Understanding the different kinds of fat in our diet

These changes are mirrored in the Food-Based Dietary Guideline (FBDG) on fat. The original South African FBDG published in 2002 recommended that “fats be used sparingly”. The revised FBDG was changed in 2012 to state, “Use fats sparingly: choose vegetable oils rather than hard fats.” 

To understand this FBDG it is necessary to keep in mind that some fats are liquid at room temperature while others are hard. Liquid fats are mainly, but not exclusively, made up of monounsaturated and polyunsaturated fatty acids, as found in plant oils.

Fats that are hard at room temperature such as hard margarine and animal fats, like lard, have a higher saturated fat content. Hard or block margarine which is still sold in supermarkets and is used in the manufacture of biscuits, pastries, cakes etc., is manufactured from liquid plant oils by a process which involves changing the structure of the fat from poly- or monounsaturated to saturated by means of a process called hydrogenation.

Read: Saturated fats bad for the memory

Some naturally occurring foods, including certain plant oils, also contain saturated fats. Coconut oil, which is all the rage at the moment, has a 92% saturated fat content, which is 6% more than butterfat (86%) and exceeds beef tallow (lard) by 30%.

Trans fats

Trans fats are currently regarded as the most harmful to human health if they are of artificial origin. The hydrogenation process mentioned above may also alter some of the fats in margarine to “artificial” trans fats, which are linked to heart disease and cancer.

Legislation in South Africa restricts the quantity of trans fats in manufactured or processed foods, and most commercially produced foods in this country now have a very low or zero trans fat content. The trans fat content of manufactured foods imported from countries that do not restrict trans fat levels can be a problem, so check the labels on the foods you buy.

Read: Healthier Fats Replacing Trans Fats, Study Finds

Foods of animal origin (which include meat, butter, cream, full cream milk and dairy products), also contain trans fats. Trans fats derived from ruminant animals are not included in the legislation, and naturally occurring conjugated linoleic acid (CLA) is regarded as potentially beneficial with regards to cancer, diabetes and heart disease. Recently it has been suggested that any trans fatty acids in the diet should be limited to the natural ruminant fats in meat and dairy products.

Essential fatty acids

Essential fatty acids, which include Omega-3 and Omega-6 fatty acids, or long-chain polyunsaturated fatty acids (LCPUFAs), are very important to human health because we are not able to manufacture these fats in our bodies and are often at risk of developing LCPUFA deficiencies, particularly of omega-3. The LCPUFAs have been linked to more than 14 major functions and health-promoting aspects of human metabolism, ranging from prevention of cancer to the development of the human eyes before birth.

Consequently, the emphasis nowadays is on how to improve the quality of fats in the diet and ensuring that we have sufficient LCPUFAs to promote optimum development of the brain, the eyes and the central nervous system during pregnancy and the vital first two years of life.

Read: Calamari: a source of omega-3

Prof Smuts emphasises that 60% of the brain consists of DHA (docosahexaenoic acid – an omega-3 fatty acid), which is why women of childbearing age who are planning a pregnancy, have fallen pregnant or are breastfeeding, should make sure that they eat foods rich in DHA or take omega-3 supplements.

Dietary goals for fat intake

Prof Smuts suggests the following quantitative goals for total fat and LCPUFA intake:

In total, our fat intake should provide 20-30% of our total energy intake. If you are an adult woman who requires a total energy intake of 8,400 kJ (2000 kcal) per day, then 20% = 1680 kJ (400 kcal) should be provided by fat intake. Each gram of fat provides 37 kJ (9 kcal), so 1680/37 (400/9) = 44 g of fat per day.

If we do this calculation for the higher intake of 30% of total energy, then 30% of 8,400 kJ (2000 kcal) = 2,520 kJ (600 kcal) per day which works out to 2,520/37 (600/9) = about 68 g of fat per day. The adult woman would, therefore, be able to eat between 44 and 68g of fat in total per day.

This recommendation will probably have “Banting” adherents passing out over their bowls of cream, but as hominids who were hunter-gatherers for millennia, we should keep in mind that 68g of fat is not easy to obtain on a daily basis. Venison was scarce and super-lean, fish was only available at the coast, and the plant foods our ancestors ate contained very little fat.

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In addition, Prof Smuts recommended that polyunsaturated fats (PUFA) should comprise up to 10% of the total energy of the diet, so on the higher 30% of energy fat intake, the above mentioned adult woman would need to ensure that up to 22 g of her 68 g fat intake is PUFA.

The LCPUFA recommendation is no less challenging – 5g of the PUFA should consist of Omega-6 fats and 2 g should be Omega-3 fats. EPA and DHA intake should be about 500 mg per day. 

To obtain 500 mg of EPA and DHA a day, the adult woman, for example, would have to eat the following quantities of the following type of fish: