2.9b Fats or Lipids in Foods

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The main headings of this chapter in the order of the text are here in the descending column just below. Use it to search & find.  Update 21 Nov. 2021

What Are lipids?
 Glycerol is the central chemical structure of lipids 
Fatty Acid (FA) 
Carbon Atom Bonds Saturated vs. Unsaturated 
Methane CH₄. 
Triglycerides
Essential Fatty Acids 
Fatty Acid Energy Production 
Lipid Transport and Lipoproteins 
The Cholesterols 
Total Cholesterol (TC) in the Blood Plasma 
Digestion of Lipid  
Three GI sources go into affecting blood cholesterol 
Effective anti-cholesterol chemicals  

What Are lipids?  They are caloric nutrients that neither fit the formula for carbohydrate (Carbon plus water molecule) nor for proteins (Amino acid nitrogen plus carbon, nitrogen, sulfur, or phosphorus) and that on hydrolysis (Boil in acid water) yield fatty acids and alcohols. Lipids are also called fats or oils.  Most lipids are soluble in organic fluids like benzene but insoluble in water, making them important in cell membrane separation. Some lipids are attached to ionic chemical groups like sulfate (SO4--) or phosphate (PO4---) and have affinity also for water and organic solvents, which makes these lipids important in intestinal digestion (Emulsification of fat by bile). Lipids resist passage of electric current and so are good insulators (Myelin sheaths of nerve fibers). Lipids as triglyceride fat are an important source of calorie storage, more so than starches or glycogen for carbohydrates as we all too often see in life. The brain is mostly lipids.

         Glycerol is the central chemical structure of lipids like glucose in carbohydrates. Also note that the three basic chemicals that structure carbohydrates, lipids, and proteins start with the "g", glucose, glycerol, and glycine, with 6, 3 and 2 carbons in each respectively. Glycerol also connects with glucose through its close 3-C structure glyceraldehyde which is a central part of the glycolytic cycle that starts glucose oxidation energy production as ATP-producing fuel.

The Glycerol Molecular Structure

H

|

H-C-OH

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H-C-OH

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H-C-OH

|

H

Lipids are built up from the 3-C chemical unit in glycerol, an alcohol ("Alcohol", note the -ol suffix, here is a general term for a carbon structure attached to OH radical).

Glycerol is one half of the glucose molecule. The two are inter-convertible in the reaction : glycerol + glycerol ====> glucose + water (H₂O) through the glycolytic cycle and get stored as fat, and the fat can be broken down to produce ATP energy packets making lipids a good source of energy storage.

Fatty Acid (FA) molecules result from boiling lipids in acid water but in the body this reaction (hydrolysis) needs an enzyme. The FA formula is R-COOH where R stands for a carbon chain that has at core (C-C-C-….). The –COOH (“carboxyl” group) can be seen in the below as a C with double-bond O coming up out of the plane of this paper and, the C connecting both O atoms and a single bond –O-, which can attract an H+ to make the COO-H+. ionic bond   

Carboxylate ion

Above: The 3D structure of the carboxyl group, often simplified to -COOH (note that COOH is acidic in that it liberates H+, in contrast to NH2, which is alkaline because it picks up an H+, or H+ ion.  This becomes very notable when we deal with protein amino acids, which are linear molecules that have -COOH at one end and -NH2 at the other end; (See next chapter on Proteins)

 The Fatty Acids

FA₁, formic acid ( H₃C-COOH; formica, the Latin word for the insect Ant)

FA₂ , acetic acid in vinegar: C(H₃)-C(H₂)-COOH. The (H₃  & H₂) signifies the number of H atoms attached to each C atom at its free bonding sites, usually written without bracket, and the bonding site relationship is understood, e.g., CH₃-CH₂-COOH.

FA₃, propionic acid gives sweat its odor: CH₃-CH₂CH₂-COOH.  

FA₄, butyric acid, FA of butter: CH₃-CH₂CH₂CH₂-COOH, note that 3-gamma amino butyric acid. GABA, is the famous neuro-transmitter that inhibits nerve impulses. 

Note the -ic suffix for fatty acid naming.

And on up (Check Google for Fatty Acid).

Three distinct groups of the FA are the 2 to 4 C (short-chain; just shown above); the 6 to 10 C (medium-chain); and the 12 to 26 C (long-chain). The long-chain even-numbered FA's are particularly important in nutrition and metabolism. (The number of C in a chain is indicated by subscript)

   Most fat in food is in FA₁₈ to FA₂₁ and is further classified by absence or presence of carbon double bonds (-C=C-).

