bile helps with this process. Bile salts cluster around the products of fat digestion to form structures called micelles , which help the fats get close enough to the microvilli of intestinal cells so that they can be absorbed. The products of fat digestion diffuse across the membrane of the intestinal cells, and bile salts are recycled back to do more work emulsifying fat and forming micelles.
Lipid digestion and absorption in the small intestine. Once inside the intestinal cell, short- and medium-chain fatty acids and glycerol can be directly absorbed into the bloodstream, but larger lipids such as long-chain fatty acids, monoglycerides, fat-soluble vitamins, and cholesterol need help with absorption and transport to the bloodstream.
Long-chain fatty acids and monoglycerides reassemble into triglycerides within the intestinal cell, and along with cholesterol and fat-soluble vitamins, are then incorporated into transport vehicles called chylomicrons. Chylomicrons are large structures with a core of triglycerides and cholesterol and an outer membrane made up of phospholipids, interspersed with proteins called apolipoproteins and cholesterol. This outer membrane makes them water-soluble so that they can travel in the aqueous environment of the body.
This finding suggests, but does not prove, that solid fats may present more digestive issues. It is important to note, however, that the study did not look directly at humans and used only one type of fat emulsion. The type of fat is not the only factor determining how hard a food is to digest. Certain foods , such as fried foods, are more difficult for the body to digest and more likely to cause digestive problems. Digestive issues can also result from food sensitivities in some cases, so people with a history of digestive problems may wish to try keeping a food diary to track their diet and symptoms.
Effective fat digestion is critical for overall health, as the body needs fat to carry out many of its functions. Effective fat digestion may even play a role in maintaining a moderate body weight. People may be able to improve their fat digestion by:. Although some natural and alternative medicine proponents argue that taking certain digestive enzymes or supplements may improve fat digestion, there is not enough scientific evidence to support this claim.
Fat digestion is a complex process that takes time and requires a functioning liver, pancreas, stomach, and small intestine, as well as numerous digestive enzymes. People who worry that they may not be properly digesting or absorbing fat should contact a doctor, as no home treatment can reliably improve fat digestion. Fats are an essential component of nutrition, alongside carbohydrates and protein.
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