We are told that high intakes of dietary cholesterol raise plasma levels of cholesterol carrying lipoproteins. In turn this is said to cause atherosclerosis which causes cardiovascular disease. This cholesterol theory of cardiovascular disease is widely believed to be true, by laymen and experts alike. However, evidence does not support the contention that dietary cholesterol is the cause of cardiovascular disease.
What Is A Theory?
A scientific theory can be thought of as a supposition with a chain of supporting evidence. In order for the theory to be valid, observations must fit with that theory. If any observations contradict the theory, then the theory is invalid and must be rejected. When we attempt to gather a chain of evidence for the cholesterol theory of cardiovascular disease, we find that much of the evidence actually contradicts the theory.
The Maasai are a group of tribal shepherd who live in Kenya and Northern Tanzania. They herd sheep and goats which are their main sources of food. They eat the flesh, and drink the milk and the blood of the animals. This gives them a very high dietary cholesterol intake. However, the plasma cholesterol levels of the Maasai are quite normal and they do not suffer from cardiovascular disease. Despite eating little but flesh and milk, they are quite healthy.
Studies have tried to raise plasma cholesterol levels by feeding eggs to healthy subjects. However, as with the Maasai, such high cholesterol diets do not raise plasma levels of cholesterol. Eating up to six eggs per day has no effect on the plasma cholesterol levels of healthy people because as dietary intake increases, liver synthesis rates fall. Therefore the body is able to regulate plasma cholesterol levels perfectly in healthy individuals.
Some studies have shown that eating higher intakes of dietary cholesterol does cause increases in plasma cholesterol. However, such studies have often made methodological errors. For example, large scale epidemiological studies often include subjects with a genetic defect called familial hypercholesterolaemia. This defect allows dietary cholesterol to influence plasma cholesterol levels. Including such subjects biases the results and is of questionable value.
Healthy People And Cholesterol
The studies investigating egg consumption and the observations of the Maasai show that in normal healthy individuals, dietary cholesterol does not influence plasma cholesterol. Only those with genetic defects are affected. In fact, early research deliberately used individuals with familial hypercholesterolaemia to assess the effects of dietary treatments. While interesting, such studies are completely irrelevant to people without the genetic defect.