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Fats, oils and lipids: Combinations of organic compounds, including FA
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Monday, 17 November, 2014, 08 : 00 AM [IST]
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Rashmi H Poojara
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fiogf49gjkf0d Fats, oils and lipids consist of a large number of organic compounds, including fatty acids (FA), monoacylglycerols (MG), diacylglycerols (DG), triacylglycerols (TG), phospholipids (PL), eicosanoids, resolvins, docosanoids, sterols, sterol esters, carotenoids, vitamins A and E, fatty alcohols, hydrocarbons and wax esters.
Classically, lipids were defined as substances that are soluble in organic solvents. However, over time this definition was thought to be no longer adequate or accurate and a novel definition and comprehensive system of classification of lipids was proposed in 2005 (Fahy et al., 2005).
The new definition is chemically-based and defines lipids as small hydrophobic or amphipathic (or amphiphilic) molecules that may originate entirely or in part through condensations of thioesters and/or isoprene units.
The proposed lipid classification system enables the cataloguing of lipids and their properties in a way that is compatible with other macromolecular databases.
Lipids can be defined as a wide variety of natural products, including fatty acids and their derivatives, steroids, terpenes, carotenoids and bile acids, which have in common a ready solubility in organic solvents, such as diethyl ether, hexane, benzene, chloroform or methanol.
The classification of lipid structures is possible based on physical properties at room temperature (oils are liquid and fats are solid), their polarity (polar and neutral lipids), their essentiality for humans (essential and nonessential fatty acids), or their structure (simple or complex).
Neutral lipids include fatty acids, alcohols, glycerides, and sterols, while polar lipids include glycerophospholipids and glyceroglycolipids. The separation into polarity classes is rather arbitrary, as some short-chain fatty acids are very polar.
A classification based on structure is, therefore, preferable. Based on structure, lipids can be classified as derived, simple, or complex.
Derived lipids include fatty acids and alcohols, which are the building blocks for the simple and complex lipids.
Simple lipids, compose of fatty acids and alcohol components, include acylglycerols, ether acylglycerols, sterols, and their esters and wax esters. In general terms, simple lipids can be hydrolysed to two different components, usually an alcohol and an acid.
Complex lipids include glycerophospholipids (phospholipids), glyceroglycolipids (glycolipids), and sphingolipids. These structures yield three or more different compounds on hydrolysis.
Fatty acids are the building blocks of most lipids and come in a variety of configurations. Fatty acids can vary in length and amount of saturation.
Based on the chain length, they can be classified as short, medium and long. Based on the saturation, they are classified as saturated (SFA) and unsaturated. The unsaturated may be mono-unsaturated (MUFA) and poly-unsaturated (PUFA) with a cis or trans-configuration.
Trans-fatty acids include any unsaturated fatty acid that contains double-bond geometry in the E (trans) configuration. Nomenclature differs only from normal cis fatty acids in the configuration of the double bonds.
The three main origins of trans-fatty acids in our diet are bacteria, deodorised oils and partially-hydrogenated oils.
The preponderance of trans-fatty acids in our diets are derived from the hydrogenation process. Hydrogenation is used to stabilise and improve oxidative stability of oils and to create plastic fats from oils.
The nutritional properties of trans-fatty acids have been debated for many years, particularly with respect to the amounts of low-density and high-density lipoprotein (LDL and HDL) contained in serum.
Some studies have shown that trans-fatty acids elevate levels of serum LDL cholesterol and lower HDL cholesterol.
Such results drew a great deal of media attention, which led to several requests for the US Food and Drug Administration (US FDA) to make labelling of trans-fatty acids mandatory on food products.
There have also been requests to either ban these substances or to impose strict limitations on their use.
Acylglycerols are the predominant constituent in oils and fats of commercial importance. Glycerol can be esterified with one, two or three fatty acids, and the individual fatty acids can be located on different carbons of glycerol.
The terms monoacylglycerol, diacylglycerol, and triacylglycerol are preferred for these compounds over the older and confusing names mono-, di-, and triglycerides.
Another source of dietary lipids is phospholipids which forms cell membranes. Cholesterol is a type of lipid distinct from fatty acids,triglycerides or phospholipids. It is a waxy sterol compound with critical roles in metabolism. There is endogenous synthesis of cholesterol in the body in addition to dietary intake.
Dietary fat includes all the lipids in plant and animal tissues that are eaten as food. The most common fats (solid) or oils (liquid) are glycerolipids, which are essentially composed of TG. The TG are accompanied by minor amounts of PL, MG, DG and sterols/sterol esters.
Fatty acids constitute the main components of these lipid entities, and are required in human nutrition as a source of energy, and for metabolic and structural activities.
The most common dietary fatty acids have been subdivided into three broad classes according to the degree of unsaturation - saturated fatty acids (SFA) have no double bonds, monounsaturated fatty acids (MUFA) have one double bond and polyunsaturated fatty acids (PUFA) have two or more double bonds.
In general, these fatty acids have an even number of carbon atoms and have unbranched structures. The double bonds of naturally-occurring unsaturated fatty acids are very often of the cis orientation.
