Not all cholesterol is bad; in fact, our bodies need cholesterol to be able to function effectively. Cholesterol is a type of lipid (fat) that makes up the structures of our cell membranes, helps in hormone synthesis and maintains bodily functions. It also plays a key role in helping the liver produce bile, which is useful in the breakdown of dietary fats and Vitamin D, which helps keep our bones healthy. However, if our body is producing and taking in more cholesterol than it needs to function, it can cause serious health problems and is why it is one of the number one causes of coronary heart disease.
Having LDL in moderation isn't likely to be harmful, because it takes cholesterol where it's required, but if the cholesterol isn't used it is stored in the arteries, where it can cause a build up of lipids also known as plaque. This plaque can cause blockages and lead to arteries hardening, which is known as atherosclerosis, all of which can lead to serious cardiovascular complications. This is why LDL is also known as 'bad cholesterol'.
Ideally the production of HDL should be stimulated, because this is cholesterol that is being taken away from the arteries and back to the liver. High levels of HDL can reduce the risk of plaque build up in the arteries, which is why it is considered to be 'good cholesterol'.
Triglycerides are fats that can be found in the blood. They come from foods such as dairy products, meats and cooking oils. These fatty substances are either produced by the liver or come directly from the body's fat stores. Triglyceride levels tend to be high in people who are overweight, eat lots of fatty foods and consume a lot of fizzy drinks. They are taken into account when a person's HDL and LDL levels are measured, because they also have an impact on a person's propensity towards developing cardiovascular disease.
It's a common myth that foods that are high in cholesterol should be avoided, and although they contribute to some extent, our livers produce up to 75% of the cholesterol we need. Saturated fats are another source of cholesterol, which is why consuming too much leads to an abundance of LDL and triglycerides in the blood. The more LDL we have in our body, the lower the level of HDL and this relationship works the other way round as well.
Eating too much food that is high in saturated fats as well as trans fats increases 'bad cholesterol' levels and triglycerides. Exercising and following a balanced diet increases the level of HDL in our blood. Taking in unsaturated fats also helps with the production of HDL.