An introduction to fat

Why do we need fat?

Most people regard all dietary fat as 'bad' or unhealthy but the body actually needs a moderate amount of fat (or fatty acids) to function properly. Besides being an energy source, fatty acids play a crucial structural role in all cell membranes; they help hold our cells together. They also help our bodies transport, break down and excrete cholesterol. They are used to make important hormone-like compounds called eicosanoids which regulate many processes in the body including inflammation and blood clotting. Fatty acids help maintain healthy hair and skin, they protect vital organs and keep the body insulated. They are also needed to transport the fat-soluble vitamins A, D, E and K, from food into the body. In summary, without some fats in the diet, we cannot function.

The chemical structure of fat:

Figure 1 shows how fatty acids are made up of chains of carbon atoms (the carbon backbone) that have hydrogen atoms attached to them. They are referred to as saturated, monounsaturated or polyunsaturated depending on the number of hydrogen atoms attached to the carbon backbone.

 

 

Figure 1: Saturated, monounsaturated and polyunsaturated fatty acids

The links between the carbon atoms are called bonds and can be single or double. Saturated fatty acids have no double bonds and are fully 'saturated' with hydrogen atoms. Monounsaturated fats have one double bond and polyunsaturated fats have more than one. The way different fatty acids behave in the body is largely determined by the number and location of these double bonds.

Saturated fat:

As stated, saturated fats have the maximum number of hydrogen atoms attached to the carbon atoms. This makes the molecule less flexible. So these fats tend to be solid at room temperature, such as lard and butter.

Unsaturated fat:

Unsaturated (monounsaturated and polyunsaturated) fats tend to be liquid at room temperature, such as olive and sunflower oil. It is their flexible nature that makes these fats so biologically useful.

Cis and trans fats:

The usual shape of naturally occurring unsaturated fats is referred to as 'cis' (Latin for 'on the same side'), as the hydrogen atoms face the same direction (see Figure 2). When the hydrogen atoms are oriented in opposing directions, the molecule is called a 'trans' fatty acid (trans means 'across' in Latin). Most trans fats are made industrially by adding hydrogen to unsaturated plant oils, in a process called hydrogenation. This process reshapes the double bonds, twisting them so that the hydrogen atoms end up on different sides of the chain.

Figure 2: Cis and trans fatty acids configuration

While trans fats are made of exactly the same building blocks as non-trans fats, their different arrangement affects the physical properties of the fat. They are harder, have a higher melting point, and crucially, a longer shelf-life than their cis-shaped equivalents.

There is increasing evidence that trans fats are harmful to health. While both saturated and trans fats increase levels of LDL cholesterol (so-called 'bad' cholesterol), trans fats also lower levels of HDL cholesterol ('good' cholesterol), thus increasing the risk of heart disease. Indeed, some evidence suggests that trans fats may be even more harmful to health than saturated fats. This position is supported by a scientific review published in the New England Journal of Medicine in 2006 which states that the consumption of trans fatty acids results in considerable potential harm but no apparent benefit (Mozaffarian et al., 2006). Many health authorities worldwide agree and recommend that consumption of trans fat be reduced or avoided altogether.  

God fat, bad fat:

We need a moderate amount of so-called 'good' fats in the diet. However, not all fats are created equal. As stated, some are beneficial, while others can be harmful. For example, as 60 per cent of the brain is made of fat, it is very important that it is provided with enough 'bendy' polyunsaturated fatty acids to stay supple and elastic in order to be able to respond to different stimuli such as hormones. If the cell membranes in the brain (and other parts of the body) are rigid they cannot react so well (Saldeen and Saldeen, 2006). This means that less flexible (more saturated) fats may take their place and normal functioning can be impaired.

Coldwater fish (salmon, mackerel, sardines and herring) need a high level of flexible polyunsaturated fats in their cell membranes to enable them to see and swim in cold, dark, watery environments. Just imagine replacing these flexible fats with those from a cold pat of butter (saturated fat), it would not work! Fish obtain their polyunsaturated fats from plankton, algae and other fish. These fats are also found in eggs and dairy foods (which also contain unhealthy saturated fats) and are plentiful in plant foods such as nuts and seeds and their oils, avocados and soya foods. Green leafy vegetables contain them too, but not much as they are a very low-fat food. Although olive oil contains some saturated fat, it is mainly made up of monounsaturated fat, which is a good substitute for butter, lard or omega-6 fats, such as sunflower oil, in cooking.

While unsaturated fats are an important part of our diets, we have absolutely no dietary requirement at all for saturated fat. Found widely in meat, dairy, eggs, poultry, processed foods and fish, this unhealthy type of fat contributes to the risk of heart disease and stroke by raising blood cholesterol levels.   

Despite its reputation for containing unsaturated 'healthy' fats, much of the fat found in oily fish is saturated (see Table 1). For example, nearly 80 per cent of the calories in smoked mackerel come from fat and over half of the calories in grilled salmon come from fat. Around one-fifth of the total fat in both these types of fish is saturated (FSA, 2002).

