Fat usually refers  to any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.

Fat in the body is almost entirely in the form of triglycerides that have a glycerol backbone and three attached fatty acids.

Saturated fats, have no double bonds, are derived from animal sources, and are typically solid at room temperatures.

Unsaturated fatty acids have double bones and include two geometric isomers, cis  with both hydrogen atoms bonded to carbon atoms on the same side with the double bond exists, and trans with the two hydrogen atoms on opposite sides.

Cis saturated fatty acids are derived from plant sources, and are liquid at room temperature.

Fat: three fatty acid chains attached to a central glycerol portion of the molecule.

The term often refers specifically to triglycerides (triple esters of glycerol), that are the main components of vegetable oils and of fatty tissue, or, even more narrowly, to triglycerides that are solid or semisolid at room temperature, thus excluding oils.

The term fat may also be used as a synonym of lipid—any biological substance of composed of carbon, hydrogen, or oxygen, that is insoluble in water but soluble in non-polar solvents.

Besides triglycerides, the term fat would include several other types of compounds like mono- and diglycerides, phospholipids (such as lecithin), sterols (such as cholesterol), waxes (such as beeswax), and free fatty acids, which are usually present in human diet in smaller amounts.

Fats are one of the three main macronutrient groups in human diet, along with carbohydrates and proteins, and the main components of common food products like milk, butter,, lard, pork, and cooking oils. 

Fats are a major source of food energy and play important structural and metabolic functions: energy storage, waterproofing, and thermal insulation.

The body can produce the fat it requires from other food ingredients, except for a few essential fatty acids that must be included in the diet. 

Dietary fats carries some flavor and aroma ingredients and vitamins that are not water-soluble.

Fats serve both as energy sources and as stores for energy.

Each gram of fat when burned or metabolized releases about 9 food calories (37 kJ = 8.8 kcal)’

Fats are also sources of essential fatty acids, an important dietary requirement. 

Vitamins A, D, E, and K are fat-soluble and 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, maintaining body temperature, and promoting healthy cell function. 

Fat also serves as a useful buffer against particular substances, chemical or biotic, by storing it in fat tissue, helping to protect vital organs, until offending substances can be metabolized or removed from the body by such means as excretion, urination, sebum excretion, and hair growth.

Adipose tissue, or fatty tissue is the body’s means of storing metabolic energy over extended periods of time. 

Adipocytes store fat derived from the diet and from liver metabolism. 

With energy distress these cells may degrade their stored fat to supply fatty acids and also glycerol to the circulation. 

These metabolic activities are regulated by  hormones: insulin, glucagon and epinephrine.

Adipose tissue also secretes the hormone leptin.

The location of the tissue determines its metabolic profile: 

Visceral fat is located within the abdominal wall.

Subcutaneous fat is located beneath the skin.

Visceral fat is a significant producer of hormones, among which several are involved in inflammatory tissue responses. 

Visceral fat produces resistin which has been linked to obesity, insulin resistance, and Type 2 diabetes. 

Pancreatic lipase acts at the ester bond, hydrolyzing it and releasing fatty acid. 

In its triglyceride form, lipids cannot be absorbed by the duodenum. 

Fatty acids, monoglycerides, and some diglycerides are absorbed by the duodenum, once the triglycerides have been broken down.

In the intestine, lipases and bile split triglycerides into monoacylglycerol and free fatty acids in a process called lipolysis. 

Theses substances are subsequently moved to absorptive enterocyte cells lining the intestines. 

The triglycerides are rebuilt in the enterocytes from their fragments and packaged together with cholesterol and proteins to form chylomicrons. 

Chylomicrons are excreted from the cells and collected by the lymph system and transported to the large vessels near the heart before being mixed into the blood. 

Certain tissues can capture the chylomicrons, releasing the triglycerides to be used as a source of energy. 

Liver cells can synthesize and store triglycerides. 

The hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids to be used as an energy source.

The glycerol component of triglycerides can be converted into glucose, via gluconeogenesis by conversion into dihydroxyacetone phosphate and then into glyceraldehyde 3-phosphate, for brain fuel when it is broken down. 

Fat cells may also be broken down for that reason.

