Fat facts

While cutting kilojoules is important in losing weight, where your heart and arteries are concerned it's the type of fat you eat that matters most.

Essentially, it's all about the effect that the different kinds of fat have on fats in the blood, including blood cholesterol. An area where many nutrition experts agree is that an increase in saturated fat in the diet is associated with an increase in coronary heart disease, and also increases in some other problems, including type 2 diabetes.

Because foods high in saturated fats are mostly high in kilojoules as well, restricting their intake will reduce the overall amount of kilojoules you consume. Some studies also show that by cutting back on saturated fat, total dietary fat tends also to be reduced.

Cholesterol

Cholesterol is necessary for essential functions, such as making sex hormones and vitamin D. The body makes most of what it needs, but sometimes excessive amounts are produced, if too much saturated fat is consumed.

There is only one type of blood cholesterol, but it's attached to different carriers called lipoproteins. The main ones are known as high-density lipoprotein (HDL) and low-density lipoprotein (LDL). HDL is the "good" form, as it transports excess cholesterol from the artery walls to the liver. "Bad" LDL cholesterol gets dumped in your artery walls, helping to clog them and restricting blood flow.

Animal products contain dietary cholesterol, so most people do eat some in their diet. However, while this cholesterol can increase LDL cholesterol, it contributes much less to the body's cholesterol than saturated or trans fat.

Fat

As for fat, it can be divided into two major types: saturated, and unsaturated (monounsaturated and polyunsaturated), which are differentiated by their chemical make-up. Most foods contain various combinations of these types of fat.

Saturated fat

Saturated fat is primarily responsible for raising "bad" LDL cholesterol levels in the blood, which ultimately lead to blocked arteries. It's found in high quantities in fatty meats and dairy foods, though it's also in some vegetable oils (palm and coconut oils).

Because saturated fats are convenient (they're solid at room temperature) and relatively cheap, they're used a lot in commercially produced foods such as cakes, biscuits, pastries and chocolate bars, as well as fried takeaway foods and potato crisps. It's mandatory for the saturated fat content to be specified in the nutrition information panel on packaged foods, but unfortunately not restaurant or takeaway meals.

Polyunsaturated fats

Polyunsaturated fats are "good" fats that have been found to lower artery-clogging LDL cholesterol.

There are two forms.

• Omega-6 fats are found in sunflower, safflower, sesame, corn and soybean oils and seeds.
• Omega-3 fats are found in high quantities in seafood, especially fish such as swordfish, salmon, gemfish, tuna and sardines, while other types of omega 3 are found in plant sources such as flaxseed, walnuts, soybeans and green vegetables.

Monounsaturated fat

Monounsaturated fat is also regarded as a "good" fat. It too can lower LDL cholesterol levels, but not quite to the extent that polyunsaturated fat does. It's found in large quantities in canola and olive oil, avocados and nuts.

So replacing saturated fat with a mixture of omega-3 and omega-6 polyunsaturated fats as well as some monounsaturated fat is the game plan for a healthy heart. Depending on the total amount you eat, it may also help you lose weight.

Trans fats

Trans fats are another type of unsaturated fat, but instead of being good for you like other unsaturated fats, trans fats (formed when vegetable oils are hydrogenated) act like saturated fat in the body - raising the level of LDL cholesterol and reducing levels of HDL.

The problematic trans fats are mainly generated when hydrogen is added to mono- or polyunsaturated fats. Partially hydrogenating them like this hardens them, making them more convenient to use on a commercial scale, and like most saturated fats they're also cheap. So in our diet we mainly get trans fat from vegetable shortening and commercial frying fat, used mostly in the food industry in the production of cakes, biscuits and other processed foods, and in takeaway meals.

Table spreads were also once a main offender, with a high trans fat content, but manufacturers have made a concerted effort to reduce levels, and these days most spreads have low levels.