Fats and Oils in the Diet of Monogastric Animals
Fats and Oils in the Diet of Monogastric Animals
Herbivorous animals would appear to have a diet fairly low in fat* when foraging on dry herbage in the wild. However, such herbage is usually mixed with seed or grain in which fats are stored in significant concentration. Green herbage could also be rich in lipids as every active cell in the biomass has a lipid bi-layer wall comprising among other components glyco-lipids and phospho-lipids (Enig 2000).
A fat comprises a glycerol molecule that links to three fatty acid molecules, to become a triglyceride. Glycerol (C3H5[OH]3) has three arms which can each link to the COOH end of a fatty acid as follows:
l l l l
H2=C-O-H + H-O-C=O = H2=C-O-C=O + H-O-H
glycerol Fatty acid arm of triglyceride water
[Each C atom is linked to a neighbouring C atom by a single bond, represented by the vertical line above each ?C? in the equation. Within the equation the ?=? sign represents two bonds; formation of each link releases a molecule of water ]
Any given natural fat such as olive or coconut oil contains triglycerides made up of several different fatty acids in fairly constant proportions overall, regardless of where the source plant was grown. A triglyceride molecule may comprise one, two or three different fatty acid components but the overall mix will remain very similar for a given plant species.
The glycerol ?core? is absolutely constant in fats but the fatty acids available to attach to glycerol are numerous and diverse. The diversity arises from: differences in ?chain length? (number of carbon atoms in the acid chain); degree of ?saturation? (or the number of pairs of H atoms attached to the C atoms - in relation to the potential or saturation number of pairs of H atoms - in the molecular chain); and the position of the double bonds in unsaturated molecules, numbering from the COOH end of the molecule.
Chain length of fatty acids
Fatty acids range in chain length from 3 carbon atoms (C) to 30 carbon atoms, although those with greater than 24C are rare. In both plant and animal fats the dominant chain length by far is 18C, comprising oleic, linoleic and stearic, which together account for 69% of all plant fats and 53% of animal fats in commonly available food sources (Table 1). At what has been designated the alpha end of the carbon chain, in all fatty acids, is a COOH or carboxyl entity (see the fatty acid component in the above equation). The chain comprises a series of linked C atoms (in single file with two H atoms each, or only one 0n some ? see unsaturated fats below) extending to the other (omega) end of the chain where the final C atom has three H atoms attached (CH3) comprising a methyl form.
*[The word ?fat? is used here to include all triglycerides, whether they be hard, soft or runny at ambient temperature. The term ?lipid? includes fats and oils but also metabolic derivatives from them such as phospholipids and glycolipids.]
Degree of Saturation of fatty acids
A fatty acid molecule is referred to as saturated if every carbon atom in the chain, apart from the two extreme end positions, has two hydrogen atoms attached. Each carbon is linked by a single chemical bond to its neighbouring C atoms. Such molecules have linear geometry and tend to pack together in such a way that they have high viscosity or are solid at ambient temperatures. Natural fats that have a sufficient proportion (greater than 50%) of saturated fatty acids to be solid in this way include dairy, coconut, palm kernel, cocoa, and tallow.
Unsaturated ? cis and trans
When one or more pairs of hydrogen atoms are absent from a molecule neighbouring pairs of C atoms are linked by a double bond. If the two H atoms, one on each of the two neighbouring C atoms are located spatially on the same side of the carbon chain this is referred to as a cisarrangement and the carbon chain forms an angle at that point. Such a deviation form a linear form makes a great difference to the melting point ? eg the oleic triglyceride (18:1) has a melting point more than 35C below that of stearic (18:0) which is the fatty acid that imparts hardness to chocolate. When there are several double bonds in the chain the molecule has a spiral shape and the melting point is very low indeed.
If the single H atoms on the C neighbours are on opposite sides of the molecular chain it is referred to as a trans arrangement. The chain retains linear geometry similar to a saturated chain even though the double bond remains. The melting point in this case is similar to a saturated fat. Trans fats are rare in nature, though traces are found in bovine milk ft (dairy cream), but the trans fats formed by industrial partial hydrogenation are referred to as ?false fats? as they are not found in nature..
It is interesting that saturated triglycerides are more prominent in the seeds of tropical crops that grow in a mean temperature around 28C, and triglycerides with the lowest melting point (well below 0C) are penta- and hexa-unsaturated, found in cold-water fish occupying waters around 5C.
This is the description of a fatty acid that has only one pair of carbon atoms connected by a double bond. It is the most abundant fatty acid in the natural world (Table 1), being present in proportions varying from 6% in coconut oil to 71% in olive oil.
