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Omega Oils - What Do They Really Mean?
Written by Dr. Tom Shurlock for and on behalf of GWF Nutrition Limited.
Copyright: GWF Nutrition Limited - Not for Reproduction.
Omega oils first appeared in the national consciousness as unsaturated fats, graduated into “heart healthy” fats, and now omega-3 or 6. But what are they? Where do they come from and how do they work?
Fat, or oil, is a broad term covering a group of nutrients called the lipids. Lipids consist of two main parts, the base and the fatty acid. In most cases the base is glycerol and it attaches to three fatty acid chains to form a triglyceride. Variations are phospholipids, where the central fatty acid is replaced by a phosphate group, glycolipids – with carbohydrate linkages – etc.
The fatty acid component consists of a chain of carbon atoms with an acid (COOH) group at one end. As the carbon atom can make four bonds with other atoms, and two are used to link a chain of carbons, two are free to link with hydrogen. If all the free carbon bonds are linked to hydrogen, then that fatty acid is saturated. If two adjacent carbons are each short of a hydrogen, they share the links with each other. This is an unsaturated link.
Figure 1. Fatty Acid Carbon ( C ) Links (Hydrogen = H)
Fatty acid chains can be of any theoretical length, from 1 to 20+ but generally, carbon links of 12 -24 are the most common. The dynamics of atomic bonding mean unsaturated bonds are more likely on the longer chain molecules and so it is a 16-chain fatty acid where we first find an unsaturated bond, and thereafter longer chains can have multiple unsaturated bonds.
We classify these fatty acids by the position of the first unsaturated bond, counting in from the carbon atom furthest from the acid group of the molecule; if it is between the 3rd and 4th carbon atom it is omega-3, between the 6th and 7th, omega-6 etc. In dietary terms, there are omega 3, 6, 7, 9 and 11, with omega-3 and omega-6 classified as essential.
It is the positioning that give these fatty acids their characteristics. One fatty acid, arachidonic (a 20-carbon chain) acid, which is one of three omega-3 or omega-6 versions of eicosatetreonic acid, is central to a whole range of metabolic and physiological mechanisms by its ability to manufacture a group of metabolites called eicosanoids.
Eicosanoids have a function in many biochemical pathways and can influence immunity, inflammation, cell oxidation and even bone metabolism. Eicosanoids can generate hormone-like substances, including the prostaglandins, as well as a host of other regulatory compounds. Equally the omega-3 eicosapenteonic acid generate similar eicosanoids and regulatory compounds but which can have antagonistic effects to arachidonic eicosanoids. For example, an omega-3 eicosanoid, insulin-like growth factor promotes bone deposition; whilst an omega-6 eicosanoid, PGE2, stimulates bone resorption. As all aspects of metabolism, including bone biochemistry, are a dynamic cycle – bone deposition and resorption is continuous and harmonised – it makes sense that there needs to be a balance between the antagonistic regulators.
Similarly, omega-3 fatty acids are regarded as anti-inflammatory, whilst omega-6 are pro-inflammatory. Omega-6 is anti-inflammatory; but less so than omega-3. Furthermore, in terms of immunity, the relationship between eicosanoids and cytokines, impacting on macrophages, describes antagonism in governing an immune response. It is also known that both omega-3 and omega-6 have a positive effect on cardiac (heart) and vascular (arteries and veins) muscle, both incorporating into the lipid layers and acting as antioxidants.
The various aspects, benefits and antagonisms of omega fatty acids have led to the recommendation of dietary ratios between omega-3 and omega-6.
Both these omega fatty acids are described as essential. Animals cannot synthesise unsaturated bonds in the C3 and C6 position (although they can do so for C9) and so need a dietary source. The main sources for unsaturated fatty acids are algae; which fish consume and they themselves become a rich source of plant oils.
It is because of this that we regard fish oil as being particularly important in human nutrition, and DHA and EPA are regarded as the primary source of omega-3 in the diet, and arachidonic for omega-6.
