Fats are part of a large group of compounds known as lipids, which also include waxes, sterols (e.g. cholesterol), fat-soluble vitamins, monoglycerides, diglycerides, triglycerides, and phospholipids. Dietary fats are predominantly triglycerides, comprising three fatty acids attached to a glycerol backbone. Each fatty acid is a chain of carbon atoms, whose number and method of attachment varies. Short-chain fatty acids (SCFA) have up to 5, medium-chain fatty acids (MCFA) have 6 to 12, long-chain fatty acids (LCFA) have 13 to 21, while very long-chain fatty acids (VLCFA) have more than 22 carbon atoms. The majority of naturally occurring fatty acids, both in the body and in the diet, are between 16 and 18 carbon atoms in length. The carbon atoms within these fatty acids may be attached via a single or double chemical bond, which determines their classification. Saturated fatty acids (SFA) contain only single bonds, monounsaturated fatty acids (MUFC) contain one double bond, and polyunsaturated fatty acids (PUFA) contain more than one double bond between carbon atoms. PUFA are further classified according to the position of the first double bond from the methyl (or omega) end of their carbon atom chains;
- Omega-3 (or n-3) fatty acids have the first double bond at the third carbon atom, examples of which include alpha linolenic acid (ALA) and its’ derivatives eicosapentanoic acid (EPA) and docosahexanoic (DHA)
- Omega-6 (or n-6) fatty acids have the first double bond at the sixth carbon atom, examples of which include linoleic acid (LA) and its’ derivative arachidonic acid (AA)
- Omega-9 (or n-9) fatty acids have the first double bond at the ninth carbon atom, such as oleic acid (OA)
Both chain length and degree of saturation influence the functional properties of fatty acids and their role in human nutrition. Increasing evidence suggests that a diet low in saturated fatty acids and high in unsaturated fatty acids of long-chain length has a positive impact on human health, particularly so in the prevention of cardiovascular disease.
How much do I need?
Fat has an energy value of 9 kcals per gram, making it the most energy dense macronutrient in the diet. The World Health Organisation (WHO) recommends a maximum dietary fat intake of 30 % of total daily energy intake. Based on this recommendation, a person consuming 2000 kcals per day should consume no more than 67 g of fat per day, and a person consuming 2500 kcals per day should consume no more than 83 g of fat per day.
While most fatty acids can be synthesised in the body, humans lack the enzymes necessary for the synthesis of the polyunsaturated fatty acids, alpha linolenic acid (ALA) (an omega-3 fatty acid) and linoleic acid (LA) (an omega-6 fatty acid). To meet requirements, they must be ingested through the diet and are thus referred to as essential fatty acids. The reference intake (RI), based on average European intakes, is 1% of total daily energy intake for ALA, and 4 % of total daily energy intake for LA. This corresponds to approximately 2 – 3 g of ALA and 8 – 12 g of LA for adults consuming 1800 – 2700 kcals per day. It is worth considering though that, due to their inclusion in polyunsaturated margarines, oils and many processed foods, a majority over-consume most omega-6 fatty acids, leading to an imbalance in the ratio of omega-3 to omega-6 fatty acids in the body. Evidence suggests that increases to the amount of omega-3 fatty acids consumed in the diet is warranted to redress this imbalance for optimum health benefits
What is their function?
Dietary fats, in addition to carbohydrates and proteins, are the main source of energy for the body, providing about 20 – 35 % of daily energy requirements. Fat is utilised as an energy source for most everyday activities and is the predominant fuel for low intensity exercise. Fats are also critical for a range of important biological functions within the body – they compose structural components of cell membranes, brain tissue, nerve sheaths and bone marrow, provide insulation and protection for organs, contribute to hormone production, act as carriers for fat-soluble vitamins, and are involved in digestive and inflammatory processes.
What are good dietary sources?
Fat intake should preferentially come from unsaturated fats, derived from vegetable oils (e.g. olive, rapeseed, sunflower), nuts (all varieties), seeds (e.g. sesame, sunflower, pumpkin), oily fish (e.g. mackerel, salmon, trout, sardines, herrings), and avocado.
Whilst the optimum source of omega-3 fatty acids are undoubtedly oily fish, with 200 – 300 g per week (about 2 servings) sufficient to meet requirements, non-fish eaters and vegetarians can also achieve reasonably good amounts of ALA from certain plant sources such as linseeds, pumpkin seeds, linseed oil, rapeseed oil, walnuts and soybeans. Dark green leafy vegetables, such as spinach and curly kale, and sweet potatoes also contain small amounts.