Triglycerides cannot pass through cell membranes freely, lipoprotein lipases must break down triglycerides into free fatty acids and glycerol. 

Fatty acids can then be taken up by cells via the fatty acid transporter.

Triglycerides, as major components of very-low-density lipoprotein (VLDL) and chylomicrons, play an important role in metabolism as energy sources and transporters of dietary fat. 

Triglycerides contain more than twice as much energy (approximately 9 kcal/g or 38 kJ/g) as carbohydrates (approximately 4 kcal/g or 17 kJ/g).

The most common type of fat, in the human diet and most living beings, is a triglyceride, an ester of the triple alcohol glycerol H(–CHOH–)

3H and three fatty acids. 

A triglyceride results  from a condensation reaction between each of glycerol’s –OH groups and the HO– part of the carboxyl group HO(O=)C− of each fatty acid, forming an ester bridge −O−(O=)C− with elimination of a water molecule H2O.

Less common types of fats include diglycerides and monoglycerides, where the esterification is limited to two or just one of glycerol’s –OH groups. 

Other alcohols, may replace glycerol. 

In the phospholipids, one of the fatty acids is replaced by phosphoric acid or a monoester thereof.

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).

Other lipids needed by the body can be synthesized from these and other fats.

Foods contain different amounts of fat with different proportions of saturated and unsaturated fatty acids. 

Some animal products, like beef and dairy products made with whole or reduced fat milk like yogurt, ice cream, cheese and butter have mostly saturated fatty acids.

Other animal products, like pork, poultry, eggs, and seafood have mostly unsaturated fats. 

Industrialized baked goods may use fats with high unsaturated fat contents as well, especially those containing partially hydrogenated oils, and processed foods that are deep-fried in hydrogenated oil are high in saturated fat content.

Plants and fish oil generally contain a higher proportion of unsaturated acids, although there are exceptions such as coconut oil and palm kernel oil.

Foods containing unsaturated fats include avocado, nuts, olive oils, and vegetable oils such as canola.

Food: Lauric acid Myristic acid Palmitic acid Stearic acid

Coconut oil 47% 18% 9% 3%

Palm kernel oil 48% 1% 44% 5%

Butter 3% 11% 29% 13%

Ground beef 0% 4% 26% 15%

Salmon 0% 1% 29% 3%

Egg yolks 0% 0.3% 27% 10%

Cashews 2% 1% 10% 7%

Soybean oil 0% 0% 11% 4%

Replacing saturated fats with unsaturated fats in the diet reduces risk of cardiovascular diseases, diabetes, or death.

No lower safe limit of saturated fatty acid intakes has been identified.

Fats and oils in most natural and traditionally processed foods contain both unsaturated and saturated fatty acids.

The complete exclusion of saturated fat is unrealistic and unwise: avoiding saturated fats could drive people to reduce the amount of polyunsaturated fats, which may have health benefits, and/or replace fats by refined carbohydrates, which carry a high risk of obesity and heart disease.

Foods rich in saturated fat, such as coconut and palm oil, are an important source of cheap dietary calories for the population in developing countries.

It is recommended  limiting the saturated fatty acids to less than 10% of daily energy intake and less than 7% for high-risk groups.

It is recommended replacing fats to reduce the content of myristic and palmitic acids, specifically.

The Mediterranean diet, includes more total fat than the diet of Northern European countries, but most of it is in the form of unsaturated fatty acids, specifically, monounsaturated and omega-3 from olive oil and fish, vegetables, and certain meats like lamb, while consumption of saturated fat is minimal in comparison. 

American Heart Association estimated that replacement of saturated fat with polyunsaturated fat in the American diet could reduce the risk of cardiovascular diseases by 30%.

There is a significant relationship between saturated fat and high serum cholesterol levels, which in turn have been claimed to have a causal relation with increased risk of cardiovascular disease.

In a study of myocardial infarctions, the ApoB/ApoA1, related to LDL and HDL, respectively, ratio was the strongest predictor of CVD among all risk factors.

Pathways involving obesity, triglyceride levels, insulin sensitivity, endothelial function, and thrombogenicity,  play a role in CVD.

In the absence of an adverse blood lipid profile, the other known risk factors obesity, triglyceride levels, insulin sensitivity, endothelial function, and thrombogenicity, have only a weak atherogenic effect.

