Dietary Fat

So much is touted in popular magazines about good and bad fats, it behooves us to do a review of dietary fat. If something in our body is out of synchrony, then we will understand better how to help ourselves to improved wellness. When a health professional speaks, we will have better odds to understand what they are saying.

Body fat is water insoluble but soluble in organic solvents. Thus, our diet is key to fat metabolism. Dietary fats are solid at room temperature, oils are liquid at room temperature.

Types of Dietary Fats/Lipids

  • fatty acids (basic unit of many lipids)
  • triglycerides (glycerol, a three carbon compound, and three fatty acids joined together)
  • phospholipids (similar to triglycerides except phosphate group and a nitrogen compound replaces one of the fatty acid molecules)
  • sterols (alcoholic or OH group on them)

Fatty Acids

A fatty acid has a chain of ten carbon atoms joined together. Hydrogen atoms are joined so that each carbon atom has four single bonds. The first carbon is part of a carboxyl group in which there is a double bond between the carbon and an oxygen molecule. Think carb = carbon; oxy= oxygen. An organic acid group, carboxyl groups are also in amino acids.

A saturated fatty acid is when there are only single bonds between adjacent carbon atoms. Unsaturated fatty acids can be arranged two ways across the double bond. Polyunsaturated indicates more than one double bond is present. Linoleic acid, with two double bonds and 18 carbon atoms, is an essential nutrient. It cannot be manufactured by the body. Linoleic acid is a major component of many dietary fats. All other fats can be manufactured by the body.

Triglycerides

  • Short chain fatty acids are attached to glycerol resulting in a liquid triglyceride. Coconut oil is short chain.
  • Long chain saturated fatty acids are attached to glycerol making a solid triglyceride. Fat in meat is long and saturated.
  • Long chain unsaturated fatty acids are liquid at room temperature. Corn oil is long chain.
  • The double bonds of unsaturated fats are vulnerable, meaning in practical terms, it can become rancid. To protect oils and margarine, refrigeration or antioxidants such as BHA (butylated hydroxyanisole) or BHT (butylated hydroxytoluene) are used, protecting the double bonds from oxidation.

The physical state depends on the fatty acids attached to it. The melting point of the triglyceride increases (i.e. becomes more solid) as the fatty acids of a triglyceride increase in length.  Conversely, the melting point of the triglyceride decreases into more a liquid-like substance as the number of double bonds in the fatty acids increase.

Hydrogenation adds hydrogen atoms to a double bond to make a single bond. Vegetable oils are turned into margarine. The hydrogenation side effect produces some trans double bonds which may be harmful to us. Transfats have been identified as the culprit to much of society’s collective growing obesity since it is very difficult to get rid of these double bond fats.

Phospholipids

Phospholipids are natural emulsifying agents enabling a molecule to contain both a water-soluble and a lipid-soluble group in membranes and in blood proteins synthesized by the body. This makes the membrane quite flexible.

Sterols

Cholesterol is the best known sterol. Sterols have a 4 ring chemical structure. The rings are made of carbon atoms joined together. A fatty acid can be added on the end. A cholesterol molecule and a fatty acid chain attached to its alcoholic OH group makes a cholesterol ester (which are found in the blood).

Lipid Digestion and Absorption

Digestion

There is no lipid digestion in the stomach. Most of the fat is in the form of large globules. Fat is the last component of the food eaten to leave the stomach, triggering the intestinal cellular release of the hormone cholescystikinin which travels to the gall bladder via the blood, to result in the release of bile.

Bile contains bile salts derived from cholesterol which help emulsify the fat. Mechanical action of small intestine plus phospholipids help in this function, making very tiny fat particles which are now capable  of being acted upon by fat-digesting enzymes.

The pancreas releases three enzymes into the small intestine (duodenum) to aid in fat digestion:

  1. Pancreatic lipase hydrolyzes off the two end fatty acids from the triglyceride containing long chain fatty acids. This leaves monoglycerides.
  2. Cholesterol  esterase hydrolyzes the fatty acid from cholesterol esters to produce free cholesterol
  3. Phospholipase hydrolyzes some phospholipid molecules.

We can see that to have pancreatic problems, as with diabetes, it is not just a carbohydrate issue.

Absorption

  • Simple diffusion (the movement of molecules from one location to another because of random thermal molecular motion) is the primary absorption mode.
  • Intestinal lipase in the mucosal cell lining of the small intestine hydrolyze triglycerides into fatty acids and glycerol.
  • Route: enter villi of capillaries to portal vein to liver.
  • Albumin blood protein binds fatty acid molecules to facilitate travel to the liver.
  • In the mucosal cells, long-chain fatty acids are re-formed into triglyceride molecules, combined with absorbed cholesterol and phospholipids and some protein.
  • Chylomicrons formed from triglyceride, phospholipid and cholesterol and some protein, entering lymph channels (lacteals), to lymph vessels to general circulation. Thus after a fat-rich meal, the blood plasma is milky because of the presence of the chylomicrons.
  • Bile salts (which aided lipid digestion and absorption) are absorbed in the ileum (the distal portion of the small intestine) and go by the portal vein to liver. Normally 95% absorbed of dietary lipids.
  • Steatorrhea is the excessive lipid in the stool, a disruption of digestion and/or absorption.

Functions of Fat

  1. Dietary lipid is the source of the essential nutrient linoleic acid.
  2. Satiation- presence of fat slows down the digestion and absorption process and delays the onset of hunger.
  3. Concentrated energy source.
  4. Absorption of vitamins A, D, E, K and their precursors.
  5. Palatability of meals: frying, spreads, salad dressing etc.
  6. Provides energy reserve in the body.
  7. Linoleic acid effective in preventing dermatitis, promotes normal growth.
  8. Structural component of nervous tissue and membranes.
  9. Insulator from cold, stored fat in body adds protection from mechanical shock  to essential organs, acts like a cushion.
  10. Prostoglandins, prostocyclins and thromoxanes potent agents are formed with the early precursor linoleic acid.

Diet and Heart Disease–Part 8

Janet Wiebe

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