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Fat provides energy; indeed it is the most energy dense of all the macronutrients with 1g providing 37kJ (9kcal). However, the constituent parts of fat, fatty acids, are required by the body for many other functions than as simply 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 (MUFAs) contain one double bond, polyunsaturated fatty acids (PUFAs) contain many double bonds. The position of the double bond relative to the omega end determines whether a polyunsaturated fatty acid is an omega 3 (n-3) or an omega 6 (n-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 omega 3 polyunsaturated fatty acid α-linolenic acid and the omega 6 polyunsaturated fatty acid linoleic acid. Although humans can elongate dietary α-linolenic acid to the long chain omega 3 polyunsaturated fatty acids 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 Europe, the major dietary sources of unsaturated fatty acids include meat & meat products, cereals & cereal products and potatoes & savoury snacks; primarily as a result of the vegetable oil used in processing. In Western diets, omega 6 fatty acids are the predominant polyunsaturated fatty acids, and this is in line with current dietary advice. The balance of omega 3 and omega 6 polyunsaturated fatty acids in Western diets has changed substantially over the last 100 years or so and as the two families of polyunsaturated fatty acids share a common metabolic pathway, concerns have been raised that this might be detrimental to health; what is becoming increasingly clear is that both omega 3 and omega 6 PUFAs have independent health effects in the body and as intakes of the omega 6 PUFAs are within the guidelines for a healthy diet, concerns about the omega 6:omega 3 ratio are driven by low intakes of omega 3 rather than high intakes of omega 6..

 

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 have been identified whereby dietary fatty acids could influence the progression of cardiovascular disease and its risk factors. These include effects on blood lipid levels, 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 LDL cholesterol concentration. Replacing saturated fatty acids with either monounsaturated fatty acids or omega 6 polyunsaturated fatty acids 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 HDL (the ‘good’) cholesterol which assists in the removal of triacylglycerols (TAGs) from the blood stream. Interest in the health effects if the long chain omega 3 polyunsaturated fatty acids 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. In recent years, the potential health benefits of α-linolenic acid has attracted attention and evidence is mounting on the role that this omega 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 polyunsaturated fatty acids. 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 omega 3 polyunsaturated fatty acids before and throughout pregnancy and lactation to support normal growth, neurological development and cognitive function of the baby. As omega 6 polyunsaturated fatty acids are more abundant in the diet, achieving an adequate intake is less problematic. However, this is not the case for the omega 3 polyunsaturated fatty acids; increasing fish consumption beyond 2 servings of oily 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 levels 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 Found (WCRF) or the World Health Organisation (WHO) 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 omega 3 and omega 6 polyunsaturated fatty acids, 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. From 1 July 2007, there is a new EU Regulation (1924/2006) on nutrition and health claims made on foods, which formally sets down the criteria that a product will have to meet in order to bear a nutrition or health claim.   

In general, we need to increase our consumption of long chain omega 3 polyunsaturated fatty acids and decrease intake of saturated fatty acids. To facilitate this, food technologists are looking at ways in which the fatty acid profile of a food can be modified in order to bring dietary improvement without requiring a major change in dietary habits. However, public health messages surrounding the optimum intakes of fatty acids must be clear and consistent to ensure that a favourable change in the fatty acid profile of the diet occurs.

 

© British Nutrition Foundation 2006