For decades, we've been told by dietitians, doctors and public health authorities to avoid saturated fats such as butter or lard in favour of unsaturated vegetable oils like safflower and soya bean. Studies conducted in the 1960s and '70s suggested that dietary saturated fatty acids raise the concentrations of total and low-density lipoprotein, or "bad" cholesterol, while polyunsaturated fatty acids do the opposite - lower cholesterol and decrease one's risk of heart disease. But research published earlier this month in the British Medical Journal has stirred controversy. Scientists from the US National Institutes of Health analysed recovered data from the Sydney Diet Heart Study conducted from 1966 to 1973 and found that substituting dietary omega-6 linoleic acid - the most abundant polyunsaturated fatty acid in a Western diet - for saturated fats increased the number of deaths from coronary heart disease and cardiovascular disease. The study, which involved 458 men, showed no evidence of polyunsaturated fatty acids' long-preached heart-healthy benefits. Participants were provided with liquid safflower oil (which is 75 per cent linoleic acid) and a safflower oil-based margarine to be used in place of animal fats for cooking, baking and spreading. Studies indicate that replacing saturated fat with polyunsaturated fat lowers the risk of heart disease Nutrition expert bill shrapnel Many experts have disputed the findings. Bill Shrapnel, deputy chairman of the Sydney University Nutrition Research Foundation, says the study used Miracle margarine as a source of polyunsaturated fat, a product which in the 1960s contained about 15 per cent trans-fatty acids, which have the worst effect on heart disease risk of any fat. "The adverse effect of the intervention in this study was almost certainly due to the increase in trans-fatty acids in the diet," says Shrapnel, who was a consultant for Unilever, makers of Miracle margarine. "Recent, well-conducted studies indicate that replacing saturated fat with polyunsaturated fat lowers the risk of heart disease, and this is widely accepted. Trans-fatty acids were largely removed from Australian margarines in the mid-1990s when their adverse effects on heart health became apparent." Faced with supermarket shelves crammed full of various oils, which ones are truly healthy and which should you avoid? Candy Sin, a dietitian with World Cancer Research Fund Hong Kong, advises avoiding animal fats, such as butter and lard, as they are high in saturated fat. Some plant oils, such as coconut oil and palm oil, are also high in saturated fat, she says, adding that healthy oils usually have a higher ratio of unsaturated fat (especially monounsaturated fat) to saturated fat. These include most plant oils, such as olive, canola and peanut oil. How you cook with oils also matters. According to Sin, heating oil at high temperature will change its chemical structure and form harmful trans fats. Canola, safflower or grapeseed oils have a higher smoke point and are preferred for stir-frying or sautéing. But even so, she advises against deep-frying, since it increases fat intake. Sin says the amount of oil used is as important as which one you choose: "Healthy oil can also contribute to weight gain and harm your health if you overuse them." All fats contain the same number of calories - nine calories per gram - regardless of whether they are "good" or "bad" fat. "A high fat intake increases the risk of becoming overweight/obese, which, in turn, increases the risk of different chronic diseases and obesity-related cancers," Sin says. The recommended daily intake of oil for most people, says Sin, is five to six tablespoons. To help you decide which to use, here's a look at some of the popular and trendy oils around. Olive oil The best known of the healthy oils, olive oil has been associated with potentially protective effects that guard against many cardiovascular risk factors, such as diabetes, high blood pressure, high cholesterol and obesity. A study published in August in the Journal of Clinical Endocrinology and Metabolism shows consumption of a Mediterranean diet enriched with olive oil for two years is linked with a protective effect on bone. Refined olive oils have been chemically treated to reduce its strong earthy flavour. Extra virgin olive oil, on the other hand, results from pressing olives without the use of heat or chemical treatments. It contains phytochemicals, which studies show can trigger cancer cell death, that are otherwise lost in the refining process. Recent studies suggest a link between extra virgin olive oil and stroke prevention in older people, and protecting the liver from oxidative stress. Due to its lower smoke point, extra virgin olive oil is best used as a dressing and drizzle. The more refined an olive oil, the better it is for all-purpose cooking. It is thought that daily consumption of about two tablespoons of olive oil may have a beneficial effect on blood cholesterol levels. Coconut oil This oil has been in the limelight in the last couple of years for its alleged health-giving properties. Coconut oil had been demonised in the past by nutrition experts for its mega-dose of saturated fat, but not all coconut oils are created equal. The key is to use virgin coconut oil that, unlike regular coconut oil, is not hydrogenated, as the process creates trans fats and destroys beneficial nutrients. Virgin coconut oil is high in saturated fat (about 77 per cent), but it's mainly lauric acid, a medium-chain fatty acid that's quickly metabolised by the liver and, therefore, less likely to be stored as body fat. Naturopathic physician