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Fat

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This article is about the type of nutrient in food. For fat in humans and animals, see Adipose tissue. For other uses, see Fat (disambiguation).
A space-filling model of an unsaturated triglyceride.
A fat, or triglyceride, molecule. Note the three fatty acid chains attached to the central glycerol portion of the molecule.
Composition of various fats

Fat is one of the three main macronutrients: fat, carbohydrate, and protein.[1] Fats, also known as triglycerides, are esters of three fatty acid chains and the alcohol glycerol.

The terms "oil", "fat", and "lipid" are often confused. "Oil" normally refers to a fat with short or unsaturated fatty acid chains that is liquid at room temperature, while "fat" may specifically refer to fats that are solids at room temperature. "Lipid" is the general term, as a lipid is not necessarily a triglyceride. Fats, like other lipids, are generally hydrophobic, and are soluble in organic solvents and insoluble in water.

Fat is an important foodstuff for many forms of life, and fats serve both structural and metabolic functions. They are necessary part of the diet of most heterotrophs (including humans). Some fatty acids that are set free by the digestion of fats are called essential because they cannot be synthesized in the body from simpler constituents. There are two essential fatty acids (EFAs) in human nutrition: alpha-linolenic acid (an omega-3 fatty acid) and linoleic acid (an omega-6 fatty acid).[2][3] Other lipids needed by the body can be synthesized from these and other fats. Fats and other lipids are broken down in the body by enzymes called lipases produced in the pancreas.

Dietary Considerations

Dietary fat and oils entered into controversy and manipulation soon after the shortage of fats and oils in Europe during the 1820's spurred adulteration, the growing market for textiles created tons of waste cottonseeds, Wesson invented deodorized cottonseed oil in 1899, and Proctor and Gamble launched partially hydrogenated cottonseed oil as Crisco in 1911. The battle lines hardened in Sept. 1955 when the USA's President Eisenhower suffered a heart attack, and the controversial Dr. Ancel Keyes declared war on dietary fats as the alleged cause.

Today, we still suffer from mis-information, superstition, hunches being touted as medical facts, and slip shod questionable research promoted at the latest "scientific proof". Undeclared conflicts of interest, bias, political interference, government funding awarded to "friendly" researchers, revocation of prior government funding, and marketing propaganda has severely colored the type of research done, how the research was conducted, and the interpretation of results for dietary fat. Almost all of these problems are caused or contributed to by the huge prior investments made, and future profits that stand to be won or lost based on the outcome of the dietary fat controversy.

It is little wonder why the public is confused and jaded at each new pronouncement on dietary fat, or the alternatives thereto.

The following are some of the least disputed "facts" about dietary fat:

