The fat in foods is made up of many types of fatty acids, defined by their chemical structure as saturated or unsaturated. Each category has many different fatty acids, with the unsaturated group subdivided into two sub-groups: monounsaturated or polyunsaturated.
To simplify things, we usually refer to the fat in a particular food by the major type of fatty acid present. Extra virgin olive oil, for example, contains saturated, monounsaturated and polyunsaturated fatty acids but its fat is described as monounsaturated because that is the dominant type.
Health effects
Within each category of fatty acids, some are more beneficial to health than others, but all types contribute the same high level of 37 kilojoules/gram. For comparison, proteins and carbohydrates each have 17 kJ/g and alcohol has 29 kJ/g.
Some foods that are high in saturated fat contribute little of value except kilojoules. That’s a valid role for babies who need to triple their birthweight in their first 12 months, but extra kilojoules can be worse than useless for those trying to lose weight.
Other foods contain various fatty acids that are valuable for reducing ‘bad’ types of cholesterol in the blood, and some of the polyunsaturated fatty acids are essential for the structure and health of many body tissues.
To complicate the picture, other components of a food (referred to as the food matrix) can influence whether their fat exerts favourable or unfavourable effects. For example, the saturated fats found in yoghurt and cheese, and to a lesser extent in milk itself, are much less likely to increase the risk of cardiovascular disease than the same fats in butter. It’s not totally clear why, but the current theory is that the calcium and protein in yoghurt and cheese, or the fact that they are fermented products, may confer protection. Milk has a neutral effect whereas butter, which lacks calcium or protein, can increase the risk of heart disease if consumed in large quantities.
Healthy fats
Foods that get top marks for the healthiness of their fats include fish and other seafood, nuts, seeds, extra virgin olive oil and avocado. The fat in all these foods is dominated by unsaturated fatty acids. Each of these foods also has many other nutritional attributes.
Some of the polyunsaturated fats in fish and seafood fall into a class known as omega 3 fatty acids. There are other fatty acids in this category, but the particular ones found in seafood have been well studied and show a number of health benefits.
I get annoyed when I see advertisements claiming some food or supplement contains ‘omega 3’. This is on a par with claiming something contains ‘vitamin’. Using the plural term ‘omega 3s’ is better, although the types found in fish and seafood are quite different from those in nuts and some seeds.
Nuts contain a range of mono and polyunsaturated fatty acids known to be healthy. The health benefits of particular types of nuts are often reported. The supporting research is almost always funded by the marketers of the particular nut. That doesn’t necessarily invalidate the claims, but independent research shows that almost all nuts contain valuable components and healthy types of fat. Walnuts stand out a little because they contain an omega 3 fatty acid, although not the same as the ones found in fish, but valuable for vegetarians. Basically all nuts are an excellent addition to the diet. The only exceptions are for people with a specific allergy (for example to peanuts – a legume rather than a nut but with similar nutritional virtues to nuts) and coconut, where 92% of the fat consists of saturated fatty acids, dominated by one called lauric acid. Claims that coconut oil is healthy are not correct and rely on aspects of an old study as well as a lack of understanding of saturated fatty acids (see below).
The good fats in seeds such as sesame, sunflower, pepitas and linseeds (also known as flaxseeds) include a mixture of mono and polyunsaturated fatty acids. Linseeds are particularly high in the same omega 3 fatty acid as walnuts. In theory, that makes them valuable. In practice, omega 3 fatty acids go rancid rapidly. Few people would eat bad fish but some do not identify the rancidity of this class of fats in walnuts and linseeds. Fortunately, if linseeds are kept cool, the coating of linseeds protects the fatty acids. If you buy ground linseeds, keep them in the fridge. Linseed oil goes rancid so quickly that it is best kept for its other use – in paints and varnishes, where its rapid oxidation is valuable.
Extra virgin olive oil has mainly monounsaturated fat. Its fat is good but its unique value among oils lies in the 30 or so ‘minor’ components. These act as antioxidants, have beneficial anti-inflammatory effects and can help keep blood pressure normal.
Avocados also feature mainly healthy monounsaturated fat, but earn their reputation from other nutritional attributes – and the fact that they can often be used in place of yellow spreads.
‘Trans’ fats
‘Trans’ fatty acids occur naturally in animal foods but can also be made in the laboratory from vegetable oils. They are used in processed products because they have a long shelf life and produce crisp texture in fried foods, chips, biscuits, pastries, donuts and snack foods and can be used to solidify oils into margarine. They can technically be described as ‘unsaturated’, but increase ‘bad’ cholesterol, decrease ‘good’ cholesterol and increase the risk of heart disease and type 2 diabetes. The World Health Organisation recommended they contribute no more than 1% of kilojoules. Margarine manufacturers and most major fast food companies have now minimised them but Australia has taken little action to adequately monitor their use. I won’t retire until we get proper labelling of these nasty fatty acids so people can avoid them.
Coconut
A study done in the 1960s (reported in a scientific journal in 1981) looked at the diet and health of two groups of people – one in Samoa and one in the Cook Islands. Their diets consisted mainly of fish, octopus, taro or breadfruit, bananas and coconuts (direct from the tree, not as processed products or coconut oil). The people were lean and active and consumed very little sugar or processed foods so it was not surprising that they had very little heart disease.
People in Samoa and the Cook Islands still eat coconuts but have added many highly processed foods to their diet and now rate as two of the fattest nations on earth. This is related to their total diet and exercise patterns which are far removed from what they were in the 1960s. It is not valid to assume adding coconut oil to a modern western diet equates to the sustenance diets of these islanders 50-60 years ago.
A study from world-class researchers compared coconut oil, butter and safflower oil. Coconut oil raised LDL (bad) cholesterol less than butter but much more than safflower oil. Other studies have found that coconut also raises HDL (good) cholesterol. This makes coconut oil better than some other fats, but doesn’t negate its overall undesirable effect in raising LDL cholesterol.
Dr Rosemary Stanton