Fats in foods, or, more correctly, their fatty acids, are of three main types, saturated, monounsaturated and polyunsaturated.

Saturated fatty acids carry a full quota of hydrogen atoms in their chemical structure. This is the type that increases the amount of cholesterol in the blood and is considered a risk factor in heart disease; animal fats are the main source.

When one pair of hydrogen atoms is missing, the fatty acids are termed monounsaturated. They do not raise blood cholesterol and may even be beneficial. The main sources are olive oil and rapeseed oil (used in some margarines and low fat spreads).

When more than one pair of hydrogen atoms is missing, the fatty acids are termed polyunsaturated. They predominate in most vegetable oils. Most appear to have no effect on blood cholesterol levels but are useful if they replace saturates in the diet. However, those found in fatty fish and fish oils (termed omega-3 polyunsaturated) are considered to help to lower cholesterol and therefore to be beneficial.