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In nutrition, biology, and chemistry, fat usually means any ester of , or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to (triple esters of glycerol), that are the main components of and of fatty tissue in animals; or, even more narrowly, to triglycerides that are solid or semisolid at room temperature, thus excluding . The term may also be used more broadly as a synonym of lipid—any substance of biological relevance, composed of carbon, hydrogen, or oxygen, that is insoluble in water but soluble in non-polar solvents. In this sense, besides the triglycerides, the term would include several other types of compounds like Monoglyceride and , (such as lecithin), (such as cholesterol), (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 and , and the main components of common food products like milk, butter, tallow, lard, salt pork, and . They are a major and dense source of food energy for many animals and play important structural and metabolic functions in most living beings, including energy storage, waterproofing, and thermal insulation. The human 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 are also the carriers of some Flavoring and aroma ingredients and that are Lipophilicity.
Fats are also sources of essential fatty acids, an important dietary requirement. Vitamins Vitamin A, Vitamin D, Vitamin E, and Vitamin 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 against 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.
In the intestine, following the secretion of and bile, triglycerides are split into monoacylglycerol and free fatty acids in a process called lipolysis. They 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 . These 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. Various tissues can capture the chylomicrons, releasing the triglycerides to be used as a source of energy. Liver cells can synthesize and store triglycerides. When the body requires as an energy source, the hormone glucagon signals the breakdown of the triglycerides by hormone-sensitive lipase to release free fatty acids. As the brain cannot utilize fatty acids as an energy source (unless converted to a ketone), 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 if the brain's needs ever outweigh the body's.
Triglycerides cannot pass through cell membranes freely. Special enzymes on the walls of blood vessels called lipoprotein lipases must break down triglycerides into free fatty acids and glycerol. Fatty acids can then be taken up by cells via fatty acid transport proteins (FATPs).
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. They contain more than twice as much energy (approximately 9kcal/g or 38Joule/g) as (approximately 4kcal/g or 17kJ/g). (2025). 9780470052426, John Wiley & Sons. ISBN 9780470052426
Other less common types of fats include and , where the esterification is limited to two or just one of glycerol's –OH groups. Other alcohols, such as cetyl alcohol (predominant in spermaceti), may replace glycerol. In the , one of the fatty acids is replaced by phosphoric acid or a monoester thereof. The benefits and risks of various amounts and types of dietary fats have been the object of much study, and are still highly controversial topics.
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, , and such as canola.
Many scientific studies have found that replacing saturated fats with cis unsaturated fats in the diet reduces risk of cardiovascular diseases (CVDs), diabetes, or death. These studies prompted many medical organizations and public health departments, including the World Health Organization (WHO), to officially issue that advice. Some countries with such recommendations include:
A 2004 review concluded that "no lower safe limit of specific saturated fatty acid intakes has been identified" and recommended that the influence of varying saturated fatty acid intakes against a background of different individual lifestyles and genetic backgrounds should be the focus in future studies.
This advice is often oversimplified by labeling the two kinds of fats as bad fats and good fats, respectively. However, since the 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 possibly unwise. For instance, some foods rich in saturated fat, such as coconut and palm oil, are an important source of cheap dietary calories for a large fraction of the population in developing countries.
Concerns were also expressed at a 2010 conference of the American Dietetic Association that a blanket recommendation to avoid saturated fats could drive people to also 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.
For these reasons, the U.S. Food and Drug Administration, for example, recommends to consume less than 10% (7% for high-risk groups) of calories from saturated fat, with 15-30% of total calories from all fat. A general 7% limit was recommended also by the American Heart Association (AHA) in 2006.
The WHO/FAO report also recommended replacing fats so as to reduce the content of myristic and palmitic acids, specifically.
The so-called Mediterranean diet, prevalent in many countries in the Mediterranean Sea area, 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. A 2017 review found evidence that a Mediterranean-style diet could reduce the risk of cardiovascular diseases, overall cancer incidence, neurodegenerative diseases, diabetes, and mortality rate. A 2018 review showed that a Mediterranean-like diet may improve overall health status, such as reduced risk of non-communicable diseases. It also may reduce the social and economic costs of diet-related illnesses.
A small number of contemporary reviews have challenged this negative view of saturated fats. For example, an evaluation of evidence from 1966 to 1973 of the observed health impact of replacing dietary saturated fat with linoleic acid found that it increased rates of death from all causes, coronary heart disease, and cardiovascular disease. These studies have been disputed by many scientists, and the consensus in the medical community is that saturated fat and cardiovascular disease are closely related. Still, these discordant studies fueled debate over the merits of substituting polyunsaturated fats for saturated fats.