Reviews have found limited evidence for a positive relationship between consuming animal fat and incidence of colorectal cancer, and breast cancer.

Other meta-analyses increased risk of ovarian cancer by high consumption of saturated fat.

Studies have indicated that the intake of saturated fat has a negative effect on the mineral density of bones. 

Substituting monounsaturated fatty acids for saturated ones is associated with increased daily physical activity and resting energy expenditure. 

The most common fatty acids in human diet are unsaturated or mono-unsaturated. 

Monounsaturated fats are found in animal flesh such as red meat, whole milk products, nuts, and high fat fruits such as olives and avocados. 

Olive oil is about 75% monounsaturated fat.

The high oleic variety sunflower oil contains at least 70% monounsaturated fat.

Canola oil and cashews are both about 58% monounsaturated fat.

Beef fat is about 50% monounsaturated fat, and lard is about 40% monounsaturated fat.

Other sources of mono saturated fat include: hazelnut, avocado oil, macadamia nut oil, grapeseed oil, peanut oil, sesame oil, corn oil, popcorn, whole grain wheat, cereal, oatmeal, almond oil, sunflower oil, hemp oil, and tea-oil Camellia.

Polyunsaturated fatty acids can be found mostly in nuts, seeds, fish, seed oils, and oysters.

Food source (100g) Polyunsaturated fat (g):

Walnuts 47

Canola Oil 34

Sunflower seeds 33

Sesame Seeds 26

Chia Seeds 23.7

Unsalted Peanuts 16

Peanut Butter 14.2

Avocado Oil 13.5 

Olive Oil 11

Safflower Oil 12.82

Seaweed 11

Sardines 5

Soybeans 7

Tuna 14

Wild Salmon 17.3

Whole Grain Wheat 9.7

Insulin resistance and sensitivity:

Monounsaturated fatty acids, especially oleic acid, have been found to lower the incidence of insulin resistance.

Polyunsaturated fatty acids, especially large amounts of arachidonic acid, and saturated fatty acids, such as arachidic acid, increase insulin resistance.

The connection  between dietary fats and insulin resistance is presumed secondary to the relationship between insulin resistance and inflammation, which is partially modulated by dietary fat ratios.

Omega 3 and 9 thought to be anti-inflammatory, and omega 6 pro-inflammatory.

Both pro- and anti-inflammatory types of fat foods are biologically necessary in our diet.

The fat dietary ratio in most US diets are skewed towards Omega 6, with subsequent disinhibition of inflammation and potentiation of insulin resistance.

Polyunsaturated fats are shown as protective against insulin resistance.

Studies find  that increasing monounsaturated fatty acids and decreasing saturated fat  intake could improve insulin sensitivity, but only when the overall fat intake of the diet was low.

Some monounsaturated  fatty acids  may promote insulin resistance, whereas polyunsaturated fat may protect against it.

Levels of oleic acid along with other monounsaturated  fatty acids in red blood cell membranes are positively associated with breast cancer risk. 

Polyunsaturated fat intake and cancer have been inconsistent and vary by numerous factors of cancer incidence.

Polyunsaturated fat supplementation has no effect on the incidence of pregnancy-related disorders, such as hypertension or preeclampsia.

Polyunsaturated fat may increase the length of gestation slightly and decreased the incidence of early premature births.

Experts recommend that pregnant and lactating women consume higher amounts of polyunsaturated fats than the general population to enhance the DHA status of the fetus and newborn.

Unsaturated fatty acids generally have double bonds in cis configuration, with the adjacent C–C bonds on the same side, as opposed to trans.

Trans fatty acids (TFAs) occur in small amounts in meat and milk of ruminants typically 2–5% of total fat.

Natural TFAs, which include conjugated linoleic acid (CLA) and vaccenic acid, originate in the rumen of these animals. 

Trans fat contents in various natural and traditionally processed foods, in g per 100 g

Food type Trans fat content

butter 2g to 7 g

whole milk 0.07g to 0.1 g

animal fat 0g to 5 g

ground beef 1 g

Margarine, can contain trans fatty acids

Trans fatty acids found to be an 

unintentional byproduct of the partial hydrogenation of vegetable and fish oils. 

