Under the Microscope: Most people believe that you should eat a diet low in fat and low in cholesterol in order to reduce weight and prevent heart disease and cancer. But this advice is out of date.
The total amount of fat in the diet doesn't closely correlate with disease. The important thing is the type of fat in the diet. "Good" fats reduce risk and "bad" fats increase risk.
Cholesterol in itself is not bad. On the contrary, cholesterol is an essential chemical in the body without which we would die. About 80 per cent of cholesterol in the body is made by the body itself. Cholesterol is an essential component of the membranes in every cell of our bodies. Also, certain hormones, eg sex hormones, are derived from cholesterol, as is Vitamin D However, problems arise if we have too much cholesterol in the blood.
Cholesterol in the body is made in the liver where it is linked to carrier proteins called lipoproteins that dissolve in blood and carry the cholesterol all around the body. Low density lipoproteins (LDL) carry cholesterol from the liver to the rest of the body and high density lipoproteins (HDL) carry cholesterol back to the liver from where it is eliminated from the body. When LDL is too high in the blood cholesterol tends to deposit on the walls of coronary arteries and, hence, LDL cholesterol has attracted the title "bad" cholesterol.
Higher levels of HDL reduce the risk that cholesterol will be deposited in coronary arteries and HDL cholesterol is often called "good" cholesterol.
Deposits of cholesterol (plaque) can narrow the artery enough to slow or block blood flow and this process, called atherosclerosis, often occurs in arteries that feed the heart. When a section of heart muscle fails to get enough nourishing blood, chest pain (angina) may result, or, more seriously, heart attack, stroke or sudden death. The good news is that cholesterol build up can be slowed, stopped or reversed.
The higher the LDL and the lower the HDL in blood the greater the risk of atherosclerosis and heart disease. It is recommended that, if you are over 20 years old, you should maintain your total blood cholesterol at less than two milligrams per millilitre (ml), HDL cholesterol levels greater than 0.4mg/ml and LDL less than 1mg/ml.
We ingest cholesterol in our diet, but there is only a weak relationship between dietary intake of cholesterol and blood cholesterol levels or risk of heart disease. For some, reducing dietary intake of cholesterol has a small but helpful effect on levels of blood cholesterol, for others dietary intake has little effect on blood cholesterol levels. Some foods are particularly rich in cholesterol, for example egg yolks, but studies have shown that in healthy individuals, eating up to one egg a day does not increase risk of heart disease. People with diabetes however should eat eggs more sparingly.
As regards other fats in our diet we can divide them into three categories - saturated and non-saturated fats and trans fats. Saturated fats in the diet raise total blood cholesterol more than dietary cholesterol does because it boosts both HDL and LDL. On the other hand, unsaturated fats in the diet are good because they decrease LDL levels and increase HDL levels in blood.
Saturated fats are mostly animal fats, found in meat, whole-milk diary products and egg yolks. Unsaturated fats are found in plant sources such as vegetable oils, nuts, seeds and in fish. Chemically speaking, unsaturated means that the fats contain double bonds. If a fat has one double bond it is monounsaturated. If it has more than one it is polyunsaturated. Polyunsaturated fats are present in sunflower, corn and soybean oils and in fish. Monounsaturated fats are found in canola, peanut and olive oils.
Trans fats are produced when hydrogen is bubbled at high temperature through unsaturated liquid vegetable oils. Hydrogen is absorbed by some of the double bonds thereby eliminating them and rendering the oil solid. This is how margarine is made. The more hydrogen is absorbed, the harder the product and the more trans fats are produced. Most trans fats in the diet are found in baked goods, margarines and snack foods. Trans fats are bad for cholesterol levels because they tend to raise LDL levels and lower HDL levels in blood. It is wise to pay some attention to minimising trans fats in your diet. Trans fats are worse than saturated fats in the diet.
About 25 years ago margarine became popular as a "healthy" substitute for butter and quickly killed off butter sales. As already described, margarine contains some trans fats. In fact the hard margarines used for cooking contain a considerable amount of trans fats and are far less healthy than butter.
Butter, unlike margarine, is an entirely natural product made from milk, and it tastes delicious. Butter does contain cholesterol and saturated fat, but is also an excellent source of the fat-soluble vitamins A, D, E and K. Margarine is higher than butter in unsaturated fats.
Margarine by law must have a fat content between 80 per cent and 90 per cent. Low-fat spreads on the other hand, have their fat content highly diluted by water and are also very low in transfats.
For a normal person, eating butter in moderation, poses no health risk. Indeed, the golden rule regarding all aspects of diet for the normal person is very simple - eat a wide variety of foods in moderation and take regular aerobic exercise. This takes care of everything.
William Reville is associate professor of biochemistry and public awareness of science officer, UCC - http://understandingscience.ucc.ie