Omega-3 Fatty Acids - Nutrition

What are Omega-3 Fatty Acids?

Omega-3 fatty acids are a family of polyunsaturated straight-chain hydrocarbons that have a double bond in the third carbon-carbon bond (n-3) when counted from the methyl end of the molecule. FIGURE to illustrate this, pointing out the methyl end and the omega bond Only plants have the ability to add double bonds in the n-3 position. There is speculation that fatty acids belonging to the n-3 family, specifically alpha-linolenic acid and docosahexaenoic acid, exhibit essential fatty acid activity in dogs and cats.

Alpha-linolenic acid (ALA) is an 18 carbon chain with double bonds at the n-3, n-6 and n-9 positions (written as 18:3n-3) that accumulated in skin phospholipids; docosahexaenoic acid (DHA) is a 22 carbon chain with doubles bonds at the n-3, n-6, n-9, n-12, n-15, and n-18 positions (written as 18:6n-3) that accumulates in the retina during growth and development.

Alpha-linolenic acid undergoes an initial desaturation by hepatic -6 desaturase followed by elongation and another desaturation to form eicosapentaenoic acid (EPA; 20:5n-3).3 Dogs have the ability to convert modest amount of ALA to EPA and supplementing with dietary ALA has been shown to increase plasma EPA levels in dogs;4 cats are unlikely to synthesise significant amounts of EPA from ALA given their limited hepatic -6 desaturase enzyme activity.5 Eicosapentaenoic acid is further elongated to form docosapentaenoic acid (DPA), which is released into circulation and taken up by the retina and brain where it is converted to docosahexaenoic acid (DHA).6,7 Why are they Important? Alpha-linolenic acid works in conjunction with linoleic acid (LA) to help maintain skin and hair coat quality;8 EPA is incorporated into the lipid bilayer of cell membranes along with arachidonic acid (AA) and may dampen inflammatory reactions;9 DHA is conditionally essential for normal brain and retinal development in puppies and kittens.2,10,11 Omega-6 and omega-3 fatty acids are in competition for the same metabolic enzymes and the ratio of omega-6 to omega-3 fatty acids as well as the total amount of each polyunsaturated fatty acid is an important consideration when evaluating dietary balance or when attempting to modify concentrations of these nutrients for health and disease management. Roles in the Body Skin and Hair Coat: Epidermal keratinocytes secrete the LA-enriched phospholipid, ceramide, in both dogs and cats, which enhances cell cohesion and create an effective water barrier.12 Increased intake of ALA may blunt the conversion of LA to AA,1,4 allowing for more LA to be incorporated into ceramide. Ratios of LA:ALA of 2.6:1 to 26:1 has been proposed for optimal skin and coat health.13 Inflammation: Eicosapentaenoic acid is incorporated into cellular membrane and competes with AA for cyclooxygenase (COX) and lipoxygenase (LOX) enzymes. Inflammatory eicosanoids produced from EPA (series-3 prostaglandins prostaglandins, prostycyclins and thromboxanes and series-5 leukotrienes) are less pro-inflammatory than those produced from AA metabolism .13 Though not considered an essential fatty acid, increased intake of EPA in conjunction with a decrease in AA intake has been suggested as an aid in the management of inflammatory condition such as allergic dermatitis,14 nephritis,15 and osteoarthritis16 in dogs. Supplementation of EPA has also been shown to be beneficial in older healthy dogs17,18 as well as those with heart disease.19 Supplementation with higher levels of EPA in people undergoing cancer treatment may also result in a decrease in circulating IL-6 levels and improvement in lean body mass and appetite.20, 21 This specific effect of EPA on lean body mass and appetite has not been studied in dogs and cats, but EPA supplementation may be beneficial to help decrease inflammation secondary to radiation therapy in dogs with nasal tumours.22 Additionally, dogs with lymphoma (stage 3a) had an improvement in short-term survival when fed a diet supplemented with EPA and DHA among other nutrient modifications.23 These effects have not been demonstrated in cats or in dogs with other types of cancers. Growth: Docosapentaenoic acid is taken up by brain and retinal tissues where it is converted to DHA. Docosahexaenoic acid is incorporated into retinal rods and brain tissue,2,10 and is conditionally essential for normal brain and retinal development. Consequences of Omega-3 Deficiency a. Growth: Puppies fed DHA deficient diets will grow normally, but show poor response to learned behaviour and visual acuity tests compared with puppies fed diets enriched with DHA.2,10 Kittens fed DHA deficient diets during gestation and lactation also have poor retinal and visual development.11 b. Adult Maintenance: Clinical signs of ALA deficiency in adult dogs and cats are similar to LA deficiency (i.e., seborrheic dermatitis), but are relatively mild and easy to miss.13

Toxicity Excess intake of ALA without a concurrent increase in LA resulting in a ratio below 2.6:1 (LA:ALA) may interfere with LA uptake into ceramide and the conversion of LA to AA,2 but toxicity studies are lacking. Excess EPA intake can cause platelet abnormalities and bleeding disorders in cats,24 though this effect has not been demonstrated in dogs.25 Dogs fed excessive amount of the omega-3 long-chain polyunsaturated fatty acids, EPA and DHA, have shown impaired wound healing26,27 as well as impaired cellular immunity.28

Dietary Sources Specific seed and seed oils have varying concentrations of ALA; for example flaxseed oil provides 50% of the total fat as ALA while rapeseed (i.e., canola oil) provides 20% ALA.28 Terrestrial animal fats (associated with muscle, organ meat, eggs) are poor sources of ALA. Cold-water algae produce high concentrations of DHA, which is converted to EPA by marine fish after consumption; there is variation in EPA concentration between wild and farmed fish species. Recommendations for optimal ratios of omega-6 to omega-3 fatty acids are provided to pet food manufacturers, but an optimal amount of ALA has not been determined. Diets with higher levels of polyunsaturated fat (ALA, EPA and DHA) are prone to oxidative damage and require larger concentrations of dietary antioxidants to prevent rancidity (fat oxidation) during processing and storage. Diagnosing Omega-3 Fatty Acid Deficiency Diagnosis of ALA and DHA deficiency is difficult to make clinically and evaluation of dietary intake is often used to demonstrate inadequate intake. Plasma lipid profiles may show low circulating concentrations of ALA and DPA though normal reference intervals have not been established.