Trans fats have been implicated in many health problems.

The hydrogenation process made it possible to turn relatively cheap liquid fats such as whale or fish oil into more solid fats and to extend their shelf-life by preventing rancidification. 

Full hydrogenation of a fat or oil produces a fully saturated fat. 

Hydrogenation is generally interrupted before completion, to yield a fat product with specific melting point, hardness, and other properties. 

Partial hydrogenation turns some of the cis double bonds into trans bonds by an isomerization reaction.

The trans configuration is favored because it is the lower energy form, and accounts for most of the trans fatty acids consumed today.

Previously some industrialized foods   were found to have up to 30% trans fats in artificial shortening, 10% in breads and cake products, 8% in cookies and crackers, 4% in salty snacks, 7% in cake frostings and sweets, and 26% in margarine and other processed spreads.

Reformulation has markedly reduced transfats.

Replacing TFAs and saturated fats with cis monounsaturated and polyunsaturated fats is beneficial for health.

Consuming trans fats has been shown to increase the risk of coronary artery disease in part by raising levels of low-density lipoprotein, lowering levels of high-density lipoprotein. increasing triglycerides in the bloodstream and promoting systemic inflammation.

The primary health risk identified for trans fat consumption is an elevated risk of coronary artery disease (CAD).

Trans fats appear to increase the risk of CAD more than any other macronutrient, conferring a substantially increased risk even at low levels of consumption of 1 to 3% of total energy intake.

Effect of trans fat information on CAD comes from the Nurses’ Health Study – a cohort study that has been following 120,000 female nurses since its inception in 1976: found that a nurse’s CAD risk roughly doubled for each 2% increase in trans fat calories consumed.

For each 5% increase in saturated fat calories, instead of carbohydrate calories, there was a 17% increase in risk of CAD.

The replacement of saturated fat or trans unsaturated fat by cis unsaturated fats was associated with larger reductions in risk than an isocaloric replacement by carbohydrates.

Replacing 2% of food energy from trans fat with non-trans unsaturated fats more than halves the risk of CAD (53%). 

Replacing a larger 5% of food energy from saturated fat with non-trans unsaturated fats reduces the risk of CAD by 43%.

Trans fat has been found to act like saturated in raising the blood level of LDL but, unlike saturated fat, it also decreases levels of HDL.

The net increase in LDL/HDL ratio with trans fat, a widely accepted indicator of risk for coronary artery, is approximately double that due to saturated fat.

The effect of eating a meal on blood lipids of cis and trans-fat-rich meals showed that cholesteryl ester transfer was 28% higher after the trans meal than after the cis meal and that lipoprotein concentrations were enriched in apolipoprotein(a) after the trans meals.

A study of nurses showed that those in the highest quartile of trans fat consumption had blood levels of C-reactive protein (CRP) that were 73% higher than those in the lowest quartile.

Transfats in human breast milk fluctuate with maternal consumption of transfat, and that the amount of trans fats in the bloodstream of breastfed infants fluctuates with the amounts found in their milk.

The risk of type 2 diabetes may be increased with trans fat consumption.

Trans fat may increase weight gain and abdominal fat, despite a similar caloric intake.

TFAs enhance intra-abdominal deposition of fat, even in the absence of caloric excess, and are associated with insulin resistance.

A study found,that each 2% increase in the intake of energy from trans unsaturated fats, as opposed to that from carbohydrates, was associated with a 73% greater risk of ovulatory infertility.

After analyzing  the diets of 12,059 people over six years it was found that those who ate the most trans fats had a 48 per cent higher risk of depression than those who did not eat trans fats:trans-fats may be substituted for docosahexaenoic acid (DHA) levels in the orbitofrontal cortex (OFC): suicidal brains were found to have 16% less (male average) to 32% less (female average) DHA in the OFC. 

The orbital frontal cortex controls reward, reward expectation, and empathy, all of which are reduced in depressive mood disorders, and regulates the limbic system.

A study found a strong relation between dietary trans fat acids and self-reported behavioral aggression and irritability, suggesting but not establishing causality.

Trans fatty acid consumption is linked to worse word memory in adults during years of high productivity, adults age <45.