  • Many cultures and regions have historically had low or non-existent histories of obesity, heart disease, and/or cancer. This has lead some people to search for the one genetic, lifestyle, environmental, or dietary factor that caused or contributed to this better health outcome, in the hope that that one factor can be exported to all other cultures as the "magic solution" to better health. These included fish oil capsules because of Eskimo's low heart disease and cancer and high fish consumption, Okinawa Japanese longevity allegedly through yams and brown rice consumption, Mediterranean Diet high in olive oil for low obesity and cardiovascular disease, Paleo Diet. and others.
  • Dietary fats contribute 9 kCal per gram, which is significantly more energy than protein or carbohydrates. Therefore, a small change in dietary fats consumed can have a significant effect on the total calories consumed.
  • Consuming dietary fats tend to cause satiation, which helps prevent over-consumption of food and calories. Processed food billed as "Low Fat" or "Fat Free" tend to substitute additional carbohydrates, spices, salt, and/or flavorings to make the food palatable and/or addictive (see Bliss point food engineering), resulting in consumption of excessive calories, salt, or carbohydrates.
  • The heart and major muscles tend to prefer (and operate more efficiently) when powered by dietary fat, as contrasted to glucose. Even the brain, after 3 to 5 days to acclimatize itself to a 'low carb, high fat' diet, can get about 70% of its energy from fat that has been metabolized into ketone bodies, with the liver supplying the other 30% of the brain's energy as glucagon (ie. glucose in a concentrated form) that has been produced by the metabolizing of dietary fat. Therefore the body can operate in a safe and healthy manner on dietary fats with minimal or no carbohydrates.
  • Body fat is often used as the body's dumping ground for toxins and other waste products found in the blood that can't be excreted or eliminated in another manner. The composition of the stored body fat (ie. the percentages of trans, PUFA, MUFA, SFA, Omega-3 fatty acid, Omega-6 fatty acid, Omega-7 fatty acid, Omega-9 fatty acid, etc.) is dependent on the person's prior dietary habits, integrated over time. These stored body fats (with the other toxins and waste products stored with it) can rapidly re-enter the bloodstream due to minute-by-minute variations in the energy expended and the food consumed. This can significantly skew the results of any dietary study on the impact of dietary fats, as the results tends to be influenced as a blend of what was eaten during the study, as well as the stored fat that waxes and wains during the study.
  • Short term studies of dietary effects are often misleading, as the long term effects can be significantly different.
  • Studying just one factor at a time (eg. hold everything else constant while asking subjects to take a fish oil capsule once per day for 3 months) often do not adequately nor accurately examine the issues, and so can be misleading.
  • Hydrogenation of oils, deodorization, and artificially created trans-fat from excessive and/or aggressive processing methods (ie. high heat, high temperature, highly reactive chemicals and/or environments, highly oxidizing, fine grinding, excessive solvent or physical extraction and "purification" steps, long storage times, etc.) are not good choices for a healthy diet, as they have been associated with significant metabolic impacts and increased risk of disease.
  • Fresh, whole foods with minimum processing are usually more nutritious than highly processed foods, or the extraction of essential parts of a food into pills, powders, liquids, or capsules.
  • The use of Concentrated Animal Feeding Operations CAFO techniques for farming have significantly changed the fats in farm animals and the resulting meats, as compared to traditional farming method.
  • Polyunsaturated fats PUFA rapidly oxidize and denature over time, or cooking. The rancid or oxidized forms of these fats are generally regarded as significant problems if they are allowed into your diet. If fresh PUFA is included in your diet, similar issues from the metabolites and AGE's Advanced glycation end-product) produced can have considerable long term adverse health effects.
  • Monosaturated fats MUFA are less likely to oxidize or denature, and are therefore usually better than PUFA fats.
  • Saturated fats SFA are the most stable and resistant to oxidation and denaturing.
  • Omega-6 fatty acid are a PUFA that tend to cause or contribute to systemic inflammation (eg. irritable bowel syndrome, rheumatoid arthritis, etc.), and therefore dietary PUFA should be significantly reduced, or minimized.
  • Omega-3 fatty acid fats, even though they are a MUFA, tend to overcome some of the negative effects from Omega-6 dietary fats
  • Since the 1900's, the ratio of the dietary fats has seen dramatic shifts. Today in North America, we have 8 times more PUFA's (Omega-6 fats) in our diets, our subcutaneous fats, and mother's breast milk. This has been mainly caused by the addition of vegetable oils, and far more processed foods that contain significant quantities of PUFA. These dietary changes are spreading more and more around the world due to food costs, multi-national brand marketing, shifting food costs, and similar economic reasons.
  • It is generally believed that the dietary fat ratio between Omega-6 PUFA to Omega-3 MUFA should be 4 or less, with 1:1 ratio ideal.
  • North American diets currently have Omega-6:Omega-3 ratios as high as 20; significantly different from the maximum proposed ratio of 4.
  • The correlation between childhood and adult obesity has been studied many times over the previous 2 centuries, but there was never found to be a statistically valid correlation (ie. being fat as a baby did not predispose that they would become an obese adult). Today in the USA, if a 4 month old baby is obese, there is a 90% chance that child will still be obese when 7 years old. If a 7 year old is obese, there is a 90% change that child will become an obese adult.[2] Therefore being obese at 4 months old predicts with 81% accuracy that the baby will be an obese adult. It is believed that this correlation has suddenly appeared since the 1950's due to the dramatic shift towards increasing dietary carbohydrates and reducing dietary fat consumption in the USA.
  • There is a world-wide epidemic of obese 6 month old babies. It is likely this was not caused by parents refusing to get the baby a gym membership. The type of fats in the human breast milk that has dramatically changed over the last 100 years, compounded by the growing use of infant formula (which is high in carbohydrates, and uses different fats than breast milk), and the mother's changing diet over the decades (both during and prior to the pregnancy, especially the fat and carbohydrate consumption) are all postulated to be the probable causes.