A cis configuration means that the hydrogen atoms attached to the double bonds are on the same side. If the hydrogen atoms are on opposite sides, the configuration is termed trans.
Fats enhance the taste and acceptability of foods; lipid components largely determine the texture, flavour and aroma of foods.
In addition, fats slow gastric emptying and intestinal motility, thereby prolonging satiety. Dietary fats provide essential fatty acids (EFA) and facilitate the absorption of lipid-soluble vitamins.
Dietary fats have two components, namely invisible fat and visible fat.
The fat that is present as an integral component of each and every food item is referred to as invisible, the fat in processed and ready-to-eat foods is called hidden fat and the oil, butter or ghee used in cooking is called visible fat.
Vegetable oils used in cooking are the major type of visible fat consumed .India has a wide variety of edible vegetable oils consumed in different parts of the country.
The fatty acid composition of some of the oils is depicted below:
Omega-3 and Omega-6 series
The position of the first C-C double bond within an unsaturated fatty acid affects its metabolism by the body, and this feature is used to further classify unsaturated fatty acids.
Omega-3 fatty acids are those that have their first C-C double bond between the third and fourth carbon atom from the methyl group or omega end.
Similarly, omega-6 fatty acids are those that have their first C-C double bond between the sixth and seventh carbon atom from the omega end, and omega-9 fatty acids are those with their first C-C double bond between the ninth and tenth carbon atoms from the omega end.
Omega-6 fatty acids are converted primarily to arachadonic acid, and omega-3 fatty acids are converted to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
Further, biochemical modification results in the production of eicosanoids, including substances called prostaglandins, prostacyclin, thromboxanes, and leukotrienes.
Unlike regular hormones such as insulin or the thyroid hormones, these local hormones are used where they are produced and are not transported to their site of action in the blood.
The omega-3 and omega-6 fatty acids follow different biochemical pathways to produce distinct types of prostaglandins and thromboxanes, each of which has very different effects in the body.
The eicosanoids are potent regulators of vital body functions such as blood pressure, blood clotting, and immune and inflammatory responses.
In general terms, the eicosanoids produced from omega-6 fatty acids tend to increase inflammatory processes and blood clotting.
Eicosanoids produced from omega-3 fatty acids tend to decrease blood clotting and inflammatory response.
The physical and functional properties of cell membranes are affected by the relative fatty-acid composition of membrane-bound phospholipids, which can be altered according to the fatty-acid composition of dietary triglycerides.
The different biochemical pathways involved in eicosanoid production utilise and therefore compete for the same enzymes, so the degree of inflammation is influenced by the relative proportions of omega-6 and omega-3 fatty acids present in cell membranes (Baur, 1994).
Many of the health benefits realised by modifying lipid intake involve altering patterns of consumption - reducing intake of saturated fats, trans-fats and cholesterol and increasing intake of mono-and polyunsaturated fats.
Omega-3 intake, in particular, has been the subject of hundred of studies in human and animals, in part of its ability to successfully reduce the risk of several diseases by different mechanisms.
Some of these are discussed below.
Reducing chronic low-level inflammation
Diets high in saturated fats have been associated with an increase in proinflammatory markers in some studies, particularly in diabetic or overweight individuals.
The intake of synthetic trans-fats has been associated with increase in markers of inflammation, although this data is conflicting and may be more pronounced in individuals who are overweight.
Omega-3 fatty acids have been studied for their prevention of cardiovascular disease and mortality and the anti inflammatory effects of omega-3 fatty acid consumption have demonstrated their beneficial against other inflammatory diseases, particularly asthma, inflammatory bowel disease (IBD) and rheumatoid arthritis.
These clinical trials are supported by several large observational trials encompassing thousands of patients, which have revealed inverse relationships between fish oil, omega-3 consumption and markers of systematic inflammation within diverse populations.
Promoting healthy blood pressure
Omega-3 fatty acids consumption has led to significant reductions in blood pressure across several clinical trials.
In a survey of 36 clinical trials on the effects of omega-3 supplementation in over 2,000 individuals, a median intake of 3.7g a day of fish oil demonstrated an average reduction in blood pressure of 2.1mm Hg (systolic) and 1.6mm Hg (diastolic).
In hypertensive individuals, the average reductions in blood pressure were much greater, amounting to -4mm Hg (systolic) and -2,73mm (diastolic).
When compared to low fat diets, Mediterranean diets demonstrated average reductions in blood of 1.7mm Hg and 1.5mm Hg over six studies including more than 2,600 individuals.
While these reductions may be very small, it is important to remember that even modest reductions in blood pressure can have significant effects on cardiovascular health.
For example, lowering diastolic blood pressure by 5mm Hg has been estimated to lower the risk of heart stroke death by 40 per cent and the risk of death by heart disease or other vascular causes by 30 per cent.
There is convincing evidence for a blood pressure-lowering effect of combining the replacement of saturated fatty acids with monounsaturated fatty acids as part of a healthy lifestyle diet (Dash/Omniheart) that includes an increased proportion of fruit and vegetables and whole grains and a reduced salt intake.
