Fats and Oils: The importance of temperature





Have you ever wondered why polyunsaturated margarine has to be kept in a fridge, yet coconut oil can be kept out at room temperature for a year or more without any untoward effects? All fats and oils in Nature are a mixture of saturated, monounsaturated and polyunsaturated fatty acids. The only difference between them is the proportions of each. Whether they are in plant or animal tissues, this is governed by the temperature at which the different fats and oils are designed to operate. This point, which is often neglected when discussing the healthiness or otherwise of fats and oils, is actually the most important consideration. The degree of saturation or unsaturation determines not only a fat's melting point, but also its chemical stability and its likelihood of auto-oxidising and creating harmful free radicals. The higher the proportion of saturated fatty acids a fat is, the less likely it is to go rancid; the more polyunsaturated fatty acids it contains, the more difficult it is to stop it going bad.



In plants, oils are usually found in their seeds. The degree of saturation of plant oils and fats is entirely dependent on the temperature in which they are grown. These oils provide a store of energy for the seeds' germination, usually in early spring when the weather is cool. For this reason, the energy contained in the oils must be accessible when ambient temperatures are low. Unsaturated oils melt at lower temperatures, and the more unsaturated they are, the lower the temperature at which they are viable. So we find oils that are highly saturated, such as coconut oil, in the tropics; palm oil, which grows slightly further from the equator, is a little less saturated; monounsaturated oils are found in olives grown in Mediterranean regions; and polyunsaturated oils in the seeds of plants grown in cooler climates. It has also been shown that the same plant species grown in a warm climate will be more saturated than if grown in a cooler region.[1]



The same is true of animals. Pigs dressed in sweaters were also found to have more saturated fat than unclothed pigs.[2] Animals must have body fats which are liquid otherwise they would be too stiff to move. So cold blooded animals such as fish, which also live in cold water, contain highly polyunsaturated fatty acids with many double bonds: the EPA and DHA of fish oils have five and six double bonds respectively. But as body or environmental temperatures rise, so we find fats tending to become more saturated. The fats of all warm blooded animals contain mixtures of saturated and unsaturated fatty acids, but the degree of saturation is quite high. Human body fat is naturally about 40% saturated, 57% monounsaturated and only 3% polyunsaturated.



This temperature aspect is highly relevant because any fat or oil must be stable at the temperature at which it is going to be used. If it is attacked by oxygen and goes rancid, as polyunsaturated margarines do if they are not refrigerated, then they become unfit for consumption if they are outside the body, and extremely harmful if they are inside it.



All polyunsaturated fatty acids will auto-oxidise at body temperature unless they are protected in some way. Let's look at how Nature makes sure it doesn't happen.



Coconuts are found in equatorial regions where the ambient temperature may be well over 40°C (104°F). Coconut oil contains a small percentage of polyunsaturated fatty acids — but significantly, coconut oil doesn't go rancid at this temperature as a polyunsaturated margarine very quickly would. This is because the polyunsaturated fatty acids in coconut oil are protected by the very high percentage of saturated fatty acids.



Our body temperature at 37°C (98.6°F) is not really much lower than the coconut's environment. Our fat must also be both liquid and stable at this temperature. So it, too, contains a high proportion of saturated fat and only a small amount of polyunsaturated fat. Just like the coconut, the saturated fatty acids in our bodies protect the polyunsaturated fatty acids from oxidation. However, if we eat a diet that contains high levels of polyunsaturated fats, as 'healthy eating' tells us we should, those fats will be incorporated in our body cells. And that, as we will see later, makes them a recipe for disaster.



Fish, whether in the cold arctic or in warmer waters, contain a higher proportion of polyunsaturated fats than do warm blooded animals. In the tropics, where diets tend to include lots of fish, coconut oil is also eaten in plenty. South Pacific islanders are protected from the harmful effects of the polyunsaturated fats in the fish by the saturated fats in the coconuts. It is also noticeable that human cultures in the tropics eat high levels of saturated coconut oil and they don't suffer heart disease.



We who live in cooler climates have the same body temperatures as Pacific islanders and we eat animals such as cattle, pigs and sheep. The natural fat of those food animals is very similar to our own fat. If the animals are allowed to feed naturally, the saturated fatty acids in animal fat protect the polyunsaturated element in our diet. This makes the fat of these animals entirely healthy for both them and us. There is one caveat, however: Nowadays, the establishment is trying to make fats 'healthier' by feeding animals with commercially produced foodstuffs that contain high levels of polyunsaturated seed oils, notably from soya and maize (corn).



Table I: Fatty Acid Composition of Selected Fats[3] Fat or oil Saturated (%) Monounsat (%) Polyunsat (%) Coconut 91 6 3 Palm kernel 83 16 1 Butter 60 34 6 Human milk 54 39 8 Lamb 53 41 5 Beef 45 51 5 Pork 43 48 8 Human (body fat) 40 57 3 Hen's Egg 39 47 14 Chicken 35 48 16 Cod 26 16 59 Margarine (polyunsat) 24 21 55 Soya oil 18 24 58 Olive oil 17 74 9 Corn oil 13 24 59 Sunflower oil 10 20 66 Safflower oil 9 12 75 Canola oil 6 67 27

In the table above, the figures for food animals are based on animals eating their natural diet as they did when these values were determined. Today, however, many food animals are fed on foods which are more polyunsaturated — and the figures can be very different. For example, pork fat which should be about 8% polyunsaturated, can now be well over 30%.[4] And as many of our intensively farmed food animals are now fed large amounts of grains and soya, it is no longer accurate to speak of their fats as 'animal fats'; in many cases they are more akin to vegetable oils. In the same way that these are taken up by the animals, when we eat them they are also incorporated in our body cells. That turns what should be a healthy fatty acid profile into a decidedly unhealthy one, with serious implications not only for those animals, but for our health as well.



References

1. Wolf RB. Effect of temperature on soybean seed constituents. J Am Oil Chem Soc 1982; 59: 230-2.

2. Wolfe R. Chemistry of nutrients and world food. Univ of Oregon Chem. October 16, 1986; 121.

3. Paul AA, Southgate DAT. McCance & Widdowson's The Composition of Foods. Fourth revised extended edition of MRC Special Report, No 297. HMSO, London, 1979.

4. McHenry EW,Cornett ML. The role of vitamins in anabolism of fats. Vit Horm 1944; 2: 1-27.

Last updated 1 june 2008

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