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Getting the right fatty acids goes to the heart of the matter

Sarah Schenker
BSc SRD PhD
Dietitian

Sara Stanner
MSc RPHNutr
Senior Nutrition Scientist
British Nutrition Foundation
London

Fat is often perceived to be the "baddie" of all nutrients, linked to obesity, heart disease and cancer.  However, as the most energy dense of all the macronutrients, with 1 g providing 37 kJ (9 kcal), it plays an important part in ensuring that we meet our daily energy requirements and enables the absorption of fat soluble vitamins. There is also increasing recognition that 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 greater awareness of the potential health benefits of specific types of fatty acids. This article discusses the role of the different fatty acids in health and describes the possible implications of recent changes in the fatty acid profile of the UK diet.

The main fatty acids in our diets
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 end. They are classified according to the number of double bonds in the chain; saturates have no double bonds, while unsaturated fatty acids have at least one. Of these unsaturated fatty acids, monounsaturates contain one double bond and polyunsaturates have more than one. There are two main families of polyunsaturates - omega-3 and omega-6 fatty acids. In Western diets, omega-6 fatty acids are the predominant polyunsaturates (see Table 1 for main dietary sources).

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Most fatty acids can be made in the body, but humans lack the enzymes required to produce two polyunsaturates - the omega-3 polyunsaturate alpha-linolenic acid and the omega-6 polyunsaturate linoleic acid. These are known as the essential fatty acids, which must be acquired from the diet. Alpha-linolenic acid can also be used by the body to synthesise the long-chain omega-3 fatty acids promoted for heart health - eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). However, our ability to do this appears to be limited and it is therefore recommended that good sources of these fatty acids, namely oily fish, are also included in the diet.

Transfatty acids
Transfatty acids are unsaturated fatty acids that occur naturally in small amounts in foods produced from ruminant animals, such as milk, beef and lamb. However, most of the transfatty acids in the diet are produced during the process of partial hydrogenation (hardening) of vegetable oils into semisolid fats. They are therefore found in hard margarines, partially hydrogenated oils, and in some bakery products, fried foods and other processed foods that are made using these. The process of partial hydrogenation causes carbon atoms to bond in a straight configuration similar to that of saturates, and as a consequence transfatty acids appear to have a similar effect on cardiovascular health to saturates. While the diets of most people in the UK are sufficiently low in these fatty acids not to exceed recommended levels (see Table 2), some low-income groups can still have high intakes.

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Fatty acids and heart health
Epidemiological evidence has established that the amount and type of dietary fat consumed is related to blood cholesterol levels, mortality rates from heart disease and the development of type 2 diabetes. On the basis of this, restriction of total fat, saturates, transfatty acids and dietary cholesterol has been advocated.(2) Low-fat, high-carbohydrate diets have become the mainstay of dietary advice for cardiovascular disease prevention, but there is accumulating evidence that it is fat quality, rather than quantity, that is particularly important for cardiovascular disease (CVD).(3)
There is overwhelming evidence that diets high in saturated and transfatty acids are associated with greater risk of heart disease through their ability to increase levels of low-density lipoprotein (LDL)-cholesterol, a well-established risk factor for heart disease and stroke. Adverse effects on other CVD risk factors have also been demonstrated (eg, vascular function, free radical damage and thrombogenesis). Low-fat, high-carbohydrate diets can help to lower total and LDL-cholesterol levels. However, replacing energy from saturates with the same amount from polyunsaturates, and to a slightly lesser extent monounsaturates, also reduces both total and LDL-cholesterol, and many investigators have suggested that this may be preferable to replacement by carbohydrate.(4-6) This is because unsaturated fatty acids do not have an undesirable effect on triglyceride levels (high levels of which are associated with increased risk of heart disease, decreased insulin sensitivity and type 2 diabetes) or the protective HDL-cholesterol as has been shown with carbohydrate.(3,7) A moderate-fat diet, rich in unsaturated fatty acids, may therefore offer an alternative to the standard low-fat/high-carbohydrate diet for cardiovascular disease prevention, at least for those with a healthy body weight. Compliance with such diets is also more likely for Western populations.

