A comprehensive review (meta-analysis) of 60 diet studies lead by Dutch researcher Ronald Mensink examined the effects of the amount and type of dietary fat on cholesterol levels. The effect of substituting carbohydrate for fats in the diet was also assessed. Rather than just total cholesterol and LDL (bad) cholesterol, this review measured effects on HDL (good) cholesterol as well. This is important as higher HDL levels have a protective effect against Coronary Artery Disease (CAD). The ratio of total cholesterol to HDL cholesterol7 is important in predicting the risk of CAD and is a more reliable predictor of this risk than is the total cholesterol level. A higher value for this ratio means you have a higher risk of developing CAD. One study reviewed suggested that lowering this ratio by 1 unit is associated with a 53% change in the risk of myocardial infarction (heart attack).

The review showed the best cholesterol results and risk reduction is achieved when saturated and trans fats8 are replaced with unsaturated fats (such as sunflower, canola, soybean and olive oils).

Plant sterols lower blood cholesterol by partially blocking the absorption of cholesterol from the gut (Jones et al). Cholesterol in the gut comes mostly from bile used to help us digest food but about a third of the cholesterol comes from food itself. Normally about 40-50% of the cholesterol is absorbed from the gut into the body. However, when sufficient plant sterols are present, the amount of cholesterol absorbed is decreased and an increased amount of cholesterol passes down the gut and out of the body. With less cholesterol entering the body, the level of blood cholesterol falls.

The fall in cholesterol is substantial. A review (meta-analysis) of 41 diet trials shows 2g/day of plant sterols lowers LDL (bad) cholesterol by an average of 10% (Katan et al). With long term use, this would be expected to translate to approximately 20% lower risk of coronary heart disease. Combining plant sterols with a diet low in saturated fat and cholesterol can lower LDL cholesterol by 20% (Skeaff). The cholesterol-lowering effects of plant sterols are also additive to the effects of statin9 drugs (Simons et al). Studies on people with diabetes show the same response to plant sterols compared with the general population.

This question was addressed by Canadian researcher David Jenkins and his team who compared a diet of known cholesterol-lowering foods with a cholesterol lowering drug9 in a group of people with high cholesterol. Coined the 'portfolio diet', the study showed the combined effects of adding plant sterols (margarine spread), soy protein foods (soy milk, soy burgers), soluble fibre (oats, barley, psyllium, okra, eggplant) and almonds, compared with drug treatment. The 'portfolio' diet lowered LDL (bad) cholesterol by 29.6%, that is, to a similar degree to the drug which lowered LDL by 33.3%.

Yes, according to a study conducted by the University of Otago, New Zealand. A group of 50 people with high cholesterol followed a series of study diets in which only the spread they used was changed: butter, polyunsaturated margarine spread, or polyunsaturated margarine spread with plant sterols. Participant's cholesterol levels were measured at the end of each diet. Blood cholesterol levels were highest on the butter diet, which was the highest in saturated fat. Compared with the butter diet, polyunsaturated margarine spread lowered LDL (bad) cholesterol by 5.5% and the plant-sterol spread lowered LDL by 12.3%. Compared with butter, using either margarine spread resulted in 10% lower levels of blood triglycerides – a favourable outcome. HDL (good) cholesterol levels did not change. Thus the reductions translate directly to a lower risk of coronary heart disease.

The authors concluded that the beneficial effect on blood lipids of margarine spread versus butter was fully explained by the reduction of saturated fat in the overall diet. The further reduction in LDL cholesterol seen in participants using the plant sterol spread was due totally to the plant sterols, as all other dietary aspects were the same.

Yes, but the effect is quite small. Soluble fibre is found in oats, fruits such as apples, psyllium and legumes such as lentils, chick peas, soya beans and other types of beans. The type of fibre they contain is known to affect the re-absorption of cholesterol from the gut but the results of scientific studies have produced very varied results. In 1999, a major review (meta-analysis) of 67 of these studies found the effect of soluble fibre on blood cholesterol was 'small but significant' (Brown et al). At practical levels of intake of soluble fibre a fall in blood cholesterol of about 2% can be expected. It should be remembered that sources of soluble fibre such as oats, fruit and legumes are also very low in saturated fat and therefore important foods to consider including in your cholesterol-lowering diet.

7 The ratio of total cholesterol to HDL cholesterol is measured like this.

Total cholesterol = LDL (bad) cholesterol + HDL (good) cholesterol + (triglycerides / 5).

For example, if your total cholesterol = 6 and your HDL cholesterol = 2 your ratio = 3. If your total cholesterol = 6 and your HDL cholesterol = 3 your ratio = 2 which is 1 unit lower than the previous example.

8 Trans fats are produced when liquid oils are partially hydrogenated to make them solid at room temperature. In Australasia, these are found mostly in fast foods and commercially baked products.

9 "Statins" are a class of drugs that lower cholesterol.