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Cholesterols are types of fat found in the blood. The liver produces sufficient cholesterol for the body’s requirements, but it can also be acquired from foods, particularly animal products, such as meat, egg yolks, butter, cheese and milk. Cholesterol is necessary for the body to function properly, as it is involved in the production of hormones and vitamin D, and in the digestive process. Cholesterol is not soluble in the blood and has to be transported through the bloodstream by carriers called lipoproteins (complexes of fatty acids and proteins). There are two types of proteins that transport cholesterol to and from cells, high-density lipoproteins (HDL), and low-density lipoproteins (LDL). The levels of cholesterol bound to these lipoproteins in the bloodstream can be an important indicator of the risk of heart disease. A blood test commonly used by doctors to determine your ‘cholesterol level’ calculates the overall cholesterol level by adding HDL and LDL to 20% of the amount of triglyceride (another fat present in the blood).  LDL is commonly known as ‘bad’ cholesterol. This is because it contributes to the formation of plaques within blood vessels, which are thick hard deposits that can clog arteries and decrease their flexibility, resulting in atherosclerosis.  If a blood clot forms in one of these narrowed arteries, it can result in a heart attack or a stroke. Excess LDL can also contribute to peripheral artery disease, which is the result of plaque narrowing an artery that supplies blood to the extremities, such as the legs. HDL is known as ‘good’ cholesterol as it assists in the removal of LDL by acting as a scavenger, carrying LDL back to the liver where it is broken down and excreted. 20% to 30% of cholesterol is carried by HDL.,  However, despite its beneficial actions, studies have shown that the level of HDL in the plasma may not reliably indicate the degree of reduction of LDL and so research into this area is continuing. A satisfactory level of HDL can, however, decrease the risk of heart attack and stroke and, conversely, a level that is too low increases the risk. Triglycerides are another type of fat also present in the blood. They are also produced by the liver and are additionally found in oils, meat and dairy products. Their purpose is to provide energy to the body’s cells and tissues. When excess calories are ingested they are converted to triglycerides and stored as body fat and as fatty deposits within organs and tissues. Like LDL, high levels of triglycerides are also associated with atherosclerosis, and many people with diabetes or heart disease have high triglyceride levels within their blood. It is also common for people with high triglyceride levels to have a high ‘overall’ cholesterol level (i.e. a high LDL level and a low HDL level). One study has shown that a healthy lipid profile (i.e. low LDL, high HDL and low triglycerides), can reduce the risk of chronic heart disease by 71%. Some common causes of elevated triglyceride levels include obesity, high alcohol consumption, cigarette smoking and a diet high in saturated fats. Underlying diseases, e.g. poorly controlled diabetes, an underactive thyroid (hypothyroidism), kidney disease, and alcoholism, and genetic disorders such as familial hypertriglyceridemia can also cause increased levels of triglycerides. The best way to maintain healthy cholesterol levels is by leading a healthy lifestyle: a varied and balanced diet, low in saturated fat and plenty of pesticide-free fruit and vegetables, in addition to sustaining a healthy weight. Exercise is another key factor in reducing or maintaining healthy cholesterol levels, although the exact mechanism for this is not yet clear. Exercise is also beneficial in controlling weight. Smoking should be avoided and alcohol consumed in moderation; as well as increasing cholesterol levels, the adverse effects on health of both smoking and excess alcohol are well documented.  NIH (2016) What is cholesterol? Retrieved May 2016 from, http://www.nhlbi.nih.gov/health/health-topics/topics/hbc  Barter, P. et al. (2007) HDL Cholesterol, Very Low Levels of LDL Cholesterol, and Cardiovascular Events. The New England Journal of Medicine. 57. 1301-1310.  Kyle, D. (2004) Understanding lipoproteins as transporters of cholesterol and other lipids. Advances in Physiology Education. 28(3). 105-106.  Lee-Rueckert, M. et al. (2016) HDL functionality in reverse cholesterol transport - Challenges in translating data emerging from mouse models to human disease. Biochim Biophys Acta. 1861(7). 566-83.  Schwartz, CC. et al. (2004) Lipoprotein cholesteryl ester production, transfer, and output in vivo in humans. J Lipid Res. 45(9). 1594-1607.  Rothblat, G.H. et al. (2010) High-density lipoprotein heterogeneity and function in reverse cholesterol transport. Current Opinion in Lipidology, 21(3), 229-238.  University of Pennsylvania. (2016) Fat and ketoacids, cholesterol, repair. Retrieved June, 2016, from https://www.med.upenn.edu/biocbiop/faculty/vanderkooi/chap7-9.pdf  Toth, P. (2005) The “Good Cholesterol”. Circulation. 111(5). 89-91.  Borge, G. et al. (2007) Nonfasting Triglycerides and Risk of Myocardial Infarction, Ischemic Heart Disease, and Death in Men and Women. Journal of the American Medical Association. 298(3). 299-308.  Manninen, V. (1992) Joint effects of serum triglyceride and LDL cholesterol and HDL cholesterol concentrations on coronary heart disease risk in the Helsinki Heart Study Implications for treatment. Circulation. 85(1). 37-45.  University of Colorado, Denver (2016) Cholesterol and Triglycerides Retrieved August, 2016, from http://www.ucdenver.edu/academics/colleges/medicalschool/departments/medicine/EndocrinologyMetabolismDiabetes/clinical/Documents/Lipid_Brochure.pdf  NIH (2016) Familial hypertriglyceridemia. Retrieved June, 2016, from https://www.nlm.nih.gov/medlineplus/ency/article/000397.htm  Meissner, M. et al. (2010) Exercise enhances whole-body cholesterol turnover in mice. Med Sci in Sports Exercise. 42.1460-1468.