WHAT WE KNOW ABOUT FATS AND OILS IN FOOD

One of the general health messages of the 1980s was to reduce fat consumption and in particular, saturated fat and cholesterol. A high concentration of cholesterol in the blood is one of several factors associated with a greater risk of heart disease.

Broadly, there are three types of fat: saturated fat (for example, depot fat from cud-chewing animals such as sheep and cattle, and the milk and dairy products from these animals); monounsaturated fat (for example, olive oil, peanuts and avocado fat) and polyunsaturated fat. The polyunsaturated fats can be divided into two families of essential fatty acids (ones which the human body cannot make): the 'omega-6' and the 'omega-3' family. The omega-6 family is found particularly in oils and spreads derived from seeds, like safflower, corn and soya. The omega-3 family is found especially in fish oils, and in the structural or membrane fat of animals including fish and land animals, even sheep and cattle. Lean meat is a reasonable source of omega-3 fatty acids, but fish is the best source.

Both types of essential fatty acid contribute to the regulation of blood fats (cholesterol and triglycerides), form part of cell membranes, and are used to make compounds called prostaglandins which regulate metabolism and leukotrienes which regulate inflammation. These effects on inflammation seem to be important in a range of conditions from rheumatoid arthritis to psoriasis (a skin condition) and ulcerative colitis (a disease of the large bowel).

In general saturated fats in the diet raise the concentration of cholesterol in blood while monounsaturated and polyunsaturated fats lower blood cholesterol levels.

Cholesterol exists in different forms in blood: HDL and LDL cholesterol. Both polyunsaturated and monounsaturated fats lower LDL cholesterol, while only omega-6 polyunsaturated fats lower the other form, HDL cholesterol. While lowering the former is important in reducing overall cholesterol, the latter form is considered to protect against heart disease. Monounsaturated fats therefore have an advantage in that they do not cause a decrease in the beneficial form of cholesterol (HDL cholesterol).

The visible fat on beef and lamb is high in saturated fat and low in unsaturated fat while, in contrast, the 'invisible' fat, which is distributed in the lean part of meat, has a high ratio of unsaturated fat to saturated fat. The fat of chicken and pork has a higher degree of unsaturation than beef and lamb and, while it is desirable to reduce the intake of all types of fat, this is not as crucial as fats containing predominantly saturated fat. Trimming visible fat from meat or removing the skin from chicken not only reduces the total fat and cholesterol intake but, in the case of beef and lamb, it also produces a beneficial increase in the unsaturated to saturated fat ratio.

As well as saturated fat, a high intake of dietary cholesterol also contributes to high levels