While fat does not make you fat per se, consuming an excess of these naturally calorie dense macronutrients may lead to weight gain for two reasons. Containing 9kcal per gram, fats are the most calorie dense macronutrients.
However, in spite of their high energy value, their effect on appetite is poor, making it easy to consume excessive amounts without the usual feeling of satiety that accompanies eating a meal rich in complex carbohydrate or protein. Nonetheless, fats are essential to any diet although the form of fat influences its metabolic fate and relative ‘healthiness’.
Types of fat
The majority of fats in the diet come from triglycerides, which are made up of fatty acids joined by a glycerol backbone. Fatty acids are chains that can vary in length from a few units long to over 20 units in length. Some may be essential, while others can be made from other fatty acids in the diet.
Saturated fatty acids have no double bonds between the individual units of the chain. They have been associated with an increased risk of developing cardiovascular disease, when consumed in excess.
However, medium chain length saturated fatty acids (between 8-14 carbon units long) do not display the same effects on the cardiovascular system as longer chain saturated fats. In fact, they have been linked to an increase in the ‘good’ HDL cholesterol. As they are not damaged on heating (being saturated), they are ideal for use as cooking oils.
Unsaturated fats contain one or many double bonds in the cis configuration, creating a ‘kink’ in the chain. They are less associated with the cardiovascular problems seen with saturated fats. However, they contain the same number of calories as saturated fats so can contribute to weight gain if eaten in excess.
Trans monounsaturated fatty acids are unsaturated but because of their trans structure around the double bond, they behave more like a saturated fat. They are predominantly industrially produced, during the creation of ‘hydrogenated’ fats, used in margarines and manufactured foods like cakes, pies and biscuits. They confer texture and shelf life benefits on manufactured foods.
They have the same or possibly worse effects on cardiovascular health than saturated fats. However, it must be borne in mind that some trans fats are naturally present in meat and dairy products. Avoidance of industrially produced trans fats is possible by choosing products that do not contain hydrogenated vegetable oil. However, there is no need to avoid meat and dairy products.
Omega 3 and omega 6 fats
The omega 3 and omega 6 fatty acids are both types of polyunsaturated fatty acid. Omega 3s are derived from the seed oils linseed, rapeseed and walnut oil, meat from grass-fed animals e.g. beef and green leafy vegetables. Omega 6 fatty acids are derived from other seed oils, such as sunflower, and olive oil. Both types of fatty acid are essential to humans, as they cannot be made in the body from other nutrients.
It is thought that the balance of intake between the omega 3 and omega 6 fatty acids is more important for health than the actual amounts consumed. Currently, far more omega 6s are consumed than omega 3, when they should ideally be consumed in equal quantities or a ratio of 2:1 (omega 6:3).
The length of the fatty acid chain of omega 3 can vary. Very long chain omega 3s are known as EPA and DHA and are predominantly found in oily fish, fish oils and the algae eaten by the fish.
It is thought that long chain omega 3s, derived from fish or marine algae have particular health benefits, concerning cardiovascular health and brain development. These do not appear to be seen at equivalent intakes of omega 3s derived from seed oils, nuts and plant sources. It may be that at high levels of seed oil consumption, similar benefits are conferred and this is of particular importance to those who do not eat oily fish.
Studies have shown that fish oils may decrease blood pressure. They have been shown to decrease the level of blood lipids, improve the diameter of blood vessels and decrease inflammation within of the blood vessels.
The retina and brain contain a lot of DHA. Some studies have shown that by supplementing with long chain omega 3s during pregnancy, the mental development of children at 4 years old may be improved. They may also play a role in the healthy development of the eye.
Metabolic fate of fats
- Storage – one of the metabolic uses of fat, given its high energy density, is as a mode of energy storage, as adipose tissue. This can be visible in the form of external fat, located on the abdomen (common in men) or hips, bottom and thighs (more common in women).
Excess stored fat can affect health in many ways, by increasing amounts of circulating fatty acids that in turn may increase risk of cardiovascular disease and cancer, decreasing mobility and therefore affecting fitness and affecting self esteem.
Fat stored in the abdominal region is thought to be the worst in terms of the effects on health. Fat can also be internal, located around organs such as the liver or heart. This type of fat is a significant health risk.
- Cellular structure – the membrane of every cell and many sub-cellular compartments in the body is composed of a type of fat known as phospholipids. These molecules form a malleable layer that allows the passive or active transfer of molecules from outside the cell to the inside and vice versa. They also form a water resistant barrier between the cell and its external environment.
- Hormones - such as the steroid hormones testosterone and oestrogen are made from fats known as sterols.
Sources of fats in the diet
- Hard fats and oils – the composition of these most obvious sources of fat will depend upon the origin. Hard animal fats such as lard and suet will contain a greater percentage of saturated fats. Butter contains milk fat and will also contain a high proportion of saturated fats because it is hard at room temperature. Margarines and lower fat spreads have variable amounts of the different types of fat, some being high in poly- or monounsaturates. Historically, these unsaturated spreads were artificially hardened by an industrial process known as hydrogenation, where polyunsaturated oils have hydrogen bubbled through at high temperatures, to create a hard spread (while the original oils would have been liquid at room temperature).
However, this process leads to the production of trans fatty acids, which have since been linked to cardiovascular disease. Manufacturers of unsaturated spreads have now begun to develop processes that do not result in the formation of trans fats.
Vegetable oils, from plant seeds (linseed, rapeseed, sunflower seed, etc) or from the fruit (avocado, olive and most nuts) have variable fatty acid composition. Many are good sources of omega 6 fatty acids and monounsaturated fatty acids. Only linseed oil is a good source of omega 3 fatty acid. Notably, palm oil and coconut oil are rich in medium chain saturated fatty acids.
- Milk and dairy products – The fats present will depend upon whether the product is made from skimmed, semi-skimmed or full fat milk. Milk fat tends to have a high proportion of short and medium chain saturated fatty acids plus long chain saturated and monounsaturated fatty acids. A small proportion of the fats are naturally in the trans configuration.
• Eggs – provide a significant source of fat (mainly saturated and monounsaturated fatty acids) and cholesterol, most of which is housed in the yolk. This explains the preference by bodybuilders to use only a few yolks and to consume more whites, for a high protein, low fat food source.
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