However, there is a potential problem when it comes to sourcing these fatty acids for our animals. Many people may find it unethical to feed fish oils to herbivores, such as horses, or potentially unsustainable algal oil. So how do plant sources stack up.
Firstly, plant sources tend to peak at 18 in terms of carbon chain length. Within that there are good sources of omegas, linoleic for omega-6 and linolenic for omega-3. However, there are no dietary plant sources of EPA (20 carbons) or DHA (22 carbons), and the EPA appear to be central to the omega effects.
Secondly, the ratio of omega-3 to omega-6 does not stack up in most commonly used raw materials in animal feeding. While we are being told that a 3:6 ratio should be in the region of 1:4 plant oils, such as rapeseed and sunflower, the ratio is far wider; from 1:2 up to 1:200. Fish oils tend to have a ratio of 3:6 in the region of 6:1 to 15:1 so intake would need to be calculated against background levels to achieve the correct ration.
However, if we don’t want to use fish or algae oils, for whatever reason there are some questions that need to be answered.
In either case of fish or plant oil, is there any use in supplying a C18 or C22 fatty acid when C20 is the active one? To answer this, we need to look at the biochemistry of fatty acid metabolism.
One of the most remarkable characteristics of fatty acid metabolism is that it involves pairs of carbon atoms; that is fatty acids consists mainly of chains of even numbers of carbon. Although interesting, it also means that both linoleic and linolenic acids and DHA are but one step away from their corresponding EPA.
One process, when fatty acids are broken down for energy, is called beta-oxidation. It involves cleaving of a pair of carbon atoms as a product called acetyl CoA. It is a reversal of fatty acid synthesis where acetyl CoA adds a pair of carbons. Simply put C22 can be broken down to C20 and C18 can be built up. What is important is this cleaving/building occurs at the acid end, and so the omega status remains unchanged.
So, although there are no Arachidonic and EPA in plant oil sources, the supply of linoleic and linolenic can be synthesised into them by the addition of a pair of carbon atoms, by acetyl CoA.
Looking at plant sources then, what would be a suitable oil for animals? When it comes to ruminant livestock and camelids, degree and type of unsaturation is a moot point due to a process called biohydrogenation. Basically, the fermentative capacity in the foregut saturates fatty acids, and little escapes unchanged. Any omegas must be supplied as protected fats.
For all other classes of animal, the oil that provides the optimal ration is hemp, whose ratio of omega-3: omega-6 is 1:3.5.
One other factor in the supply of these fatty acids is their bioavailability. Generally speaking, the greater the degree of unsaturation, the greater the absorbability of the fatty acid. But there are other considerations. Fat absorption can take several passive pathways, being absorbed between and across the cells of the gut wall. If presented as a polar lipid, such as a phospholipid, trans-cellular transport is more efficient (please see article on the oatinol delivery system) and the components of hemp/oat oil are particularly rich in polar lipids.
So, omega fatty acids are unsaturated and the number is simply a notation of where the first unsaturated bond sits on the carbon backbone. Omega positions 3 and 6 have specific properties in that the C20 (EPA) versions are precursors of a family of regulatory factors, the eicosanoids. Often antagonistic, sometimes complementary, they are essential nutrients as they are derived from plants or algae and the specific (3 or 6) positioning cannot be synthesised by animals. Whilst fish oil (derived ultimately from algae) provides high proportions of omega-3, and so is favoured in human nutrition, it may not be ethically suitable for herbivores, such as horses. Plant sources tend to have high levels of omega-6 and therefore not suitable to achieve the ratios desired to maintain optimal balance in the biochemical processes.
Only a few plant oil sources are relevant, and hemp oil is probably the best. Coupled with a beneficial 3:6 ratio, the high level of fats as polar lipids ensures good bioavailability and so can readily provide quality omega fatty acids that will be synthesised into the relevant EPA and eicosanoids.