Fats and oils belong to a family of organic compounds called lipids, and the role they play throughout human physiology makes them an essential component to the diet, and indeed they form one of the recognised macronutrients.
The aim of this chapter is to classify the most common dietary lipids, and to give an overview of their structure and function, and thereby provide a firm foundation for further reading. At present dietary recommendations for lipids, and their subsequent effect on human health, is the subject of great debate. Clearly, the role played by lipids within the body cannot be under estimated.
Key physiological functions of lipids
- formation of virtually all cell membranes
- formation of myelin sheath within the nervous system
- constitutes majority of the CNS and spinal cord
- synthesis of steroid hormones
- assists in the regulation of enzymes
- insulation through subcutaneous adipose tissue
- protection of internal organs
- transportation, storage and utilisation of fat soluble vitamins A, D, E, K.
- fuel source during lower intensity work loads
- storage of energy within the adipose tissue
The Structure of Lipids
At room temperature lipids which are liquid are called oils, and those which are solid are called fats.
Most dietary lipids consist of chains or rings of carbon atoms joined together along with other atoms, most commonly hydrogen and oxygen. There are many different kinds of fats that vary mainly due to their structure. The smaller units of fats are called fatty acids. These are seen in differing molecular lengths and with the presence of double strength bonds between some carbon atoms. The double bonds change the shape of the molecule and enable lipids to become very versatile. A different shape means a different function in the body.
Both short chain and medium chain fatty acids such as those commonly found in butter are small enough to be absorbed quickly into the blood, thus they provide a faster source of energy than the long chain fatty acids. For this reason they are less likely to cause weight gain (Portillo et al, 1998).
Fatty acids naturally occur as triglycerides, where three fatty acids attach to a carbohydrate backbone called glycerol. During digestion the fatty acids are broken off and then used in the body as required.
These fats comprise of chains of carbon atoms which are ‘saturated’ or full with hydrogen, which gives them distinct properties .They do not contain any double bonds which means they have a straight structure. The shape of fatty acids is highly significant in understanding both their resultant properties and the way they function within the body. Saturated fatty acids are straight, which accounts for their solid structure at room temperature, since they are able to pack tightly together with little space between them.
|Common sources of saturated fat|
|meat – beef, pork, lamb, venison|
poultry – chicken, duck
dairy – milk, cheese, yoghurt, cream, butter
Some studies strongly suggest that much of the original research and subsequent follow up work relating to saturated fat and its effect on health have been subjected to both misinterpretation and misrepresentation (Enig, 1993; Ravnskov, 1995).
Further to this, an increasing number of researchers have failed to find a correlation between saturated fat intake and the development of clogged arteries, more commonly termed as atherosclerosis (Ravnskov, 1998). In fact, an evaluation of the fat found within the arterial walls as a result of CHD, reveals that only 26% is saturated, the rest is unsaturated or plant-based fats (Felton, 1994).
Research reveals there is a need for saturated fat to be included within the daily diet. Functions include:
- enhancement of the immune system (Cohen, 1986)
- provision of energy and structural integrity to the cells (Mead et al, 1986)
- enhancement of liver function (Nanji et al, 1995; Cha and Sachan, 1994)
- the ability of coconut oil to act as an antimicrobal and antiviral agent (Projan et al, 1994;Hornung et al, 1994)
It should be remembered that diets too high in any kind of fat are also very energy dense, which may lead to a positive energy balance and thereby promote an increase in body fat. An excess of body fat is inherently damaging to our health and is closely linked to the development of disease, including coronary heart disease, diabetes and cancer.
These fatty acids are described as unsaturated because some hydrogen atoms are absent from the chain of carbons. This causes two effects:
- a double bond is formed between one or more of the carbon atoms
- the fatty acid bends at each double bond
Remember a different shape means a different function within the body. Unsaturated fats come in two main categories:
- monounsaturated – a single double bond – single bend in molecule
- polyunsaturated – several double bonds – several bends in molecule
Monounsaturated fatty acids
These oils contain fatty acids with only one double bond thus the term ‘mono’ or one. This means the molecule has a single bend in it. The body is able to recognise the distinct shape and length of the various monounsaturated fatty acids, and utilise them accordingly. Diets high in monounsaturated fats have been shown to lower both LDL cholesterol and plasma triglycerides, and are therefore thought to reduce the risk of CHD (Kris-Etherton et al, 1999). The body’s tissues are also able to synthesise monounsaturated fatty acids from saturated fatty acids where necessary.
|Sources of monounsaturated fatty acids|
Polyunsaturated fatty acids:
These fatty acids are long chains of carbon atoms, again with missing hydrogen but this time they posses more than one double bond. As with the monounsaturated fatty acids, there is a distinct bend at the point of each double bond, which again means a different shape and a different function.
