Fat


In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food.
The term often refers specifically to triglycerides, that are the main components of vegetable oils and of fatty tissue in animals; or, even more narrowly, to triglycerides that are solid or semisolid at room temperature, thus excluding oils. 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 mono- and diglycerides, phospholipids, sterols, waxes, 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, tallow, lard, salt pork, and cooking oils. 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 flavor and aroma ingredients and vitamins that are not water-soluble.

Biological importance

In humans and many animals, fats serve both as energy sources and as stores for energy in excess of what the body needs immediately. Each gram of fat when burned or metabolized releases about nine food calories.
Fats are also sources of essential fatty acids, an important dietary requirement. 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 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.

Adipose tissue

In animals, adipose 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. 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. Adipose tissue also secretes the hormone leptin.

Production and processing

A variety of chemical and physical techniques are used for the production and processing of fats, both industrially and in cottage or home settings. They include:
The pancreatic lipase acts at the ester bond, hydrolyzing the bond and "releasing" the fatty acid. In 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, following the secretion of lipases 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 chylomicrons. 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 fatty acids 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, 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.
Triglycerides, as major components of very-low-density lipoprotein and chylomicrons, play an important role in metabolism as energy sources and transporters of dietary fat. They contain more than twice as much energy as carbohydrates.

Nutritional and health aspects

The most common type of fat, in human diet and most living beings, is a triglyceride, an ester of the triple alcohol glycerol and three fatty acids. The molecule of a triglyceride can be described as resulting from a condensation reaction between each of glycerol's –OH groups and the HO– part of the carboxyl group of each fatty acid, forming an ester bridge with elimination of a water molecule.
Other 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, such as cetyl alcohol, may replace glycerol. In the phospholipids, 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.

Essential fatty acids

There are two essential fatty acids in human nutrition: alpha-Linolenic acid and linoleic acid. The adult body can synthesize other lipids that it needs from these two.

Dietary sources

Saturated vs. unsaturated fats

Different 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.
Many scientific studies have found that replacing saturated fats with cis unsaturated fats in the diet reduces risk of cardiovascular diseases, diabetes, or death. These studies prompted many medical organizations and public health departments, including the World Health Organization, to officially issue that advice. Some countries with such recommendations include:
  • United Kingdom
  • United States
  • India
  • Canada
  • Australia
  • Singapore
  • New Zealand
  • Hong Kong
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% 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 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 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.