Where more than one pair of carbon atoms have a double bond between them the term polyunsaturated is applied. Unfortunately in common usage and food labelling no information about the degree of poly-unsaturation is given. It would be useful to introduce the prefixes di-, tri-, tetra-, penta- and hexa-unsaturated - where 2, 3, 4, 5 or 6 pairs of C atoms, respectively, are linked with a double bond. All of these categories except tetra (4) occupy important places in a healthy mix of dietary fats.
Table 1 Four dominant fatty acids that are found in all plant and animal lipids. Chain length (C atoms) and number of double bonds are shown in brackets (C:DB)
Plant source (n=23)
Animal source (n=5)
Palmitic (16:0) saturated
Stearic (18:0) saturated
Oleic (18:1) mono-unsaturated
Linoleic (18:2) poly-unsaturated
Table 1.(continued) **Cocoa fat (35% stearic) is excluded to avoid distorting the mean
Essential and non-essential fatty acids (Enig 2000)
The liver and adipose tissues are capable of synthesising fats from a carbohydrate substrate, mostly generating saturated and mono-unsaturated fats. These fats are important in the metabolism of the animal but not essential in the diet.
Essential fatty acids
For normal metabolic functioning some fats are required that the body cannot fabricate requiring that such fats be supplied in the diet in sufficient amount. Until they were recognised by science around 40 years ago there was great confusion about the efficacy and metabolism of some dietary fat components. There are two groups of essential fatty acids: omega 6 and omega 3. The designation ?omega? is used to refer to the position in the carbon chain of a particular carbon atom sharing a double bond with its neighbour. Omega 6 identifies the carbon atom that is sixth from the CH3 end of the chain, having a double bond with the seventh carbon atom from the end. Likewise for omega 3, where the double bond is between the 3rd and 4th carbon atoms.
There are other double bonds in the chains of the essential fatty acids but the presence of those at the 6 and 3 positions are associated with vital metabolic functions.
Linoleic acid - omega 6 group
Linoleic acid, although universally available in fats from plant and animal sources, is generated only in plants whence it is consumed and stored by animals. Cod liver oil, tallow and coconut oil contain only small traces of linoleic, whereas there is more than 40% present in soy, sunflower, corn, cotton-seed, trans-genic safflower and sesame (Table 2). Linoleic is converted into several derivative omega 6 fatty acids of 20 and 22 C chain length (through the action of desaturase [H removing] and elongase [C adding] enzymes) that are critical to cell function.
Alpha linolenic acid ? omega 3 group
The second type of essential fatty acid has an omega 3 double bond, and there are three well-known members of this group: alpha linolenic acid (ALA) and its derivatives: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). ALA is the basic omega 3 molecule that is found in a small number of oil seeds, notably flax, black-currant, soy, canola, walnut and candlenut (Aleurites moluccana). There is also a trace in butter, lard, tallow and poultry fat. The fat of grass-fed beef (tallow) is known to have a higher level of ALA than the tallow of lot-fed beef.. When a mammal consumes ALA the same enzyme system that works on linoleic acid [desaturase and elongase] converts it into EPA (20:5) and DHA (22:6) whilst preserving the integrity of the critical omega 3 double bond). It is the EPA and DHA which perform essential functions in cell metabolism at the level of the mitochondria, rather than ALA which appears to be basically the feed-stock for their generation in the metabolic system. DHA is also critical to neurological development in the fetus and the newborn (Williams and Burdge 2006).
FORMULATING A HEALTHY DIET ? THE RIGHT BALANCE OF FATS
The aim in formulating a diet is to meet the animal?s energy needs while ensuring that there are no deficiencies of essential components, and fats with special health and performance benefits are also included. Availability and price are going to affect the choice of components, recognising for example that while some fatty acids may have no particular health benefit they are relatively abundant in some low-cost food sources.
Guidance from research on fats in the human diet
Hayes (2002) in his review suggests that between 30% and 40% of dietary energy should be in the form of fat. The mix of zero-, mono- and poly-unsaturated is proposed by Hayes at between 1:1:1 and 1:1.5:1. He also suggests that within the poly-unsaturated group the maximum amount of omega 6 should not exceed a ratio of 7:1 with omega 3 components.
Research by Goyens et al (2002) shows that when the principal source of omega-3 is alpha linolenic (18:3) its conversion to EPA was better when the ratio of 7:1 with linoleic is present but the overall percentage of polyunsaturated fat is lower than one third of dietary energy. It was inferred that the enzyme conversion systems are ?over-loaded? when linoleic is more abundant. One response to this result is to reduce Hayes suggested ratio of 1:1.5:1 to a lower level of polyunsaturated energy as in 2:3:1. Then 40% total dietary energy would require 6.6% polyunsaturated fat or six parts linoleic to one part alpha linolenic. If such a low proportion of linoleic is unachievable from the feed sources available it would be necessary to add some omega 3 fat from a fish source as insurance against deficiency of EPA and DHA. Some trialling of variations of these components is called for.