Benita Perch says virgin coconut oil can reduce the risk of heart disease, cancer and other ageing-related conditions. It boosts the immune system, helps with weight loss, and improves digestion and the absorption of nutrients from food. Stable at very high temperatures, it is suitable for frying and baking. Rapeseed oil In recent years, top chefs have started using this in their culinary creations as a substitute for extra virgin olive oil. Rapeseed oil has high nutritional value because of significant amounts of omega-3 fatty acids. It comes from the seeds of the plant known as "rape", which comes from the Latin for "turnip". During the second world war, the oil was used for industrial purposes. Rapeseed oil is relatively healthy with 7 per cent saturated fat compared with 14 per cent in olive oil and 51 per cent in butter. It won't go toxic at high heat, as olive oil will, and has a higher smoke point. Flaxseed oil Produced from the seeds of the flax plant, this oil contains both omega-3 and omega-6 fatty acids. It also has the essential fatty acid alpha-linolenic acid, which the body converts into two omega-3 fatty acids found in fish oil, according to the University of Maryland Medical Centre. But studies are mixed about whether flaxseed oil has the same benefit as fish oil. Both flaxseed and flaxseed oil have been used for high cholesterol levels and in an effort to prevent cancer, says the US National Centre for Complementary and Alternative Medicine. But studies on flaxseed preparations to lower cholesterol levels report mixed results. Flaxseed oil has different folk or traditional uses, including for arthritis. Animal studies suggest that adding flaxseed oil to the diet could reduce the risk of osteoporosis in post-menopausal women and women with diabetes, according to a report published in 2009 in the International Journal of Food Safety, Nutrition and Public Heal th . Avocado oil Similar in its health-giving properties to olive oil, avocado oil is rich in antioxidants and vitamin E. Research into this oil and its antioxidant properties has been done in Mexico, the world's largest avocado producer. Studies conducted at the Morelia General Hospital in Mexico found that avocado oil helped to lower blood cholesterol and may also have a role in the prevention of type-2 diabetes. It is high in monounsaturated fats and can be used for high-temperature cooking. Rice bran oil Extracted from the germ and inner husk of rice, this oil is rich in antioxidants and vitamin E. "Rice bran oil, like sesame oil, is low in saturated fat and appears to improve a patient's cholesterol profile," says Dr Devarajan Sankar, a research scientist in the Department of Cardiovascular Disease at Fukuoka University Chikushi Hospital in Chikushino, Japan. "Additionally, it may reduce heart disease risk in other ways, including being a substitute for less healthy oils and fats in the diet." In one of Sankar's studies, people who cooked with a blend of sesame and rice bran oils saw a significant drop in blood pressure and improved cholesterol levels. This worked nearly as well as a commonly prescribed medication for high blood pressure, and the use of the oil blend with medication yielded even more impressive results. Rice bran oil also contains an antioxidant called y-oryzanol, which Japanese studies have shown to help treat hot flushes and other symptoms experienced by menopausal women. Chia oil This is the pure and natural omega-3 oil extracted from chia seeds, the superfood that originated in Mexico and South America. According to the Chia Co, Australian producers of chia products, the oil is good for vegetarians seeking a healthy source of plant-based oils, arthritis sufferers looking to improve joint function and mobility, and those who want to enhance their heart health. A teaspoon (10ml) of chia oil every day is said to meet the body's daily requirement for plant-based omega-3 alpha-linolenic acid. Fat is an essential part of our diet and should make up no more than 30 per cent of daily calorie intake. Candy Sin, a registered dietitian with World Cancer Research Fund Hong Kong, says fat has various functions: it carries fat-soluble vitamins such as E, D, E and K, protects organs against shock and is the body's energy reserve. Oils are made up of saturated and unsaturated fat in different ratios, says Sin. Unsaturated fats include monounsaturated, polyunsaturated and trans fat. Each type of fat has a different effect on blood cholesterol. Fat Facts : Saturated fat : bad fat. It increases the level of bad cholesterol (low-density lipoprotein, or LDL) in blood that causes blockages in blood vessels, which in turn increases the risk of heart diseases. Less than 7 per cent of your daily fat calories should come from saturated fat, as it is associated with raising blood cholesterol, which increases the risk of heart disease, stroke and some cancers. Found in: fats from animal sources; plant oils such as coconut oil and palm oil. Trans fat: bad fat. Trans fat is a product of a process called hydrogenation - adding hydrogen to vegetable oil - which makes the oil less likely to spoil. It carries the same health concerns as saturated fat. Found in: foods that contain hydrogenated or partially hydrogenated oils. Polyunsaturated fat: good fat. It decreases bad cholesterol levels in the blood, which helps prevent heart disease. Omega-3 and omega-6 are two well-known examples. Found in: sunflower oil, safflower oil. Monounsaturated fat: good fat. It decreases bad cholesterol and increases good cholesterol levels in the blood, which helps prevent heart diseases. Found in: olive oil, canola oil.