Carbon Atom Bonds Saturated vs. Unsaturated: Each atom has sites on its surface where it can bond with another atom to form a molecule. In the water molecule, H₂O, whose structure is more clearly shown by HOH, the H atoms have one bonding site each and the O atom has two bonding sites. In single-line bonding, each of the bonded atoms at the site is donating its outer orbital electron to make a 2-electron bond, and the energy requirements of bonding are maximally satisfied. Single-line bonds are said to be saturated because bond sites are maximally satisfied. Each element atom has characteristic number of maximal bond sites: hydrogen H has one, oxygen O has 2 sites and Carbon C has up to 4 sites to bind other atoms as in Methane CH₄.

H

H--C--H

H

Methane is the simplest saturated hydrocarbon (only H & C atoms) and the lipids formed from it are called saturated. In some carbon compound molecules, the C atom is doubly or triply bonded to another C atom (e.g., ethylene, written ₂HC=CH₂ to show double bonding C; or acetylene gas, written H-C≡C-H to show triple bonding). It means that 2- and 3-electron pairs respectively are shared electrons one from each C-atom bond site instead of the saturated one-line, 2-electron bonds. Such molecule is unsaturated and a lipid made up of these molecules is called unsaturated (unsat fat) and can be mono- or poly-unsaturated with single or multiple saturated fatty acids. A mono-unsaturated fatty acid olive oil is healthier than a saturated fat like lard or butter but healthiest are the polyunsaturated fats (PUF[A]) on food label = polyunsaturated fatty [acid]) vegetable oils. 

Triglycerides: Fatty acids (FA) react with the –OH ends of the glycerol molecule in an acid-base type neutralization so that the FA loses the H+ of its COO-H+ end and the glycerol loses its OH- end. When 3 FA molecules combine with a Glycerol molecule in this way it makes a Triglyceride (TG), which makes up the typical yellow fat of our bodies. In a TG molecule, each OH of a glycerol is replaced by an FA structure that has lost its acidic H+ so it is also called neutral (acid-base neutral) fat. Triglyceride is individualized by specific combinations of its three FA, (below subscripts x, y, and z).

   Triglycerides are the energy storage form of fats.  Much more triglyceride can be stored in the body than the carbohydrate storage glycogen.

H

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   H-C-O-FA_x

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   H-C-O-FA_y

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   H-C-O-FA_z

|

H

As mentioned above, the FA is an actual C, H and O fatty acid. Triglycerides from plants have mostly 18 C unsaturated FAs and are liquid oils at room temp while TGs from animals are up to 22 C mostly saturated and solid fats like lard.

   Another major class of lipids is constructed when one of the glycerol OH reacts with phosphoric acid while the other two –OH react with FA, and the result is a phospholipid, one of which, known as Lecithin, is an important component of cell membranes like cell walls and tissue linings. The phosphate linkage allows it to transport water-soluble molecules like glucose through the cell wall.

Essential Fatty Acids: Many fatty acids and all glycerol can be made in our bodies from glucose. The few fatty acids that cannot be made in our bodies, and must be supplied in diet are essential fatty acids (EFA) and found only in plant food, and one of the most important is polyunsaturated C₁₈ linoleic acid because it makes arachidonic acid of the prostaglandins that are important in inflammation and of myelin that sheathes nerves. Deficiency in EFA leads to weak immunity and severe defect in the nervous system. 

 The ω−3 (omega-3) oils are essential unsaturated fatty acids sold as large jelly capsules highly touted for good blood lipid levels.

Fatty Acid Energy Production: The normal level of fatty acids in human blood is 8 to 31 mg/dL and is important because FAs serve as replacement for blood glucose in energy production in starvation or other low blood sugar condition. The significance of FAs in energy production is that FA can be produced by breaking down fat and protein in a starvation condition. In starvation, the body’s reserve of carbohydrate is quickly depleted since its glycogen cannot be stored enough to go for more than a day without food and then a starving person must start to break down body fat and protein into fatty acids. Diabetes mellitus comes from a lack of insulin that prevents food carbohydrate from being used for energy production. It is like starvation without famine. In diabetes, because carbohydrate cannot be utilized, the fat and protein are broken down for FA, resulting in excess ketones and acidity in blood from the FA – diabetic keto-acidosis. And, continuing the idea, the FAs are the energy source of last resort for a body deprived of glucose and in circulatory shock like in a heart attack or stroke; it is the supply of FAs in your blood that keeps heart and brain tissue alive for the extra 5 minutes that CPR needs to restore adequate oxygen and blood-glucose supply for energy production when there is no source of oxygen. The energy produced by 1 mole of the main human FA, the 16 C palmitic acid, is twice that produced by an equivalent of blood glucose.