Table 1: Nutritional content of selected oily fish and vegetarian alternatives
100 grams of: Protein (grams) Total fat (grams) Saturated fat (grams) Cholesterol (milligrams) Fibre (grams) Energy (kcals)
Grilled salmon 24.2 13.1 2.5 60 0 215
Smoked mackerel 18.9 30.9 6.3 105 0 354
Tinned sardines 23.3 14.1 2.9 65 0 220
Smoked tofu 10.9 7.1 1.5 0 0.5 112
Red lentils, boiled 7.6 0.4 Trace amounts 0 1.9 100
Kidney beans, boiled 8.4 0.5 0.1 0 6.7 103
Tinned chick peas 10.3 2.5 0.2 0 4.3 119
Vegetarian style fish fingers 13.3 15.2 1.9 0 1.0 279
Sources: Food Standards Agency, Sainsbury's, Redwoods and Cauldron Foods

Fish and shellfish are also significant sources of cholesterol; some species containing nearly as much as steak. The amount of cholesterol in prawns is estimated to be around 280 milligrams per 100 grams; nearly four times as much as is found in a rump steak (FSA, 2002). Whichever way it is dressed up, fish is not a low-fat food and the high levels of cholesterol and saturated fat found in some fish foods make them an unhealthy option.

It could be argued that diets containing oily fish are associated with a lower risk of heart disease and stroke compared to diets rich in red meat and dairy foods. However, a vegetarian or vegan diet is the healthiest option. Dr Neal Barnard, president and founder of the Physicians Committee for Responsible Medicine in Washington DC, says that fish are not low enough in fat or cholesterol to achieve what vegetarian diets can in terms of promoting health. He points out that although the fat content of fish varies widely, all fish contain cholesterol and more fat than is generally found in the foods that make up a well-balanced plant-based diet (wholegrains, pulses, fruit and vegetables) most of which contain well under 10 per cent fat (Barnard, 2003).

In summary, plant-based diets inevitably contain considerably lower amounts of saturated fat (and either less or no cholesterol) compared to typical Western diets. Research shows that vegetarians consume a third less saturated fat and only half as much cholesterol as omnivores. Vegans consume even less: half the amount of saturated fat eaten by omnivores and no cholesterol (Davis and Kris-Etherton, 2003). This contributes significantly to their lower risk of heart disease, stroke and many other diseases.

Getting to know the EFAs:

In the late 1920s the scientists George and Mildred Burr first introduced the idea that specific components of fat could be necessary for normal growth and development (Burr and Burr, 1929). Since then two specific polyunsaturated fatty acids have been classified as 'essential' as they cannot be manufactured within the body; they must be provided in the diet. Hence, they are called 'essential fatty acids' (EFAs). The two EFAs required for good health are the omega-3 fatty acid alpha linolenic acid (ALA) and the omega-6 fatty acid linoleic acid (LA).

Figure 3: Fatty acid family

ALA is described as an omega-3 fatty acid because the first double bond occurs at the third carbon atom along the carbon backbone of the molecule. As shown in Figure 3, ALA can be converted in the body into the longer-chain omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). LA is an omega-6 fatty acid (as the first double bond occurs at the sixth carbon atom). Similarly, our tissues can convert it into other longer chain omega-6 fatty acids, such as gamma-linolenic acid (GLA) and arachidonic acid (AA).

Although AA, GLA, DHA and EPA are physiologically important, they are not referred to as 'essential' as they can be produced in the body from LA and ALA. The essentiality of ALA and LA in humans is best explained by our inability to introduce double bonds in positions before carbon nine (Uauy et al., 2003). In other words, they are classed as essential as we can't make these fats in our bodies.

EFAs are important for the normal functioning of all tissues of the body. They are necessary for the formation of healthy cell membranes, the proper development and functioning of the brain and nervous system and for the production of eicosanoids, important regulators of vital body functions such as blood pressure, blood-clotting, and immune and inflammatory responses.

How much fat?

The UK Dietary Reference Values for the total amount of fat and different fatty acids in the diet were published in 1991 (Department of Health, 1991). Additional recommendations were made for omega-6 and omega-3s by the Committee on Medical Aspects of Food (COMA) in 1994 in a review on the nutritional aspects of CVD (COMA, 1994).

Taken together, current UK recommendations are that total fat intake should contribute no more than 33 per cent of daily total energy intake and that saturated fats should contribute an average of no more than 11 per cent of food energy, monounsaturated fats an average of 13 per cent, polyunsaturated fats 6.5 per cent and trans fats no more than two per cent. COMA recommended that among the general population there should be no further increase in average intakes of omega-6 fats but that intakes of long-chain omega-3 (EPA and DHA) fats should increase from about 0.1 grams to 0.2 grams per day (COMA, 1994).

These recommendations are similar to those given by the World Health Organisation (WHO) and Food and Agriculture Organisation (FAO). These are summarised along with the actual amounts of each type of fat required to meet these recommendations in Table 2.

Table 2: WHO recommendations for daily fat intake
Type of fat Percentage of total energy Grams required for women (aged 19-50)
consuming 1,940 calories/day
Grams required for men (aged 19-50)
consuming 2,550 calories/day
Total fat 15-30 32.3-64.7 42.5-85.0
Saturated fat Less than 10 Less than 21.6 Less than 28.3
Polyunsaturated fats 6-10 13.0-21.6 13.3-28.3
Omega-6 5-8 10.8-17.2 14.2-22.7
Omega-3 1-2 2.2-4.3 2.8-5.7
Source: WHO/FAO, 2003.

In summary, total fat intake should contribute between 15-30 per cent of total energy intake with less than 10 per cent from saturated fat. Those obtaining more than 10 per cent of their dietary energy from omega-6 fats should not increase their intake of this fat. Most people need to increase their intake of omega-3s to provide two per cent of dietary energy. Less than one per cent of calories (but preferably none), should come from trans fats.