While high serum cholesterol is a risk factor in human health it can be avoided, when lauric and myristic acids are present, by the mitigating influence of oleic and linoleic acids. If saturated fats such as coconut and palm oil (92% and 50% saturated respectively) are available at low cost, there would appear to be a low risk of health problems in raising the proportion of saturated fat in the diet above the levels
proposed by Hayes (2002). Coconut oil has positive effects on the risk of tissue inflammation and infection, as well as stimulating a raised level of energy use, making it highly desirable in the feed mix. Use of some variations of the proposed ratios of the different fat types, and careful observation of the outcomes, would be valuable research.
Some detailed references to the literature on fats in the human diet, which provide useful background to the general issues of dietary fat metabolism as presented in Appendix A.
Omega 3 sources
There is great variability in the efficiency of conversion of ALA to EPA and DHA, depending on the availability of the conversion enzymes that also have a role, mentioned above, in the conversion of linoleic acid into its derivatives . As many common feed/food component fats are high in linoleic (soy 53%; sunflower 68%; cottonseed 53%) this fatty acid tends to monopolise the enzyme system when these components are prominent in the ration, such that ALA conversion is limited (Gerster 1998; Burge and Calder 2005). It is recommended that a ratio of no higher than 6:1 of linoleic to ALA should be provided for a balanced ration. As ALA is expensive because of the few oil-seeds that provide it, the amount of ALA available (ie affordable) is usually small, thereby imposing the need for constraint of the amount of linoleic included in the ration. The fat component of the ration therefore needs to include a generous quota of saturated and mono-unsaturated fat to achieve an appropriate proportion of fat to achieve a desired energy level in the overall mix.
The other members of the omega 3 group, EPA and DHA can be added to the ration directly from fish oil sources such as cod liver oil which contains 7% each of EPA and DHA. As fish oils are becoming increasingly expensive, however, it is preferable to finely ?tune? the relative amounts of omega 3 and omega 6 based on the minimum omega 3 needed to maintain good health. A mix of fish and oil-seed sources of omega 3 in therefore indicated.
The most common fat by far in both plant and animal fats is oleic acid (18:1) ? Table 1. The only other mono fat in fruit and other feeds is palmitoleic (16:1) which is absent from all common crop plant sources except macadamia (18%) and avocado (3%), and is also present in candlenut (8%) which has crop status in south-east Asia. There is a small amount in all animal fats and 12% in cod liver ? Table 2. These two fatty acids have some standing as anti-microbial agents (Enig 2000).
This is the largest group of fatty acids, ranging in carbon chain length from butyric (4 C) - present in milk - in steps of two C atoms up to stearic (18 C). The group of three fatty acids with 8, 10 and 12 C atoms is referred to as the medium-chain triglyceride group (MCT). This has achieved high nutritional status due to its use in the specialised nutrition of intensive care human patients, but it is also the source of some very effective antibiotic agents (mono-glycerides) including a commercial product - mono-laurin (Kabara et al 1972).
The attack on saturated fats
Saturated fat has been demonised by the marketers of poly-unsaturated fats, in particular, on the basis of the rise in blood cholesterol of experimental animals (rabbit, rat, gerbil, several primate species) and humans in response to a significant proportion of saturated fat in the diet. It is mainly the lauric (12 C) and myristic (14 C) acids that consistently increase total cbolesterol in the short term (Hegsted and Gallagher 1967; Nicolosi et al 1981, Khor 2004). In addition palmitic (16 C) acid raises serum cholesterol when cholesterol is present in the diet (Hayes 1995).
The cholesterol issue
As a result of recent research, a clear distinction is made between LDL cholesterol (bad) and HDL cholesterol. (good), which appear to be mutually antagonistic with respect to artery inflammation. The ratio of these two components provides a critical indicator of cholesterol risk to heart health, rather than the total (combined) serum cholesterol concentration. Retrospective assessment of past research results shows that when saturated fats raised total cholesterol, HDL was frequently raised proportionately, so that the ration of LDL to HDL (and the risk to heart health) were not raised (Fife 2005).
Lauric and myristic acid have been generally placed in the category of cholesterol worsening fats, but apart from the new insight provided by the ration of total cholesterol to HDL it is also well to remember that Blood Cholesterol is just one of several risk factors for heart disease. For the record these other risk factors include: tissue and joint inflammation (for which C-Reactive protein is an indicator), high blood pressure, overweight, diabetes, smoking, genetic inheritance, alcohol consumption, age, gender and stress. Given this large number of potential contributors to risk of heart disease it is clear that the great hostility of marketers of unsaturated fats towards saturated fats is based on other objectives than a balanced allocation of risk to heart health.