Mononunsaturates or polyunsaturates?
Although the benefits of omega-6 polyunsaturates on LDL-cholesterol are greater than the impact of mono-unsaturates, intakes of omega-6 polyunsaturates above 10% of energy (the recommended maximum) may have adverse effects on HDL-cholesterol.(5) In addition, polyunsaturates, in comparison to monounsaturates, are more easily oxidised both in foods during cooking and processing, and in the body, where oxidised lipids are implicated in protein and DNA damage. Monounsaturates may offer additional benefits for other risk factors (such as factors associated with inflammation and haemostatis).(3) This may swing the balance in favour of monounsaturates as a better replacement for saturates.  

The benefits of omega-3 for heart health
Although not without critics, many studies have supported the idea that long-chain omega-3 polyunsaturates found in fish oils (DHA and EPA) protect against fatal heart disease. This has been suggested in cohort studies (eg, the Nurses' Health Study), intervention trials with fish (eg, the DART study) or interventions with fish oil supplements (eg, the GISSI prevention trial). These fatty acids do not appear to lower blood cholesterol levels; in fact, short-term supplementation has been shown to cause a nonsignificant increase in LDL-cholesterol.(8) However, they lower blood triglyceride levels, decrease platelet aggregation and bleeding time and may reduce blood pressure and protect against arrhythmias.(9) Currently, intakes of 0.45 g per day of EPA and DHA from oil-rich fish is recommended for the general population although higher intakes (7 g/week) are recommended for secondary prevention.(10)

Other health effects of unsaturated fatty acids
There is accumulating evidence of a role for unsaturated fatty acids in a number of other conditions.(7) Dietary fat affects a number of metabolic pathways, including those involved with glycaemic control, so the type and amount of dietary fat may have a role to play in the management of type 2 diabetes. Unsaturated fatty acids may also be associated with reduced risk of developing certain cancers, including colon, breast and prostate cancer, although the evidence has not yet been deemed sufficient for authoritative bodies such as the World Cancer Research Fund to make specific dietary recommendations.

As brain cells are especially rich in long-chain omega-3 polyunsaturates, there has been considerable speculation about their role in cognitive function and behaviour.  Research is still in the early stages, but there is some evidence to suggest improvements in cognitive function following supplementation with these fatty acids.(7)
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 omega-3 and omega-6 polyunsaturates and this can result in a reduced inflammatory effect. Whether this brings clinical symptoms, however, is still unclear.

Recent trends in fatty acid intakes in the UK
As a population, we have seen a gradual decline in average fat intake since the late 1970s and despite declining energy intakes, the percentage of energy derived from fat has fallen and is now similar to the population target of 35% of food energy (see Table 1). But the fatty acid profile of the diet has also changed considerably over this period. The population is now consuming less saturates and is deriving proportionally more energy from polyunsaturates (see Figure 1). This followed compliance with public health messages to swap butter for vegetable oils and spreads and to consume lower-fat products (eg, meats and dairy foods), and also by efforts from the food industry to modify the fatty acid profile of their foods.

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The omega 3:6 ratio
However, the increase in intake of polyunsaturates in recent years has been due to a rise in omega-6 fatty acid consumption, and our intake of omega-3 polyunsaturates has not increased over the same time period. This has led to a change in the balance or ratio of omega-3 to omega-6 polyunsaturates and, as the two families of fatty acids compete for the same enzymes within their metabolic pathways, concern has been expressed that this change may have had an unfavourable effect on our health.11 Currently the British diet provides these fatty acids in the ratio of 7:1. This has increased from the 1950s when the average diet provided a ratio more like 4:1 and is very different from the 1.5:1 suggested to be consumed by Stone Age man.(7)  
It is becoming increasingly clear that both omega-3 and omega-6 polyunsaturates have independent health effects in the body, and as intakes of the omega-6 polyunsaturates are within the guidelines for a healthy diet (see Table 2), concerns about the omega-6 to omega-3 ratio are driven by low intakes of omega-3 rather than high intakes of omega-6.  We therefore need to encourage the consumption of foods rich in omega-3 fatty acids both to improve total intake and raise the omega 3:6 ratio of our diets.

Conclusion
In conclusion, for the vast majority of people, especially those who are overweight, moderation in fat intake (30-35% of energy) needs to be emphasised. However, the partial replacement of saturates by unsaturated fatty acids should be implicit in this message, as well as the importance of avoidance of partially hydrogenated fats (transfatty acids) and daily inclusion of sources of omega-3 fatty acids (such as oily fish, rapeseed and walnut oils, walnuts, flaxseeds etc). Intakes of long-chain omega-3 fatty acids of 1 g per day whether via supplements or food sources (oily fish) are likely to be  beneficial for those at risk of cardiovascular disease.

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References

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