Fat is not always the bad guy! Knowing that there are different types of dietary fat with very different properties is key to making optimal food choices and leading a healthy lifestyle. Trans and saturated fats are guilty of inducing health issues if eaten in excess of guidelines, they are usually the bad fats that you read about in the media and so on. They can cause weight gain, cholesterol build up, heart problems and more when eaten in excessive amounts. But, monounsaturated and polyunsaturated fats have the opposite effect and are definitely not to be considered the enemy!
Fats are essential because they are a storage unit for energy. In fact, gram for gram fats are the most efficient source of energy we can consume with 1g of fat providing 9kcal of energy. This does mean that less is more when it comes to portion sizes, but it also means that small amounts of fat keep us energised! Fat is also a vital part of the membranes surrounding every cell in your body, which means that without it your cells would not be able to function properly. When levels of fat are too low, brain and nervous function is compromised which is why you hear people talking of fish being ‘brain food’.
Without fat, we would also not be able to absorb many of the vitamins we rely on to maintain health. Vitamins A, D, E and K are all fat-soluble. We could also find ourselves in a hormonal mess should fat levels fall below the recommended requirements. Fats are structural components of prostaglandins which regulate many of the body’s functions. They also regulate the production of sex hormones, which is why females who are dangerously underweight/too lean experience problems with menstruation.
If you want your skin to have a healthy, youthful glow then consuming fats is important. Fatty acid deficiency results in dry, flaky skin. In addition to the skins surface, the layer of subcutaneous fat (beneath the skin) helps to regulate body temperature. This is why those with very low body fat are more sensitive to cold and obese individuals struggle in hot weather.
So basically, if you want to avoid fluctuating moods, stay mentally alert, fight fatigue, control weight and take care of every cell and organ in your body, you need to be consuming those healthy fats, not cutting fat out of your diet completely!
Glycerides and Fatty Acids
Triglycerides make up ~95% of dietary lipids (fats). A triglyceride molecule is formed from a molecule of glycerol (a three carbon alcohol) with three fatty acids attached. Fatty acids consist of an even-numbered chain of carbon atoms with hydrogen atoms attached, a methyl group at one end and a carboxylic acid at the other. The physical and biological properties of triglycerides are determined by the nature of the fatty acids from which it is made.
Phospholipids make up a relatively small proportion of dietary fat. They occur in virtually all animal and vegetable foods. Liver, eggs, peanuts and soybeans are particularly rich sources. The base group of the phospholipid is soluble in water, whereas the fatty acids are insoluble in water. This enables phospholipids to act as an interface between water and fat soluble molecules, making it an excellent emulsifying agent. The structural integrity of cell membranes and lipoproteins is dependent on this nature of phospholipids. Phospholipids are also an important source of essential fatty acids.
Sterols are also made from carbon, oxygen and hydrogen atoms but the molecular arrangement is unlike that of triglycerides and phospholipids. Instead, the atoms are arranged in ring formations with a series of side chains. Cholesterol is the principle sterol of animal tissues and is found only in food derived from animals such as eggs, dairy, meat, fish and poultry. Cholesterol plays an important structural role in membranes and lipoproteins and also functions as a precursor of bile acid, steroid hormones (oestrogen and testosterone) and vitamin D.
Potentially Harmful Dietary Fats
These fats are ‘saturated’ with hydrogen which is where the name comes from. The carbon atoms that make up the base structure of this fatty acid are bonded to the maximum number of hydrogen atoms possible. The structure of these fats makes them solid (in general) at room temperature. Foods with high levels of saturated fat include red meat, processed meat, cheese, butter and nuts.
Trans fat can be found naturally at low levels in foods such as meat and dairy products, but on the whole they are man-made through the process of hydrogenation. Typically, this involves the addition of hydrogen atoms to a polyunsaturated vegetable oil using a nickel catalyst. This type of fat has a longer shelf life than other fats. Foods with high levels of trans fat include tinned soup, margarine, cakes, biscuits, frozen food and fast food.
Health-Promoting Dietary Fats
This fat is unsaturated as the molecular structure contains one carbon-carbon bond. These carbon atoms could bond with more hydrogen atoms if they were not in this double bond. The word ‘mono’ is used because there is only one of these bonds in the entire molecular structure. Monounsaturated fats are generally liquid at room temperature. Foods high in monounsaturated fat are avocadoes, peanut butter and olives.
Now you know what the structure of monounsaturated fats is, it isn’t difficult to work out what a polyunsaturated fat is like. ‘Poly’ means that there are two or more carbon-carbon bonds within the molecular structure. The most famous polyunsaturated fats are probably omega-3 and omega-6. The 3 and 6 denote where on the structure the double bond is (in case you were wondering).
Within this category, there are essential (EFA’s) and non-essential fatty acids (NEFA’s). Essential fatty acids are those which cannot be synthesised in the body and have to be consumed through diet.
Alpha-Linolenic Acid (ALA), short chain Omega-3’s and Omega-6’s and Linolenic Acid (LA) are all essential fatty acids.
Long-chain Omega-3’s and Omega-6’s, Arachidonic Acid (AA), Eicosapentaenoic Acid (EPA) and Docosahexaenoic Acid (DHA) are all non-essential fatty acids as they can be synthesised from other fatty acids.
Foods which are high in polyunsaturated fats include oily fish(salmon, trout, sardines), walnuts, sunflower oil, sesame oil and low levels can also be found in leafy green vegetables.
The bottom line is simple. Don’t go no-fat, focus on the good fats. Try to eliminate trans fat from your diet, limit your intake of saturated fat and ensure you include essential fatty acids in your diet every day.
How Much You Need
The amount of fat you require to maintain optimum health is dependent upon lifestyle, weight, age and current health status. Essentials of Human Nutrition (2007) recommends the following:
- Dietary fat should provide at least 15% of total energy and 20% for women of reproductive age.
- Upper limit of dietary fat should be 30% for an average adult.
- 10% or less of dietary fat consumed should be in the form of saturated fat.
- Trans-fats should only make up 1% of total dietary fat intake.
Mann J, Truswell SA, Essentials of Human Nutrition, 2007, 3:33-52.
Pirozzo S, Summerbell C, Cameron C, Glasziou P, Should we recommend low-fat diets for obesity? Obesity Review, 2003, 4(2): 83-90.
Venkatraman JT, Leddy J, Pendergast D, Dietary Fats and Immune Status in Athletes: Clinical Implications, medicine and Science in Sports and Exercise, 2000, 32(7): 389-95.