Polyunsaturates have 2 sub-divisions which are considered essential to the daily diet.
- essential fatty acids
These fatty acids are essential to the diet since the human body is unable to synthesise them itself. They are important because they act as precursors to local hormone like substances called prostaglandins, which act as potent regulators of metabolism within the cells. The combined effects of these prostaglandins are far reaching within the body, and a deficiency in these particular fatty acids can lead to ill health. They are sub-divided into two categories.
- Omega 3 fatty acids
- Omega 6 fatty acids
One end of the fatty acid chain is referred to as the omega end, omega being the last letter of the Greek alphabet. The omega number refers to the position of the first double bond from the omega end of the fatty acid. These fatty acids are very specific to the functioning of the cell, and must be eaten in the required amounts to promote good health.
Omega 3 fatty acids such as those found in oily fish are particularly beneficial. They have been shown to reduce the tendency of blood to clot, lower blood triglyceride levels, lower total cholesterol levels and in some individuals to raise HDL cholesterol, all of which are thought to lower the risk of CHD (Harris, 1989).
|The ratio of omega 3 to omega 6 essential fatty acids|
| The balance between omega 3 and 6 fatty acids found in the diet is of special importance. |
Our evolutionary diet provided a ratio close to 1:1, whilst the modern Western diet is between 1: 15-16 in favour of omega 6.
Excessive amounts of omega 6 polyunsaturated fatty acids and a very high omega 6 to omega 3 ratio, have been shown to promote the development of cardiovascular disease and cancer, as well as inflammatory and autoimmune diseases (Simopoulos, 2002).
An ideal ratio of omega 6 to 3 fatty acids is 2:1-1:1 (Okuyama et al,1997).
Fatty Acid Isomers
Unsaturated fatty acids are found in two isomers or shapes:
- cis unsaturated double bonds
- trans unsaturated double bonds
Cis unsaturated double bonds:
Most unsaturated fatty acids are naturally found with a cis configuration, where the two hydrogen atoms by the double bond are the same side. This produces the natural bend in the molecule. The body easily recognises this shape and uses these fatty acids for their appropriate purposes.
Trans unsaturated double bonds:
Although these fatty acids occur naturally in small amounts within the diet, they are mostly produced through the mass processing of unsaturated oils within the food industry. The double bond remains but due to a disruption of the molecule it has an unnatural, unsaturated, straight fatty acid chain.
The body no longer recognises these fatty acids, as their shape combined with their length is now entirely foreign, and as a result they become very damaging to the structure and function of the tissue. These are the ‘bad’ fats, which are increasingly associated with a variety of diseases, including CHD.
It is completely unnatural to have a straight long chain fatty acid with a double bond.
Some experts believe that trans fats are closer in structure to plastic than fat.
· hydrogenation and the formation of trans fats
This refers to a process used within the food industry where quantities of unsaturated vegetable oils are placed into a large vat and then:
- heated to extremely high temperatures
- subjected to high pressure
- mixed with a nickel catalyst (to increase the reaction rate)
- subjected to the forced application of hydrogen
The addition of the hydrogen during this process, results in the formation of saturated fatty acids, as the original double bonds are broken and replaced with single bonds once again full of hydrogen. The amount of hydrogen added during the process will determine the amount of saturated fatty acids in the end product. The consistency of the final product will also be determined by the degree of hydrogenation. Fully hydrogenated vegetable oil would result in a very hard waxy substance. Most hydrogenation is partial, leaving varying amounts of the vegetable oil still containing unsaturated double bonds. Many of these double bonds however, will have been converted into trans fatty acids due to the actions of this intense and prolonged process.
|Common foods which contain trans fats|
take away foods
many “low fat” processed foods (low in saturated fat but high in trans fat)
- trans fatty acids and health risks
Metabolic studies have shown that dietary trans fatty acids have adverse effects on blood lipid levels, promoting an increase in LDL or ‘bad’ cholesterol, and a decrease in healthier HDL cholesterol, and that this effect is double that produced from the ingestion of saturated fat (Mensink and Katan, 1990).