Chemistry of Fats

Fats and oils are categorized according to the number and bonding of the carbon atoms in the aliphatic chain. Fats that are saturated fats have no double bonds between the carbons in the chain. Unsaturated fats have one or more double bonded carbons in the chain. The nomenclature is based on the non-acid (non-carbonyl) end of the chain. This end is called the omega end or the n-end. Thus alpha-linolenic acid is called an omega-3 fatty acid because the 3rd carbon from that end is the first double bonded carbon in the chain counting from that end. Some oils and fats have multiple double bonds and are therefore called polyunsaturated fats. Unsaturated fats can be further divided into cis fats, which are the most common in nature, and trans fats, which are rare in nature. Unsaturated fats can be altered by reaction with hydrogen effected by a catalyst. This action, called hydrogenation, tends to break all the double bonds and makes a fully saturated fat. To make vegetable shortening, then, liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce saturated fats, which have more desirable physical properties e.g., they melt at a desirable temperature (30–40 °C), and store well, whereas polyunsaturated oils go rancid when they react with oxygen in the air. However, trans fats are generated during hydrogenation as contaminants created by an unwanted side reaction on the catalyst during partial hydrogenation. Consumption of such trans fats has shown to increase the risk of coronary heart disease[2][3]

Saturated fats can stack themselves in a closely packed arrangement, so they can solidify easily and are typically solid at room temperature. For example, animal fats tallow and lard are high in saturated fatty acid content and are solids. Olive and linseed oils on the other hand are unsaturated and liquid.

Fats serve both as energy sources for the body, and as stores for energy in excess of what the body needs immediately. Each gram of fat when burned or metabolized releases about 9 food calories (37 kJ = 8.8 kcal).[3] Fats are broken down in the healthy body to release their constituents, glycerol and fatty acids. Glycerol itself can be converted to glucose by the liver and so become a source of energy.

Chemical structure

Main article: Triglyceride
Example of a natural triglyceride with three different fatty acids. One fatty acids is saturated (blue highlighted), another contains one double bond within the carbon chain (green highlighted). The third fatty acid (a polyunsaturated fatty acid, highlighted in red) contains three double bonds within the carbon chain. All carbon-carbon double bonds shown are cis isomers.

There are many different kinds of fats, but each is a variation on the same chemical structure. All fats are derivatives of fatty acids and glycerol. The fat molecules are called triglycerides (triesters of glycerol). Three chains of fatty acid are bonded to each of the three -OH groups of the glycerol by the reaction of the carboxyl end of the fatty acid (-COOH) with the alcohol. HOH (water) is eliminated and the carbons are linked by an -O- bond through dehydration synthesis. This process is called esterification and fats are therefore esters. As a simple visual illustration, if the kinks and angles of these chains were straightened out, the molecule would have the shape of a capital letter E. The fatty acids would each be a horizontal line; the glycerol "backbone" would be the vertical line that joins the horizontal lines. Fats therefore have "ester" bonds.

The properties of any specific fat molecule depend on the particular fatty acids that constitute it. Fatty acids form a family of compounds are composed of increasing numbers of carbon atoms linked into a zig-zag chain (hydrogen atoms to the side). The more carbon atoms there are in any fatty acid, the longer its chain will be. Long chains are more susceptible to intermolecular forces of attraction (in this case, van der Waals forces), and so the longer ones melt at a higher temperature (melting point).