Trans fats are one of several components of Western diets which promote acne, along with carbohydrates with high glycemic load such as refined sugars or refined starches, milk and dairy products, and saturated fats, while omega-3 fatty acids, which reduce acne, are deficient in Western diets.

Intake of dietary trans fat perturbs the body’s ability to metabolize essential fatty acids (EFAs, including Omega-3) leading to changes in the phospholipid fatty acid composition of the arterial walls, thereby raising risk of coronary artery disease.

Trans fatty acids may impair the metabolism of long-chain polyunsaturated fatty acids.

Maternal pregnancy trans fatty acid intake has been inversely associated with long-chain polyunsaturated fatty acids levels in infants at birth thought to underlie the positive association between breastfeeding and intelligence.

Trans fats are processed by the liver differently than other fats, and may cause liver dysfunction: impairing 

the conversion of  essential fatty acids to arachidonic acid and prostaglandins, both of which are important to the functioning of cells.

Butter contains about 3% trans fat.

A Dutch meta-analysis that all trans fats, regardless of natural or artificial origin equally raise LDL and lower HDL levels.

Nutritional authorities consider all trans fats equally harmful for health and recommend that their consumption be reduced to trace amounts.

The World Health Organization recommends that trans fats make up no more than 0.9% of a person’s diet.

Trans fatty acids are not essential and provide no known benefit to human health: whether of animal or plant origin.

Dietary trans fatty acids are more deleterious with respect to coronary artery disease than saturated fatty acids.

There is no safe level of trans fat consumption.

It is recommended that trans fatty acid consumption be limited to less than 1% of overall energy intake.

Interesterification, a process that chemically scrambles the fatty acids among a mix of triglycerides achieves results similar to those of partial hydrogenation without affecting the fatty acids themselves; in particular, without creating any new trans fat.

Omega-three and omega-six fatty acids:

The most abundant omega-3 polyunsaturated fatty acid in erythrocyte membranes, were associated with a reduced risk of breast cancer.

The DHA obtained through the consumption of polyunsaturated fatty acids is positively associated with cognitive and behavioral performance.

DHA is vital for the grey matter structure of the human brain, as well as retinal stimulation and neurotransmission.

High levels of triglycerides in the bloodstream have been linked to atherosclerosis, heart disease and stroke.

There is a strong inverse relationship between triglyceride level and HDL-cholesterol level.

High triglyceride levels increase the quantity of small, dense LDL particles.

Triglyceride levels:

(mg/dL) (mmol/L)

< 150 < 1.70 Normal range – low risk

150–199 1.70–2.25 Slightly above normal

200–499 2.26–5.65 Some risk

500 or higher > 5.65 Very high – high risk

These levels are tested after fasting 8 to 12 hours. 

Triglyceride levels remain temporarily higher for a period after eating.

The American Heart Association recommends an optimal triglyceride level of 100 mg/dL or lower to improve heart health.

Elevated triglyceride management includes:

Weight loss, dietary modification, moderate exercise as first-line lifestyle modification treatments.

 For hypertriglyceridemia, restriction of carbohydrates, specifically fructose and fat in the diet and the consumption of omega-3 fatty acids from algae, nuts, fish and seeds are recommended.

Medications are recommended in those with high levels of triglycerides that are not corrected with lifestyle modifications, with fibrates being recommended first.

Omega-3-carboxylic acids is another prescription drug used to treat very high levels of blood triglycerides.

Very high levels of triglycerides that would increase the risk of pancreatitis is treated with a drug from the fibrate class. 

Niacin and omega-3 fatty acids as well as drugs from the statin class may be used in conjunction, with statins being the main medication for moderate hypertriglyceridemia when reduction of cardiovascular risk is required.

Fats are broken down in the healthy body to release constituents of glycerol and fatty acids. 

Glycerol itself can be converted to glucose by the liver and so become a source of energy. 

Fats and other lipids are broken down in the body by lipases, enzymes produced in the pancreas.

Many cell types can use either glucose or fatty acids as a source of energy for metabolism. 

The heart and skeletal muscle prefer fatty acids.

Fatty acids can also be used as a source of fuel for brain cells through mitochondrial oxidation.



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