Dietary fat and coronary heart disease
The best ecological study of diet and coronary heart disease (CHD) is the Seven Countries study, which consisted of 16 cohorts in seven countries involving a total of 12,763 middle-aged men that were examined between 1958 and 1964 (Keys, 1980).
The results showed that a substantial proportion of the variation in CHD death rates between geographical regions was explained by differences in intake of SFA and MUFA (Keys et al, 1986).
Moreover, the Seven Countries study also demonstrated strong associations between mean intakes of SFA and mean levels of total cholesterol (Keys, 1980).
The study prompted the diet heart hypothesis that high intakes of SFA and cholesterol and low intakes of PUFA increase the level of total cholesterol, and ultimately result in the development of CHD.
This result from the pooling of observational studies, along with supportive evidence from clinical trials of lower CHD risk in high P/S diets, and the effects of PUFA to lower LDL cholesterol and the total/HDL ratio, led the consultation to conclude there was convincing evidence of lower CHD risk when MUFA and PUFA replace SFA.
A global increase in total fat supply and total fat intake is evident. The significant increase in production and per capita supply per day of fat from vegetable oils, especially in developing countries, are probably contributing to the increase in total fat intake.
Fat intakes remain high in developed countries, but the increase in total fat intake in developing countries is of concern as it may be a factor contributing to the increase in non-communicable diseases.
Guidelines for fat intake should not only concentrate on high fat intakes, but also ensure that enough fat is provided in the diet to meet essential fatty acids and energy requirements. The type of fat consumed is of special importance.
The Food and Agriculture Organisation’s (FAO) 2009 guidelines regarding dietary lipids could be summarised as follows:
The minimum total fat intakes for adults is 15 per cent energy to ensure adequate consumption of total energy, essential fatty acids and fat-soluble vitamins for most individuals.
The maximum total fat intakes for adults a 30-35 per cent energy for most individuals.
Replacing SFA (C12:0–C16:0) with polyunsaturated fatty acids (PUFA) decreases LDL cholesterol concentration and the total/HDL-cholesterol ratio. A similar, but lesser, effect is achieved by replacing these SFA with monounsaturated fatty acids (MUFA). Therefore, it is recommended that SFA should be replaced with PUFA (n-3 and n-6) in the diet and the total intake of SFA not exceed 10 per cent energy.
There is convincing evidence that replacing SFA (C12:0–C16:0) with MUFA reduces LDL cholesterol concentration and total HDL-cholesterol ratio.
There is convincing evidence that linoleic acid (LA) and alpha-linolenic acid (ALA) are indispensable since they cannot be synthesised by humans. There is convincing evidence that replacing SFA with PUFA decreases the risk of CHD. The minimum intake values for essential fatty acids to prevent deficiency symptoms are estimated at a convincing level to be 2.5 per cent E LA plus 0.5 per cent E ALA. Based on epidemiologic studies and randomised controlled trials of CHD events, the minimum recommended value of total PUFA consumption for lowering LDL and total cholesterol concentrations, increasing HDL cholesterol concentrations and decreasing the risk of CHD events is six per cent energy. Thus, the recommended range (ADMR) for PUFA is 6–11 per cent energy.
The available evidence indicates that 0.5-0.6 per cent E alpha-linolenic acid (ALA) per day corresponds to the prevention of deficiency symptoms. The total n-3 fatty acid intake can range between 0.5-2 per cent E whereas the minimum dietary requirement of ALA (>0.5 per cent E) for adults prevents deficiency symptoms. The higher value two per cent E (ALA) plus n-3 long-chain polyunsaturated fatty acids (LCPUFA) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) (AMDR 0.250g-2.0g) can be part of a healthy diet. Whilst ALA may have individual properties in its own right, there is evidence that the n-3 LCPUFA may contribute to the prevention of CHD and possibly other degenerative diseases of aging.
An estimated average requirement (EAR) for linoleic acid of two per cent energy and an adequate intake (AI) for LA of 2-3 per cent energy are proposed. A desirable ratio between the intake of n-6 and n-3 fatty acids is required to achieve optimum health.
Trans-fatty acids from commercial partially-hydrogenated vegetable oils (PHVO) increase CHD risk factors and CHD events – more so than had been thought in the past.
There also is probable evidence of an increased risk of fatal CHD and sudden cardiac death in addition to an increased risk of metabolic syndrome components and diabetes.
In promoting the removal of TFA, which are predominantly a by-product of industrial processing (partial hydrogenation) usually in the form of PHVO, particular attention must be given to what would be their replacement. This is a challenge for the food industry. The current recommendation of TFA is less than one per cent energy.
It is advisable to ensure moderate use of edible oils and avoid trans-fat. A part of the visible fat can be substituted by nuts for optimum health.
Animal foods that are high in cholesterol and saturated fats should be consumed in limited amounts.
Omega-3 fat intakes can be improved by flax, perilla, fenugreek and mustard seeds in the diet for vegetarians and fish for the non vegetarians. It is important to be aware of the hidden fat in processed and ready-to-eat foods.
(The author is assistant professor, department of home science, St Teresa’s College, Ernakulam. She can be contacted at rashmipoojara@rediffmail.com)
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