People in the study were also assessed for signs of dementia and Alzheimer's disease.
And autopsies were carried out on 90 patients who died during the study to look for certain physical signs associated with Alzheimer's disease such as deposits of protein outside and around nerve cells.
The team found that the risk of developing Alzheimer's disease increased by 51% for each point of the loneliness score.
Those with the highest loneliness score of 3.2 had about 2.1 times the risk of developing Alzheimer's disease compared to those with a low score of 1.4.
When the researchers factored in social isolation, such as if people had a small social network, the results did not change significantly.