What is cholesterol?

Cholesterol is a waxy substance, one of the wide range 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 oestrogen and testosterone, the steroid hormones made by the adrenal gland, for example cortisol, as well as for vitamin D.

Cholesterol is also essential for making bile acids and is therefore vital to digesting fats. An adult body contains about 150g of cholesterol.

How much cholesterol comes from the diet?

Although nearly all body tissues can make cholesterol, most is made by the liver and intestine. It is therefore naturally present in the blood, irrespective of how much cholesterol is consumed in the diet. The liver must produce a certain amount of cholesterol - without it the body would cease to function properly. It is estimated that an adult makes 0.5-1.0g of cholesterol per day, which is more than would usually be absorbed from the diet.

The total amount of cholesterol in our bodies at any one time will depend on the amounts made by the body, the amount of cholesterol absorbed from the diet and the amount eliminated from the body in the faeces. Usually only about one third of the cholesterol in the body comes from the diet.

In most people, the amount of cholesterol made by the body will decline when more is absorbed from the diet.

LDL and HDL cholesterol

Fats are mainly carried around the body attached to specialised proteins, the lipoproteins. These lipoproteins are constantly changing in composition as they deposit and receive fats from blood and body tissues 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.

CHD and cholesterol

Many factors influence the risk of cardiovascular disease, coronary heart disease (CHD) and stroke. These include non-modifiable factors such as inherited (genetic) risks (reflected in family history), increasing age and male gender, and the well-established modifiable risks such as smoking, raised blood pressure, elevated cholesterol levels, lack of physical activity, obesity and Type 2 diabetes.

Coronary heart disease can result in death when the blood supply to the heart is severely interrupted by a blockage of the coronary arteries which supply oxygen to the heart. There are two main parts of this process: the build-up of fatty plaques (atheroma) in the artery (atherosclerosis) and thrombosis – when some of the fatty material breaks away and causes a blood clot to form that blocks the artery.

A high level of cholesterol-rich LDL is believed to be one of several factors that contribute to build up of atheroma in the arteries.

All information checked by an independent Registered Nutritionist/Dietitian.

Egg misconceptions

Over 30 years of prospective epidemiological surveys of CHD risk have consistently found no independent relationship between dietary cholesterol or egg consumption and CHD risk [1, 2]. In addition, there is strong evidence showing that the effects of cholesterol-rich foods on blood cholesterol are small and clinically insignificant in comparison with the effects of dietary saturated fatty acids (SFA). The amount of SFA that we consume influences the level of circulating low density lipoprotein (LDL)-cholesterol to a much greater extent than dietary cholesterol in foods such as eggs [3].

This evidence has led to major world and UK health organisations revising their guidance [4], including the British Heart Foundation which has dispensed with its recommendation to limit eggs to 3-4 a week, although people with familial hypercholesterolaemia (1 in 500 in the UK) would still be advised to restrict dietary cholesterol intake. The Department of Health also advises that there is no recommended limit on how many eggs people should eat, as long as you eat a varied diet.

The American Heart Association has also removed specific reference to eggs in their dietary recommendations for heart health (although continues to recommend a restriction of cholesterol intake to < 300mg per day). Furthermore, the 2010 Dietary Guidelines for Americans suggest that consumers should follow a healthy eating pattern that focuses on nutrient-dense foods, including eggs [5] and there is a developing awareness of the important contribution that eggs can make to a healthy diet, suggesting that the recommendations relating to dietary cholesterol should be revised [6].

Egg misconceptions research

The misconceptions around eggs and cholesterol largely stem from incorrect conclusions drawn from early research [1,3,4,6].

  • Research in the early twentieth century, on animals fed foods that were high in cholesterol and saturated fat, led researchers to an oversimplified conclusion that dietary cholesterol was the key component in coronary heart disease (CHD) risk in both animals and humans.
  • Later studies provided evidence for a definite link between raised LDL-cholesterol and increased risk of CHD, but small changes in LDL-cholesterol do not translate into clinically significant changes in CHD risk.
  • Early studies on the effects of dietary cholesterol on serum cholesterol levels produced misleading results because the diets contained high levels of saturated fat and/or extremely high amounts of cholesterol (>1000mg per day). Later studies have been able to separate the cholesterol-raising effects of dietary cholesterol from saturated fat, which often exist together in the same foods. Eggs are not high in saturated fat.
  • Studies in the 1990s began to look in more depth at the separate effects of dietary cholesterol and saturated fat, which tend to exist together in the same foods. A review of these studies in 2006 [3] concluded that although dietary cholesterol can increase serum cholesterol, both the LDL- and HDL- components are increased. The review noted that the effect was apparent at cholesterol intakes of less than 400mg per day, but was small relative to the effect of saturated fat. In addition, any impact of dietary cholesterol on LDL or CHD risk was potentially offset by a favourable increase in HDL.

References

1. McNamara DJ (2000) Dietary cholesterol and atherosclerosis. Biochimica et Biophysica Acta 1529: 310-20.

2. Rong Y, Chen L, Zhu T et al (2013) Egg consumption and risk of coronary heart disease and stroke: dose-response meta-analysis of prospective cohort studies. British Medical Journal 346: e8539

3. Lee A and Griffin B (2006) Dietary cholesterol, eggs and coronary heart disease risk in perspective. Nutrition Bulletin 31: 21–27.

4. Gray J and Griffin B (2009) Eggs and dietary cholesterol - dispelling the myth. Nutrition Bulletin 34: 66-70

5. Kanter MM, Kris-Atherton P, Fernandez ML et al (2012) Exploring the factors that affect blood cholesterol and heart disease risk: is dietary cholesterol as bad for you as history leads us to believe? Advances in Nutrition 3: 711-717

6. Eckel RH et al (2014) 2013 AHA/ACC Guideline on Lifestyle Management to Reduce Cardiovascular Risk. Circulation. 2015;131:e326. DOI: 10.1161/CIR.0000000000000155.

7. Government Dietary Guidelines