Lipid Digestion

Most natural whole foods have inherent enzymes capable of completely or partially digesting lipids if the enzymes are not destroyed through heat and processing. This widely underestimated value of whole, raw, fresh foods has been by and large ignored.

A common argument against the value of inherent food enzymes is the presumed inhospitability of the gastrointestinal tract to all complex proteins. In other words, it is argued that enzymes within the food would simply be broken down into individual amino acids and would not have a chance to effect their enzymatic action which depends upon intact secondary and tertiary structure, the folding of protein chains.

There is considerable evidence, however, that enzymes as well as other macromolecules are able to survive the rigors of digestion. Examples include the ability of bacteria, salivary ptyalin, amylase and pollen to not only survive the digestive tract but be absorbed into the circulation.1-3 Another interesting example of the importance of natural food enzymes is the capability of bile activated lipase in milk to pass into the small intestine intact. This enzyme has thus far been identified in human, dog, cat and non-human primate raw mammary milk. If the enzyme is destroyed through pasteurization, growth has been shown to be cut in half compared to those receiving the enzyme in the whole, raw, natural product. 4-6

Although inherent food enzymes may be a virtue of raw natural foods since they can assist in the digestive process, they are often the enemy in processed foods. Lipase, for example, has the capability under the right conditions to begin hydrolyzing fatty acids from their glycerol backbone. However, this is undesirable in processed foods since once the fatty acids are split they may then more readily degrade and create various off-flavors as well as potentially toxic oxidized molecules. Lipoxygenase found in some raw seeds can catalyze the oxidation of unsaturated fatty acids. Such nutrient degrading enzymes are inactivated by heat (and germination -- a better way) in processing methods. 7

Fresh "from the vine" fatty acids are still ensconced within protective cells and normally do not degrade unless separated from their natural context. Lipoxygenase in soybeans, for example, is activated as soon as the bean is split. Thus food enzymes are an enemy to the food processor who seeks shelf life, but a potential nutritional benefactor.

Food enzymes prepared from microbiological cultures can be used as supplements to replace enzymes lost during processing. These dried, or oil emulsified products, are activated only when hydrated in the digestive tract and can assist in the digestive process.8

DIGESTIVE MECHANISMS
Endogenous digestion proceeds through many steps. When a food is eaten, mastication helps separate the fats from the other components of the food. This permits digestion by enzyme systems which allow less than 5% of fats to pass undigested. Digestion can begin with the secretion of serous glands on the back of the tongue in some species, including humans, continue in the stomach through the action of gastric lipase, and be completed by pancreatic lipase excreted into the small intestine. As lipids enter the duodenum, various hormones such as secretin and cholecystokinin are stimulated. These hormones influence lipid digestion by affecting the pH of the intestinal contents, the release of pancreatic lipase, and the secretion of bile.

The increase in pH that occurs as the food bolus moves into the duodenum is necessary for the activity of pancreatic lipase. Secretion of bile salts from the liver emulsifies the products of lipolysis incorporating them into micelles which are complexes of bile salts, phospholipid molecules, and cholesterol. (Fig. 11)

[ Micelle Transport Image ]
http://www.wysong.net/articles/lipid/figures/figure11.jpg

Shorter chain fatty acids complexed with albumin are capable of being absorbed both in the stomach and in the small intestine. Longer chain triglycerides are disassembled in the small intestine by lipase, solubilized in micelles, and transported into mucosal cells (enterocytes) as free fatty acids. monoglycerides and small amounts of glycerol, diglycerides, cholesterol, and phospholipids. Once in the mucosal cell these components are reassembled into triglycerides, then complexed with protein, carbohydrate or phosphate and incorporated into chylomicrons which are a type of lipoprotein that permits transport of lipids within the watery medium of the blood. The inner core of the chylomicron is composed of nonpolar triglycerides and cholesterol esters, and the membrane is polar being made up of phospholipids, cholesterol, and proteins which permit solubility in blood. (Fig. 12)

[ Lipid Absorption Image ]
http://www.wysong.net/articles/lipid/figures/figure12.jpg

Many more details of lipid digestion have been worked out quite intricately by researchers.9-14 It is interesting to note in this process that the principle of nothing working in isolation holds particularly true. The coordination of a variety of factors within the food itself and within the organism makes possible delivering lipids to the organism for energy and myriad metabolic processes.

Fats are not simply consumed and then passively absorbed through the walls of the intestine. Rather they are prepared by intricate emulsification systems, broken down in specific patterns by enzymes, absorbed by complex mechanisms, reassembled, and prepared for delivery to the body through complexing with a variety of other nutrients. These processes make it possible for non-water-soluble lipid components to be delivered efficiently throughout organisms which are comprised primarily of water… an incredible feat.

References available within book text, click the following link to view this article on wysong.net:
http://www.wysong.net/articles/lipid/03_article_lipid_chapter_three_lipid_digestion.shtml

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