The health effects of dietary unsaturated fatty acids

Summary Introduction What are unsaturated fatty acids? Unsaturated fatty acids in the UK diet Unsaturated fatty acids in health and disease Unsaturated fatty acids and public health Conclusions Acknowledgements References Appendix 1 Summary Fat provides energy; indeed it is the most energy dense of all the macronutrients, with 1 g providing 37 kJ (9 kcal). However, the constituent parts of fat, fatty acids, are required by the body for many other functions than simply as an energy source, and there is an increasing awareness of the potential health benefits of specific types of fatty acids. Fatty acids are long hydrocarbon chains, with a methyl group at one end (the omega or n ‐end) and an acid group at the other. Unsaturated fatty acids are hydrocarbon chains containing at least one carbon–carbon double bond; monounsaturated fatty acids contain one double bond, and polyunsaturated fatty acids (PUFAs) contain many double bonds. The position of the double bond relative to the omega end determines whether a PUFA is an n‐ 3 (omega 3) or an n‐ 6 (omega 6) fatty acid. Most fatty acids can be synthesised in the body, but humans lack the enzymes required to produce two fatty acids. These are called the essential fatty acids and must be acquired from the diet. In humans, the essential fatty acids are the n‐ 3 PUFA α‐linolenic acid and the n‐ 6 PUFA linoleic acid. Although humans can elongate dietary α‐linolenic acid to the long chain n‐ 3 PUFAs eicosapentaenoic acid and docosahexaenoic acid, the rate of synthesis may not be sufficient to meet requirements, and it is, therefore, recommended that good sources of these fatty acids, namely, oil‐rich fish, are also included in the diet. Fat is found in most food groups, and foods containing fat generally provide a range of different fatty acids, both saturated and unsaturated. In the UK, the major dietary sources of unsaturated fatty acids include meat primarily as a result of the vegetable oil used in processing. Recommended intakes of both total fat and the different types of fatty acids have been set for the UK population, and it is possible to monitor fat intake from the data collected in nationwide dietary surveys. As a population, we are not currently meeting these recommendations, so there is still scope for dietary change. In Western diets, n‐ 6 fatty acids are the predominant PUFAs, and this is in line with current dietary advice to consume a minimum of 1% energy as n‐ 6 PUFAs and 0.2% energy as n‐ 3 PUFAs. The balance of n‐ 3 and n‐ 6 PUFAs in Western diets has changed substantially over the last 100 years or so, and as the two families of PUFAs share a common metabolic pathway, concerns have been raised that this might be detrimental to health; what is becoming increasingly clear is that both n‐ 3 and n‐ 6 PUFAs have independent health effects in the body, and as intakes of the n‐ 6 PUFAs are within the guidelines for a healthy diet, concerns about the n‐ 6 to n‐ 3 ratio are driven by low intakes of n‐ 3 rather than high intakes of n‐ 6. Currently in adults n‐ 6 PUFAs contribute to 5.3% energy. Detecting associations between components of the diet and risk of various diseases is notoriously complex and in many cases, the evidence is still accumulating. Cardiovascular disease, characterised by hardening and narrowing of blood vessels and/or the development of blood clots, is one of the leading causes of mortality and morbidity worldwide. The type and total amount of dietary fat has a clear part to play in affecting an individual’s disease risk, yet the precise mechanisms by which unsaturated fatty acids reduce cardiovascular disease risk are still unclear. A number of mechanisms whereby dietary fatty acids could influence the progression of cardiovascular disease and its risk factors have been identified. These include effects on blood lipid concentrations, blood pressure, inflammatory response, arrhythmia and endothelial function, along with many other effects, both known and as yet undefined. A well‐established risk factor for cardiovascular disease is an elevated plasma low density lipoprotein (LDL) cholesterol concentration. Replacing saturated fatty acids with either monounsaturated fatty acids or n‐ 6 PUFAs reduces LDL (the ‘bad’) cholesterol, and so reduces the risk of developing the disease. Unsaturated fatty acids, such as linoleic acid or monounsaturated fatty acids, also slightly raise high density lipoprotein (HDL) (the ‘good’) cholesterol, which assists in the removal of triacylglycerols from the bloodstream. Interest in the health effects of the long chain n‐ 3 PUFAs found in fish oils is also increasing. There is strong supportive, but not yet conclusive, evidence that these fatty acids protect against fatal heart disease. On the basis of this conclusion, in 2004 the Scientific Advisory Committee on Nutrition advised the UK government to adopt the population‐wide dietary recommendation to eat at least two portions of fish per week, of which one should be oil‐rich, equivalent to 0.45 g of the long chain n‐ 3 PUFAs per day. In recent years, the potential health benefits of α‐linolenic acid has attracted attention, and evidence is mounting on the role that this n‐ 3 fatty acid may play in preventing the progression of cardiovascular disease, although it is currently unclear what, if any, association exists. Brain cells are especially rich in certain long chain PUFAs. This has led to the suggestion that dietary status of these long chain fatty acids might influence cognitive function and behaviour. Research in this field is still in its early stages, but there is a small amount of evidence to suggest improvements in cognitive function following fatty acid supplementation. In contrast, it is well established that pregnant women must have an adequate supply of the long chain n‐ 3 PUFAs before and throughout pregnancy and lactation to support normal growth, neurological development and cognitive function of the baby. As n‐ 6 PUFAs are more abundant in the diet, achieving an adequate intake is less problematic. However, this is not the case for the n‐ 3 PUFAs; increasing fish consumption beyond two servings of oil‐rich fish per week or relying on fish oil supplementation is not appropriate during pregnancy due to the potential problems associated with heavy metal contamination of fish, or the high vitamin A level in some fish oil supplements. Unsaturated fatty acids have also been associated with a number of other diseases and although the evidence is by no means conclusive, it is an area that is attracting a huge amount of interest. Dietary fat affects a number of different metabolic pathways, including those involved with glycaemic control, so the types and amounts of dietary fat may have a role to play in the management of type 2 diabetes. Unsaturated fatty acids may also be associated with a reduced risk of developing certain cancers, including cancers of the colon, breast and prostate, although currently the level of evidence is not deemed sufficient by authoritative bodies, such as the World Cancer Research Fund, World Health Organization and the Department of Health, to make any specific dietary recommendations. There are a number of inflammatory conditions, such as asthma, Crohn’s disease and arthritis, which could potentially be alleviated by dietary modification. The fatty acid composition of cell membranes can be altered by consumption of both n‐ 3 and n‐ 6 PUFAs, and this can result in reduced inflammatory activity. However, whether this effect brings about a significant reduction in clinical symptoms is still unclear. It is also important to note that there are concerns that the beneficial effects on certain disease outcomes are only observed with very high intakes of unsaturated fatty acids, which could realistically only be achievable by supplementation. Few nutritionists would be comfortable recommending supplement use as the only alternative to fish, as this can be expensive and goes against the idea that all the nutrients that our bodies require can be obtained from the food that we eat if the right choices are made. Unsaturated fatty acids are now a nutritional hot topic, and their presence in foods has attracted both public and industrial interest. There is currently no specific legislation to control the use of health claims relating to the fatty acid content