In the past it was thought that people should limit the number of eggs they eat because they contain dietary cholesterol. But it is now known that saturates - in which eggs are relatively low - are more influential in raising blood cholesterol than dietary cholesterol itself. Over 30 years of prospective epidemiological surveys of coronary heart disease (CHD) risk have consistently found no independent relationship between dietary cholesterol or egg consumption and CHD risk, McNamara (2000).
Lee and Griffin (2006) suggests that, the prevention of CHD must focus on obesity, diabetes and metabolic syndrome as the prevalent sources of increased risk. It even goes as far as saying that moderate consumption of eggs (1-2 eggs per day) should be encouraged as part of an energy restricted, weight losing regimen due to eggs’ satiety index.
Recent research by Dhurandhar (2007) reported that eating eggs can help with weight loss. American studies found that by eating two eggs for breakfast, over weight and obese women ate less at subsequent meals and lost more weight than those eating a bagel-based breakfast.
Eggs contain significant amounts of protein and many other nutrients including vitamins B2, B12, and D, and the minerals phosphorus and iodine, so they can make an important contribution to a healthy diet.
Coronary Heart Disease
The biggest cause of death in adult men and women in the world is coronary heart disease (CHD), which is responsible for killing millions of peoples every year. Many factors influence the risk of CHD. These include both non-modifiable factors such as inherited risks (reflected in family history), increasing age and gender, and modifiable risks such as smoking, high blood pressure and high blood cholesterol levels. There is now increasing evidence that many other factors, including lack of physical activity, being overweight, the distribution of body fat, lack of antioxidants, an abnormal response to insulin, abnormalities in blood clotting factors and high blood homocysteine levels may also contribute to CHD.
However, research has shown that a high blood cholesterol level is one of the major modifiable risk factors of CHD and that our lifestyles, and particularly the food we eat, can influence our blood cholesterol level and therefore our risk of heart disease.
The following key facts are designed to help dispel some of the myths surrounding cholesterol.
What is cholesterol...?
Cholesterol is a waxy substance, one of the wide ranges of fats (lipids) present in the body. It is essential to life. Cholesterol is an important component of all cell membranes, with about 25% of the body’s total cholesterol in the cell membranes of the nervous system, where it is a major component of the fatty sheaths that insulate nerves. It also provides the basic skeleton for the synthesis of many hormones - the sex steroids, such as oestradiol and testosterone, the steroid hormones synthesized by the adrenal gland, for example cortisol, as well as the precursor of vitamin D (7-dehydrocholesterol). Cholesterol is also an essential precursor of the bile acids and is therefore vital to fat digestion. An adult body contains about 150g of cholesterol.
How much cholesterol comes from the diet?
Although nearly all body tissues can synthesize cholesterol, most is synthesized by the liver and intestine. It is therefore naturally present in the blood, irrespective of dietary cholesterol intake. The liver must produce a certain amount of cholesterol - without it the body would cease to function properly. It is estimated that an adult synthesizes 0.5-1.0g of cholesterol per day, which is more than the amount usually absorbed from the diet.
Cholesterol is also present in some foods including eggs, butter and cream, liver and kidney. Average western diets are estimated to contain 250-750 mg per day, approximately half of which is absorbed by the intestine to enter the circulation.
The total amount of cholesterol in our bodies at any one time will depend on the amounts synthesized and excreted and the amount of dietary cholesterol absorbed. Usually only about one third of the cholesterol in the body comes from the diet.
In most people, body synthesis will decline when more is absorbed from the diet; however some people are unable to regulate cholesterol homeostasis in this way.
What is the difference between blood cholesterol and dietary cholesterol...?
i. Blood cholesterol is the level of cholesterol in the bloodstream.
ii. Dietary cholesterol is the cholesterol present in some foods that we eat.
Is dietary cholesterol the main cause of high blood cholesterol...?
Research has shown that the amount of fat in the diet especially saturates has a greater effect in raising blood cholesterol concentration than the amount of cholesterol in the diet. Other factors, such as smoking, being overweight and physical activity also influence blood lipid and cholesterol levels. However, the overall risk profile for cardiovascular disease is far more complex and may be influenced by many other factors. Some of these, such as raised blood pressure, blood clotting factors and thrombotic tendency, and alcohol intake are well established,
whilst others e.g. blood homocysteine levels, and antioxidant status - influenced by the intake of antioxidant nutrients and phytochemicals are the subject of ongoing research.
What are LDL and HDL cholesterol...?
Fats are mainly carried around the body bound to specialized proteins, the lipoproteins. These lipoproteins are in a constant state of flux as they deposit and receive their component lipids and their chemistry and involvement in metabolism is complex.
In simple terms, most cholesterol in the blood (70%) is carried by low-density lipoproteins (LDL), which take cholesterol from the liver to body tissues. High blood levels of LDL-cholesterol (sometimes called "bad cholesterol") are associated with an increased risk of CHD. Much of the remaining cholesterol is carried as high-density lipoproteins (HDL), so-called "good cholesterol", which is involved in the disposal of cholesterol, and which is associated with a decreased risk of CHD.
High blood cholesterol levels and CHD risk
Coronary heart disease causes death when the blood supply to the cardiac muscle is severely impaired by an occlusion of the coronary arteries. There are two main aspects to this process: atherosclerosis and thrombosis.
The complex sequence of cellular events that causes the development of atheroma, essentially a chronic inflammatory condition, has been extensively investigated over the past decade. An early event is the dysfunction of the endothelial cells lining the arterial wall, resulting in a damaged area to which white blood cells (monocytes) are attracted. These cells are changed into macrophages and infiltrate the intima (the inner coat) of the arterial wall, taking up oxidized LDL cholesterol from the bloodstream. Repeated injury at the same site encourages the
continuous build-up of lipid and other cellular products that constitute the atheroma, which may ultimately cause a fatal blockage of the arteries.
A high level of cholesterol-rich LDL is believed to be one of several factors that contribute to the initiation and maintenance of the process of coronary atherosclerosis. The antioxidant status of the arterial wall is thought to be another key factor in this whole process, and there is evidence that the presence of adequate antioxidants, such as vitamin E, may help protect against cholesterol-induced endothelial injury.
References:
Dhurandhar, NV. (2007). Eggs for breakfast help promote weight loss. Presented at Experimental Biology 2007. Astrup A, Buemann B, Flint A, Raben A. Low fat diets and energy balance: how does the evidence.
Lee, A. and Griffin, B. (2006). Dietary cholesterol, eggs and coronary heart disease risk in perspective. British Nutrition Foundation. Nutrition Bulletin. 31(1): 21- 27.
McNamara, DJ. (2000). Dietary cholesterol and atherosclerosis. Biochimica et Biophysica Acta 1529: 310-320.