1.6.3 Niacin (vitamin B3 )
Niacin, which comprises two compounds, nicotinic acid and nicotinamide, also occurs widely in food and is added to many breakfast cereals. It is easily absorbed into the blood from the digestive system and plays a vital role in energy production in cells. It appears to reduce the levels of low density lipoproteins or LDLs in the blood and increase high density lipoproteins or HDLs, perhaps by affecting the proteins that carry the fats. This is important because LDLs are a way of transporting cholesterol around in the blood. Cells that need cholesterol take it up from LDLs. If cells contain excess cholesterol, it is returned to the blood packaged into HDLs. The higher the ratio of HDL to LDL in the blood, the lower the risk of developing heart disease. Thus, if niacin increases HDLs and reduces LDLs, this should give some protection against heart disease.
The deficiency disease associated with lack of niacin is pellagra. Its symptoms are the four Ds – diarrhoea, dermatitis, dementia and death, normally experienced in that order! The term ‘pellagra’ was first used in 1771 to describe the disease that was endemic at that time in poor populations in southern Europe. ‘Pellagra’ is from the Italian words pelle meaning ‘skin’ and agro meaning ‘sour’ and refers to the thickened, roughened skin, or dermatitis, which is characteristic of the disease.
It was noticed that people with pellagra subsisted on a diet that was based on maize and contained very little meat. By 1900, the disease had spread to France, Egypt and England, and in 1902 it was first reported in America. For the next 20 years, it reached epidemic proportions in the southern USA. Again, poverty and the consumption of large quantities of maize (corn) appeared to be the risk factors. Although it was soon realised that the deficiency disease could be prevented by the inclusion of meat in the diet, it was not until the late 1930s that the explanation of the link with eating a lot of maize was understood.
This link involves a molecule called tryptophan. Tryptophan is an amino acid, one of the molecules that make up proteins, and is commonly found in animal proteins. Niacin can be synthesised in the body from tryptophan. In fact, in the average UK diet, there is probably sufficient protein to satisfy all the niacin requirement of the body, and dietary niacin is therefore not essential. However, maize contains so little tryptophan that there is insufficient for the body to convert to niacin. Additionally, any niacin present in the maize itself is so tightly bound to molecules in the maize which are not digested, that it cannot be absorbed by the body. In the indigenous populations of Mexico and Central America, who also subsist on a diet of maize, there have been almost no occurrences of pellagra. It seems that their tradition of soaking the maize in an alkaline solution of lime before cooking it, releases the bound niacin, freeing it for absorption by the body. Poor peasants of the Deccan Plateau of India, however, do suffer from pellagra, although their staple carbohydrate is millet (sorghum) rather than maize. This grain contains sufficient tryptophan but it also contains very high levels of leucine, another amino acid that has been found to prevent niacin synthesis in the body, so symptoms of its deficiency occur. People with HIV infection can also suffer from a pellagra-like condition, since the infection causes the tryptophan levels in their blood to be very low. High doses of niacin can reverse the pellagra condition.