Based on available metabolic studies, one report estimated that approximately 30,000 premature coronary heart disease deaths annually could be attributed to the consumption of trans fatty acids (Willett and Ascherio, 1994).
Cholesterol is a large lipid molecule, which cannot be utilised by the body as energy; effectively it has no calories. However, it is extensively used throughout the tissues for both structure and function and is essential to life. The functions of cholesterol include:
- a vital component of cell membranes – cell membranes, or walls, are predominantly made up of lipids. Cholesterol is also incorporated within cell membranes where, along with saturated fatty acids, it helps to provide structure and rigidity. Membranes which contain greater amounts of polyunsaturated fatty acids, due to eating too much vegetable oil and processed food, become floppy and require more cholesterol to make them rigid (Jones, 1997).
- production of steroid hormones – many hormones are proteins synthesised from amino acids. The steroid hormones however, including the sex hormones such as testosterone and oestrogen and adrenal hormones such as cortisol, are synthesised from cholesterol.
- synthesis of bile acids – the liver produces bile in order to digest dietary fat.
- synthesis of vitamin D – cholesterol within the skin produces a precursor to vitamin D, when exposed to sunlight.
|Sources of dietary cholesterol|
Dietary cholesterol only plays a relatively minor role in meeting our daily needs. Cholesterol is so essential to life that the liver is able to synthesise its own supply. When dietary cholesterol intake is low, the body increases the synthesis of its own supply, and as dietary intake increases cholesterol production falls.
Cholesterol is a lipid, which means that it does not mix with the watery medium of blood. The body has developed protein based carriers, called lipoproteins that completely encase lipids for transport in the blood.
Three key lipoproteins are:
- very low density lipoproteins (VLDL). Synthesised by the liver. Contain both cholesterol and triglycerides. Transport triglycerides into adipose tissue.
- low density lipoproteins (LDL). Formed from VLDL’s once they have unloaded most of their triglycerides. Transport the remaining cholesterol to cells throughout the body that are in need.
- high density lipoproteins (HDL). Synthesised by the liver. Transport excess cholesterol from the tissues and blood back to the liver.
*Note: Although many researchers link both cholesterol and animal fats to the development of CHD, others now challenge this concept, and question its very validity (Stehbens, 2001).
Genetic factors may well play a role in establishing ideal dietary fat guidelines (Mistry et al, 1981).
Key learning pointsFunctions of lipids include:
- cell membranes
- myelin sheath
- synthesis of steroid hormones
- transportation of fat soluble vitamins A, D, E, K
- Fatty acids occur in three lengths:
- short chain
- medium chain
- long chain
- found in animal products
- found in palm oil and coconut oil
- some fatty acids are considered to be atherogenic
- saturated fats found in coconut oil considered to have health benefits
- controversy over effects of saturated fat and health
- found in olive oil, peanut oil, avocados, red meat
- linked with a reduced risk of heart disease
- should be used in preference to saturated fat
- omega 3 found in, oily fish, flax oil
- omega 6 found in vegetable, pumpkin and sesame seeds
- modern diet too high in omega 6
- ideal ratio 2:1 or 1:1
Trans fatty acids:
- damaging to the structure and function of tissue
- caused by over heating and hydrogenation
- found in processed food
- increased risk of heart disease
- the most damaging dietary fat
- Cholesterol is an important substance, functions include:
- component of cell membrane
- production of steroid hormones
- synthesis of bile acid
- synthesis of vitamin D
- found in animal produce
Cholesterol and triglycerides are transported in lipoproteins:
- VLDL’s unload triglycerides into adipose tissue
- LDL’s transport cholesterol from the liver to the tissues
- HDL’s transport cholesterol from the tissues and the blood, back to the liver
- Some studies indicate risk factors for CHD include:
- elevated VLDL cholesterol
- elevated LDL cholesterol
- reduced HDL cholesterol
- other studies question the link between cholesterol and CHD
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