A 2017 review by the AHA estimated that replacement of saturated fat with polyunsaturated fat in the American diet could reduce the risk of cardiovascular diseases by 30%.
The consumption of saturated fat is generally considered a risk factor for dyslipidemia—abnormal blood lipid levels, including high total cholesterol, high levels of triglycerides, high levels of low-density lipoprotein (LDL, "bad" cholesterol) or low levels of high-density lipoprotein (HDL, "good" cholesterol). These parameters in turn are believed to be risk indicators for some types of cardiovascular disease. These effects were observed in children too.
Several meta-analyses (reviews and consolidations of multiple previously published experimental studies) have confirmed 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 (the so-called lipid hypothesis). However, high cholesterol may be caused by many factors. Other indicators, such as high LDL/HDL ratio, have proved to be more predictive. In a study of myocardial infarction in 52 countries, the ApoB/ApoA1 (related to LDL and HDL, respectively) ratio was the strongest predictor of CVD among all risk factors. There are other pathways involving obesity, triglyceride levels, insulin sensitivity, endothelial function, and thrombogenicity, among others, that play a role in CVD, although it seems, in the absence of an adverse blood lipid profile, the other known risk factors have only a weak atherogenic effect. Different saturated fatty acids have differing effects on various lipid levels.
Polyunsaturated fatty acids can be found mostly in nuts, seeds, fish, seed oils, and .
Food sources of polyunsaturated fats include:
| 47 | |
| 34 | |
| 33 | |
| 26 | |
| 23.7 | |
| 16 | |
| 14.2 | |
| 13.5 | |
| 11 | |
| Safflower oil | 12.82 |
| 11 | |
| 5 | |
| 7 | |
| 14 | |
| 17.3 | |
| 9.7 |
The large-scale KANWU study found that increasing MUFA and decreasing SFA intake could improve insulin sensitivity, but only when the overall fat intake of the diet was low. However, some MUFAs may promote insulin resistance (like the SFAs), whereas PUFAs may protect against it.
Results from observational clinical trials on PUFA intake and cancer have been inconsistent and vary by numerous factors of cancer incidence, including gender and genetic risk. Some studies have shown associations between higher intakes and/or blood levels of omega-3 PUFAs and a decreased risk of certain cancers, including breast and colorectal cancer, while other studies found no associations with cancer risk.
Expert panels in the United States and Europe 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.
The trans fat content in various natural and traditionally processed foods is shown in the table below.
| Butter | 2 to 7 g |
| Whole milk | 0.07 to 0.1 g |
| Animal fat | 0 to 5 g |
| Ground beef | 1 g |
Several experimental studies in humans found no statistical difference on fasting blood lipids between a diet with large amounts of IE fat, having 25-40% C16:0 or C18:0 on the 2-position, and a similar diet with non-IE fat, having only 3-9% C16:0 or C18:0 on the 2-position. A negative result was obtained also in a study that compared the effects on blood cholesterol levels of an IE fat product mimicking cocoa butter and the real non-IE product. Another study found tentative evidence that interesterified fat may lower cardiovascular disease risk.
A 2007 study funded by the Malaysian Palm Oil Board claimed that replacing natural palm oil by other interesterified or partially hydrogenated fats caused adverse health effects, such as higher LDL/HDL ratio and plasma glucose levels. However, these effects could be attributed to the higher percentage of saturated acids in the IE and partially hydrogenated fats, rather than to the IE process itself.
The National Cholesterol Education Program has set guidelines for triglyceride levels:Crawford, H., Micheal. Current Diagnosis & Treatment Cardiology. 3rd ed. McGraw-Hill Medical, 2009. p19
The AHA recommends an optimal triglyceride level of 100mg/dL (1.1mmol/L) or lower to improve heart health.
Rancification
Unsaturated fats undergo auto-oxidation, which involves replacement of a C-H bond with C-OH unit. The process requires oxygen (air) and is accelerated by the presence of traces of metals, which serve as catalysts. Doubly unsaturated fatty acids are particularly prone to this reaction. Vegetable oils resist this process to a small degree because they contain antioxidants, such as tocopherol. Fats and oils often are treated with chelating agents such as citric acid to remove the metal catalysts.
Role in disease
In the human body, high levels of triglycerides in the bloodstream have been linked to atherosclerosis, heart disease
and stroke. However, the relative negative impact of raised levels of triglycerides compared to that of LDL:HDL ratios is as yet unknown. The risk can be partly accounted for by a strong inverse relationship between triglyceride level and HDL-cholesterol level. But the risk is also due to high triglyceride levels increasing the quantity of small, dense LDL particles.
Guidelines
These levels are tested after fasting for 8 to 12 hours. Triglyceride levels remain temporarily higher for a period after eating.
< 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
Reducing triglyceride levels
See also
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