Digestion, absorption and metabolism of fats
Each day the gastro intestinal (GI) tract receives on average from the food we eat, 50 -100 grams of triglycerides, 4-8 gm of phospholipids and 200-350 mg of cholesterol!.
The goal of fat digestion is to dismantle triglycerides into small molecules the body can absorb and use –namely, monoglycerides, fatty acids and glycerol.
In the mouth:
- An enzyme, lingual lipase, plays an active role in fat digestion (milk) in infants, but a relatively minor role in adults.
- The stomach’s churning action mixes fat with water and acid. A gastric lipase emulsifies a small amount of fat.
- The primary site of digestion and absorption of fats is the small intestine. When fat enters the small intestine, the gallbladder secretes bile and emulsifies the fat. Pancreatic lipase/steapsin, is the most important enzyme in fat digestion and hydrolyzes triglycerides (emulsified fat) to monoglycerides, fatty acids and glycerol (intestinal lipase plays a lesser role).
- The presence of bile is essential for normal digestion and absorption of lipids. The bile salts reduce the surface tension of large globules of unhydrolyzed fat and are broken down to smaller globules under the constant churning action of the intestine. The food fat is reduced into an ultrafine emulsion, which exposes an increased surface of fat molecules to the saponifying action of the pancreatic lipase and alkaline intestinal juice. The bile salts also facilitate the solution of the glycerides and of the fatty acids to be absorbed by the mucosal cells, over the mucosal barrier, directly into the blood and lymph vessels of the small intestine. Bile also solubilize the carotenes and fat-soluble vitamins and assist with their normal absorption.
- After bile enters the small intestines and emulsifies fat, it has two destinations: most of the bile is reabsorbed from the small intestine and recycled; the other possibility is that some of the bile is trapped by dietary fibers (fruits, grains, legumes) in the large intestine and excreted. Because cholesterol is needed to make bile, the excretion of bile effectively reduces blood cholesterol.
Small molecules (glycerol, short and medium chain fatty acids) can diffuse easily into the intestinal cells and be absorbed directly into the blood stream. Larger molecules (monoglycerides and long chain fatty acids) are emulsified by bile, forming complexes, called micelles. The micelles diffuse into the intestinal cells where the monoglycerides and long-chain fatty acids are reassembled into new triglycerides. Within the intestinal cells, the newly made triglycerides and other lipids (cholesterol and phospholipids) are packed with protein into transport vehicles, known as chylomicrons. The chylomicrons move through the lymph until they reach a point of entry into the bloodstream at the thoracic duct near the heart. The blood carries these lipids to the rest of the body for immediate use or storage. 30-40% enters the capillaries of the small intestine and is carried to the liver (via portal circulation).
In normal individuals, 95% of the ingested fat is absorbed. In the absence of bile, as in the case of malnutrition and diseases (cystic fibrosis with pancreatic impairment and celiac disease), fat absorption is poor and fatty stools result (steatorrhea). Vegetable oils and butter, with melting points below 50ᵒC, are effectively digested and utilized, compared to the saturated fatty acids like coconut oil, palm oil, animal fats and hydrogenated peanut butter.
LIPIDS IN CIRCULATION:
The transport vehicles for fat are known as lipoproteins – they transport fat through the watery bloodstream. The four main types are: chylomicrons (lowest in density), VLDL (very low density), LDL (half cholesterol, accounting for heart disease) and HDL (high density) – the more lipids, the less dense; more proteins, more dense.
Lipids are found in body tissues, but also in blood plasma e.g. triglycerides, free fatty acids, cholesterol, phospholipids, cerebrosides, carotenoids and the fat soluble vitamins A, D, E and K. Body lipids are highly dynamic: in the blood they are carried to fat depots to be stored; or they are transported to other tissues, liver and muscles, to be metabolized for energy. After a meal, fat is moved to adipose tissue for storage of excess energy. This stored fat includes absorbed dietary lipids, fat derived from excess dietary carbohydrate and protein. During fasting, lipids are transported away from the fat depots to active metabolizing tissue for energy. During extensive fasting, the fat reserves are mobilized and free fatty acids are poured out from the adipose tissues.
Therefore the steps in fat digestion are:
- Fats are emulsified in the stomach.
- Emulsified fats to fatty acids and glycerol in intestine.
Upon absorption fatty acids and glycerol are recombined to form neutral fat, which is oxidized to furnish energy; or it also may be stored as body fat for reserve; or combine with protein to form complex substances of body tissue.
FAT METABOLISM in the LIVER:
The metabolism of fatty acids take place in the liver. Fatty acids from absorbed fat or released from the adipose tissues, are converted into phospholipids (through phosphorylation) and with glycerol and choline. If insufficient choline is available, phospholipid formation is interrupted and fatty acids accumulate in the liver (fatty acids are transported from the liver to other tissues mostly in phospholipid form). Normally, sufficient choline is ingested in the form of dietary phospholipids (animal foods and eggs). Choline may also be synthesized in the body from methionine. Therefore choline and methionine are lipotropic factors – they transport and utilize fats. In their absence, phospholipids synthesis in the liver is impaired and fatty acids accumulate in the liver, resulting in an abnormal fatty liver.
Fatty acids are oxidized in the liver and muscle tissue and yield energy during this process. Muscles tend to favor carbohydrates for energy, but during extensive or prolonged exercise, fats hydrolyzed from fat depots, are an important energy source for muscle work.
ABNORMAL FAT METABOLISM:
When large amounts of fat are metabolized and there are too many ketone bodies (in excess of what can be metabolized), the concentration of nonoxidized ketone bodies in the blood increases, the blood pH become acid (acidosis) and ketosis ensues. Some of the acetoacetic acid is decarboxylated, (liberating carbon dioxide) and yield acetone. Some of the excess ketone bodies is excreted into the urine (ketonuria).
Abnormal fat metabolism happens when:
- Large amounts of fats are ingested in the absence of carbohydrates.
- Prolonged starvation – no carbs and fat alone from the fat depots, is burned for energy.
- Uncontrolled diabetes – fat metabolism is increased and the rate of ketogenesis exceeds that of oxidative disposal of the accumulating ketone bodies.
SYNTHESIS AND STORAGE:
Fat synthesis takes place in the liver and mammary glands, adipose tissues and in many other tissues. The depot fat in the adipose tissue are neutral fat (triglycerides), while phospholipids and cholesterol are synthesized in the organs. The fatty acid composition in the depot fat varies – the portion which is synthesized from carbohydrate and protein, in the liver and adipose tissue, have a fixed fatty acid composition; the portion derived from ingested fat, tends to vary with the nature of the dietary lipids.
The depot fat composition of normal lean individuals show minor differences with obese, diabetes, and coronary heart disease patients. There is a sex difference in the depot fat composition: men have a greater total of saturated fatty acids and fewer unsaturated fatty acids.