Examples of fatty acids.
trans Unsaturated (Example shown: Elaidic acid) cis Unsaturated (Example shown: Oleic acid) Saturated (Example shown: Stearic acid)
Elaidic-acid-3D-balls.png Oleic-acid-3D-ball-&-stick.png Stearic-acid-3D-balls.png
Elaidic acid is the principal trans unsaturated fatty acid often found in partially hydrogenated vegetable oils.[36] Oleic acid is a cis unsaturated fatty acid making up 55–80% of olive oil.[37] Stearic acid is a saturated fatty acid found in animal fats and is the intended product in full hydrogenation. Stearic acid is neither cis nor trans because it has no carbon-carbon double bonds.

Fatty acid chains may also differ by length, often categorized as short to very long.

  • Short-chain fatty acids (SCFA) are fatty acids with aliphatic tails of fewer than six carbons (i.e. butyric acid).[4]
  • Medium-chain fatty acids (MCFA) are fatty acids with aliphatic tails of 6–12[5] carbons, which can form medium-chain triglycerides.
  • Long-chain fatty acids (LCFA) are fatty acids with aliphatic tails 13 to 21 carbons.[6]
  • Very long chain fatty acids (VLCFA) are fatty acids with aliphatic tails longer than 22 carbons

Any of these aliphatic fatty acid chains may be glycerated and the resultant fats may have tails of different lengths from very short triformin to very long, e.g., cerotic acid, or hexacosanoic acid, a 26-carbon long-chain saturated fatty acid. Long chain fats are exemplified by tallow (lard) whose chains are 17 carbons long. Most fats found in food, whether vegetable or animal, are made up of medium to long-chain fatty acids, usually of equal or nearly equal length. Many cell types can use either glucose or fatty acids for this energy. In particular, heart and skeletal muscle prefer fatty acids. Despite long-standing assertions to the contrary, fatty acids can also be used as a source of fuel for brain cells.[citation needed]

Importance for living organisms

Fats are also sources of essential fatty acids, an important dietary requirement. They provide energy as noted above. Vitamins A, D, E, and K are fat-soluble, meaning they can only be digested, absorbed, and transported in conjunction with fats.

Fats play a vital role in maintaining healthy skin and hair, insulating body organs against shock, maintaining body temperature, and promoting healthy cell function.

Fat also serves as a useful buffer towards a host of diseases. When a particular substance, whether chemical or biotic, reaches unsafe levels in the bloodstream, the body can effectively dilute—or at least maintain equilibrium of—the offending substances by storing it in new fat tissue. This helps to protect vital organs, until such time as the offending substances can be metabolized or removed from the body by such means as excretion, urination, accidental or intentional bloodletting, sebum excretion, and hair growth.

Adipose tissue

The obese mouse on the left has large stores of adipose tissue. For comparison, a mouse with a normal amount of adipose tissue is shown on the right.
Main article: Adipose tissue

In animals, adipose, or fatty tissue is the body's means of storing metabolic energy over extended periods of time. Adipocytes (fat cells) store fat derived from the diet and from liver metabolism. Under energy stress these cells may degrade their stored fat to supply fatty acids and also glycerol to the circulation. These metabolic activities are regulated by several hormones (e.g., insulin, glucagon and epinephrine).

The location of the tissue determines its metabolic profile: visceral fat is located within the abdominal wall (i.e., beneath the wall of abdominal muscle) whereas "subcutaneous fat" is located beneath the skin (and includes fat that is located in the abdominal area beneath the skin but above the abdominal muscle wall). Visceral fat was recently discovered to be a significant producer of signaling chemicals (i.e., hormones), among which several are involved in inflammatory tissue responses. One of these is resistin which has been linked to obesity, insulin resistance, and Type 2 diabetes. This latter result is currently controversial, and there have been reputable studies supporting all sides on the issue.

See also

References

Further reading