Eggs are being rehabilitated with each new study, and are regaining their position as "the incredible, edible egg" touted in an old commercial. For 50 years, eggs were connected with heart disease, high blood pressure, and obesity — all conditions that have been linked to an increased risk of Alzheimer's disease.

New studies, however, have found that eggs only have a slight effect on cholesterol levels, and in fact, actually enhance brain function.

The newest study, which is published in the American Journal of Clinical Nutrition, is from the University of Eastern Finland. It found that a diet relatively high in cholesterol, such as eating an egg every day, didn't raise the risk of dementia or Alzheimer's.

In addition, the study found no increase in memory disorders in people carrying the APOE4 gene. The APOE4 gene, which affects the metabolism of cholesterol and increases the risk of memory disorders, is hereditary and affects about a third of Finnish people.

Researchers assessed the dietary habits of almost 2,500 men between the ages of 42 and 60. At the beginning of the study, none of the men had memory problems. During a follow-up of 22 years, 337 men were diagnosed with a memory disorder, 266 of them with Alzheimer's disease; 32.5 per cent of the study participants were carriers of APOE4.

The study found no association between a high intake of dietary cholesterol and the risk of dementia or Alzheimer's disease, even in people who carried the APOE4 gene. On the contrary, the consumption of eggs was associated with better results in certain tests measuring cognitive performance.

The Finnish research follows several studies showing that eggs don't increase the risk of coronary heart disease, including a November study published in the Journal of the American College of Nutrition which found that eating an egg a day actually reduced the risk of stroke by 12 percent.