Lipid Transport and Lipoproteins: Lipids absorbed from the intestine or made in the liver need to be transported in the blood plasma but, as the free molecules, they are insoluble. So they need processing by attaching them to molecules named “lipoproteins” (big bodies composed of triglycerides, phospholipids, various cholesterols and a binding protein called apo{lipo}protein).

Above is a lipoprotein. It functions as shuttle in blood plasma for lipid molecules absorbed from the intestines and here is shown transporting hundreds of such molecules. Three types of lipoprotein vary in mass density in centrifuge: the Chylomicron, the Low-Density Lipoprotein (LDL), and the High-Density Lipoprotein (HDL). The chylomicron forms in the intestinal absorptive cells and transports the food-digested lipids from gut to liver. Chylomicrons are in blood in the few hours after eating, not in the longer-than-8-hour fasting state. In liver, Chylomicrons are changed to LDL-cholesterol particles and transported around body in the blood.

The Cholesterols

   The LDL cholesterol (LDL-C) is called “bad” cholesterol because its elevation in blood test gives increased risk of myocardial infarction (heart attack) and cerebral infarction (brain stroke). It brings cholesterol into tissues and may overload them on the walls of coronary and cerebral arteries, narrowing the arteries and cutting oxygen to heart muscle and brain neurons. 

 The HDL cholesterol (HDL-C) in increased level gives protection against the bad effect of LDL cholesterol because it removes free cholesterol from tissues and so it is called “good” cholesterol.

Cholesterol is a Sterol which is in the general chemical class of Alcohols (Told by the -ol end suffix). The Sterols are alcohols with general formula R-OH, where R is a complex organic molecule such as seen in the cholesterol structure.  
   Use a loupe for the diagram. Also you may Google "cholesterol structure"

You see the Steroid Nucleus in the ABCD structure. The atoms indicated in the cholesterol molecule are C carbon, H hydrogen, and O oxygen. The –OH is the hydroxyl radical, which is a closely bonded H and O with a free bonding site on O, as indicated –OH. The –CH₃ is the methyl radical with one free bonding site on the C as indicated by –CH₃. Note the Steroid Nucleus structure is composed of the 3 attached hexagons (A, B and C) and the attached pentagon (D). Each numbered bare apex (of the hexagons and pentagon) should be visualized as having two H atoms attached, one up and one down at 90⁰ to the plane of this page or screen, and the vertical up-going lines from the crevice or apex at C¹⁰ and C¹³ should be visualized as attachment of a CH₃ radical, which replaces one of the two H atoms to an apex C atom. The numbers inside the hexagons and the pentagon tell the numbered order of carbon atom according to a system that runs counterclockwise with the A & B hexagons, jumps to the C hexagon, switching to clockwise, and then jumps to the D pentagon, switching back to counterclockwise. The double line, =, between C⁵ and C⁶ in the B hexagon indicates double bonded Carbon atoms and the two C atoms involved, C₅=C₆ each lack the usual 2^nd H atom and so have HC=CH bonding. The “steroid nucleus” is the common central chemical skeleton also of the steroid hormones that include the estrogen and testosterone sex steroids, the androgen sports steroids and the corticosteroids (Noted by the suffix “-stero-“, e.g., corticosteroid”.)

Cholesterol is the main sterol in animals; it is not essential in human nutrition: we produce it from glucose and water. Thus even with a fat-free, cholesterol-absent diet, one could have high cholesterol from eating high-calorie non-cholesterol food. Still, it is best to avoid food high in LDL cholesterol for a long, healthy life. 

Total Cholesterol (TC) in the Blood Plasma: Sources of your TC are the foods you eat and your internal synthesis from the simplest carbohydrate blood sugar Glucose. Recently the internal synthesis pathway is important because of the popular, effective lowering-cholesterol Statin drugs like Lipitor (All these family of drugs have the generic-name -statin suffix). These drugs selectively inhibit the rate-limiting enzyme HMG-CoA reductase in an early step in converting Glucose to Cholesterol and thereby reduce formation of Cholesterol from sugar.

Note that having a low LDL is the most important good-health factor in the cholesterol blood test. It is more important than having very low total cholesterol. The total cholesterol also contains HDL-cholesterol, which picks up excess cholesterol from tissues and transports it to liver for excretion.

