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.
As the body calls for additional energy, the adipose tissue responds by dismantling its triglycerides and dispensing 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. Lipoproteins are characterized by size, density, and composition.
As the size of the lipoprotein increases, the density decreases. What should you know about these lipoproteins? Recall that 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. For a given volume of lipid, the smaller the droplet size, the greater the surface area, which means more lipase molecules can get to work. As monoglycerides and fatty acids are liberated through the action of lipase, they retain their association with bile acids and complex with other lipids to form structures called micelles.
Micelles are essentially small aggregates nm in diameter of mixed lipids and bile acids suspended within the ingesta. As the ingesta is mixed, micelles bump into the brush border of small intestinal enterocytes, and the lipids, including monoglyceride and fatty acids, are taken up into the epithelial cells. The major products of lipid digestion - fatty acids and 2-monoglycerides - enter the enterocyte by simple diffusion across the plasma membrane.
A considerable fraction of the fatty acids also enter the enterocyte via a specific fatty acid transporter protein in the membrane. Lipids are transported from the enterocyte into blood by a mechanism distinctly different from what we've seen for monosaccharides and amino acids. Once inside the enterocyte, fatty acids and monoglyceride are transported into the endoplasmic reticulum, where they are used to synthesize triglyeride.
Beginning in the endoplasmic reticulum and continuing in the Golgi, triglyceride is packaged with cholesterol, lipoproteins and other lipids into particles called chylomicrons. Remember where this is occurring - in the absorptive enterocyte of the small intestine. Chylomicrons are extruded from the Golgi into exocytotic vesicles, which are transported to the basolateral aspect of the enterocyte. The vesicles fuse with the plasma membrane and undergo exocytosis, dumping the chylomicrons into the space outside the cells.
Because chylomicrons are particles, virtually all steps in this pathway can be visualized using an electron microscope, as the montage of images below demonstrates. Transport of lipids into the circulation is also different from what occurs with sugars and amino acids. Instead of being absorbed directly into capillary blood, chylomicrons are transported first into the lymphatic vessel that penetrates into each villus called the central lacteal.
Until recently, it was not understood how the large chylomicrons are taken up into the lacteals. Lipase from the pancreas further digests fats into monoglycerides and fatty acids.
Bile again grabs onto the fat to help move it to the tiny hair-like projections of the intestines. These projections, called microvilli, help transport the fats into the cells of the digestive system. From there, the body must absorb fats. To do this, the broken down components of the fats regroup into triacylglycerols. These can join together with cholesterol , phospholipids, and a protein to form lipoproteins.
Lipoproteins enter the lymphatic system, and the body then releases them into the bloodstream. As fat digestion requires numerous enzymes, various conditions can affect this process and, as a result, absorption. Liver disorders, small bowel syndrome, and problems with the small intestine can make it more difficult for the body to digest and absorb fat.
Due to this, some people with these conditions may notice fatty stools. A study suggests that solid fats — those that are solid at room temperature, such as butter — are harder for the body to digest than fat droplets. The study used a model of the human digestive system to see how quickly enzymes could break down the two types of fat.
The digestive model broke down solid fats about half as quickly. 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.
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