Benefits of lauric acid
As already mentioned lauric acid is a potent antibiotic, credited by many with strong suppression not only of bacterial infections but also fungal and viral infections (Fife 2005). Another favourable aspect of lauric acid is evidence that it suppresses cytokines that are responsible for tissue inflammation (Sadeghi et al 1999). Whereas LDL cholesterol is implicated in promoting inflammation of the arterial wall, leading to development of atheromas, lauric acid also has an inflammation suppressing effect, seeming to counteract its potential to inflame the artery wall by raising LDL cholesterol.
The antibiotic effect of lauric acid may be valuable in animal feed, perhaps reducing the risk of harmful infections. It is not clear if any mono-gastric domestic animals suffer ill effects from saturated fatty acids that induce an increase in plasma cholesterol, but it appears that gerbils used in experiments do not (Hegsted and Gallagher 1981). Many other species of experimental animals also experience raised cholesterol from the intake of lauric and myristic fats in particular, and continue to be used as a research tool in studies of the response of cholesterol to diet.
All fatty acids provide a similar amount of energy per unit weight, except that shorter chain molecules have slightly less metabolic energy because of their slightly higher proportion of molecular oxygen. Contrast Caproic (C6H12O2), for example, with Stearic (C18H36O2). As fats provide more than double the energy per gram than carbohydrates they are attractive for a high energy diet, either to support rapid growth or for energy-demanding performance.
A great deal of research in human health has been done on the effect of different proportions of various types of fat on some health indicators, generally in relation to heart function and vascular disease which may be less relevant to animal diets in any case. Tests with laboratory animals and human volunteers have shown that some saturated fats (notably lauric, myristic and palmitic) do raise blood cholesterol. In recent decades it has been learned that one form of cholesterol (LDL) is linked to development of heart disease while the other form (HDL) acts as an antidote to LDL. The absolute amount of cholesterol is no longer regarded as a critical indicator of risk, being replaced by the ratio of total cholesterol to HDL.
MCT as a special energy booster
One highly relevant aspect of the MCT group of fatty acids in the diet is its role in boosting the energy supply to the muscle tissues. Experimentation with human athletes (Muoio et al 1994) and with mice (Fushiki et al 1995) showed that after a period of adjustment to a diet high in MCT stamina was markedly increased when performing demanding physical ?work?.
Saturated fat versus toxic trans fat
A great deal has been written about the human diet, revealing a biased response to some data. At the same time there has been a seemingly ridiculous campaign by many food processors to lower the absolute amount of fat in the diet. The disastrous effects of trans fatty acids, that have induced high LDL, obesity and carcinomas have greatly harmed the image of dietary fats, and the fat-lowering campaign led by dieticians may be related to this. The marketers of unsaturated fats have pounced on the trans fat ?toxicity? experience, describing this as another form of saturated fat, and by association further damaging the image of the natural saturated fats (Enig 2000).
Enig, M.G. (2000) Know your fats. The complete primer for understanding the nutrition of fats, oils and cholesterol. Bethesda Press, Silver Spring, USA
Fife, B (2005) Coconut Cures. Piccadilly Books, Colorado Springs, USA
Goyens PL, Spilker ME, Zock PL, Katan MB and Mensink RP (2002) Conversion of alpha-linolenic acid in humans is influenced by the absolute amounts of alpha-linolenic acid and linoleic acid in the diet and not by their ratio. J Amer Diet Assoc. 102(4):511-517
Hayes, KC (2002) Dietary fat and heart health: in search of the ideal fat (a review). Asia Pacific J Clin Nutr 2002, 11 (Suppl): S394 to S400
Hegsted DM and Gallagher A (1981) Dietary fat and cholesterol and serum cholesterol in the gerbil. Atherosclerosis 38(3-4):359-371
Kabara JJ et al (1972) Fatty acids and derivatives as microbial agents. In: Antimikcrobial agents and chemotherapy 2:1.23-28. American Society for Microbiology.
Nicolosi RJ, Marlett JA, Morello AM, Flanagan SA, Hegsted DM (1981)Influence of dietary unsaturated and saturated fat on the plasma lipoproteins of Mongolian gerbils. Atherosclerosis. 1981 Feb-Mar;38(3-4):359-71.
Williams CM and Burdge G (2006) Long-chain n-3 PUFA: plant v. marine sources. Proc. Nutr. Soc. 65(1):42-50