Cholesterol absorbed from intestine makes first pass through the liver via the portal vein, which bypasses the general circulation. In liver cells, it is processed into the cholesterol that circulates in blood. One third of the daily production of cholesterol is immediately excreted into the bile as either free cholesterol or converted to the bile acids and excreted. But the bile acids bring more cholesterol into the system by assisting its digestion from food and absorption in small intestine wall. And about 90% of the cholesterol excreted in bile as cholesterol or bile acids is reabsorbed in the lower intestine. 

Digestion of Lipid may involve indigestion with belly cramp, bloating and diarrhea. Why? In contrast to carbohydrate, whose basic sugar unit dissolves easily in water and is quickly and easily digested and absorbed into blood for body use, the triglycerides are not soluble in water. So they need to be coated with water-soluble agent. This doubles the volume and that results in bloating. The TG in food is encased in fat cells, which are part of thick-walled connective tissue that – if not disrupted by adequate chewing or special food preparation and cooking (processing as soup, oil, butter, cream, milk) – present your digestion with hunks of tissue nearly impossible for the stomach and intestine to completely digest. This incompletely digested fat cannot be absorbed in intestine and so it passes down to colon and rectum as loose, foul-smelling, bulky feces in the bowel movements and becomes especially bad for the older person who has trouble with gallbladder or has lost it to surgery.

   Fat digestion starts on a food preparation counter by cutting fatty food into small pieces to increase surface volume and assist chewing. Then, cooking breaks down complex molecule into simpler fat molecule, easier for digestive enzyme to work on. Chewing food well will destroy fat cell wall, liberating fat to exist freely in stomach so it can be digested. In stomach the chewed fat food mixes about in the acid juice for several hours, which starts the breakdown of large fat molecules. Next, in duodenum, partly digested food is mixed with bile from gallbladder, which allows the TriGlyceride (TG) molecules to connect with water molecules, and also it mixes with digestive factor that splits each TG molecule into 1 glycerol and 3 FA. Then free glycerol molecule and the FA molecules are absorbed into blood through pores in intestine wall, transported on chylomicrons in portal vein blood to liver and metabolized in liver to LDL-cholesterol for body use. (Converted to glucose and burned for energy in physically active person or stored as layer of body fat for sluggish physically inactive person)

   When one realizes that triglycerides and cholesterol, being available by internal body synthesis from glucose and water, are not essential foods, the best advice is to have “an edge” against eating fat. Eat it infrequently and make smaller or smallest portion your rule. 

Always, when you are deciding to eat, aim for healthier, happier, long-lived you because you eat less (food and fat). And promote feeling of guilt when you give in to fat: You should know it is killing you slowly and badly.

Cholesterol digestion has interest from standpoint of stopping it. Some lower-cholesterol drugs do that. Cholesterol in food comes only from other animal product food. In meat, cholesterol exists in lipoprotein-cholesterol molecule. It is not easily digestible and for that we ought to thank Lady Luck, for if it were, we would all be dead of artery disease before age 50! 

Three GI sources go into affecting blood cholesterol: food we eat, our bile secretion from gallbladder, and the cells sloughed off from upper intestine. Meat, egg yolk, seafood and dairy product provide most cholesterol in foods. Bile from the gallbladder is important in dissolving the cholesterol molecule in water, a necessity for absorption. And bile itself contains cholesterol and its unopposed excretion is a pathway for lowering cholesterol. The effect of removing the gallbladder is mixed: it lowers cholesterol in intestine but also stops its way of getting rid of body cholesterol. Studies show that the total cholesterol falls 30% initially after gallbladder removal.

   The digestive process for food cholesterol is same as for other lipids. Freed-up pure cholesterol molecule is absorbed intact in the microscopic size pores of small intestine, mostly middle and terminal ileum. But cholesterol is poorly digested and absorbed; so under best conditions only 50% gets into blood and the other 50% goes into toilet bowl.

   Effective anti-cholesterol chemicals – β-Sitosterol (ST) – exists in nature. Sitosterol is a sterol with molecular structure resembling cholesterol. Its size is just right for it to fool the micro-pores of your intestine into dealing with it as if it were cholesterol, and that allows the ST molecule to settle like a too big golf ball into micro-pore hole meant for the cholesterol molecule. But, unlike cholesterol, it is too large to get through the hole so it just sits, stuck in it, blocking. Where can you get ST? Easy! Fresh fruit and vegetable, especially carrot! I have raw fruit and veg salad much as I can. So eating much vegetable a day keeps bad cholesterol away! And does it? You bet! My serum total cholesterol according to latest blood test was 107 mg% and my LDL below 60 mg%.

      Chapter 9 Continues next Section on Proteins. To read now, click 2.9c Proteins and Amino Acids in Nutrition