Intake of oily fish like salmon, mackerel or sardines may help one protect against Alzheimer's, scientists claim.
A new study has found that these fish are rich in a specific type of vitamin D that patients who are not taking medication have in very low quantities.
The D2 metabolite comes only from food rather than exposure to the sun as with other forms of vitamin D.
Researchers from Kingston University, Surrey, worked on the new study for six months with scientists at Brighton and Sussex Medical School and the Sussex Partnership NHS Foundation.
The Kingston team analyzed blood samples, comparing results from dementia patients not being treated with medication to those who are on drugs and a control group who did not have Alzheimer's.
They found that in patients with the degenerative brain disease who were not being prescribed medication, levels of vitamin D2 were often "non-existent".
Professor Declan Naughton, lead researcher from Kingston University's School of Life Sciences, said that even when it was found it was "in such low quantities that it could barely be measured".
But the other two groups had far higher levels.
"There are several different types of vitamin D that can be active in the body. The key point in this study was that a blood test, which we developed at Kingston University, was for the first time able to accurately measure which, if any, of the different variations of vitamin D were present in the patients," the Daily Express quoted Professor Naughton as saying.
It is thought that the acetylcholinesterase inhibitor drug, which is used to treat early stages of Alzheimer's, may "switch on" the body's ability to absorb vitamin D2.
The findings, published in the journal Current Alzheimer Research, could eventually pave the way for better dietary advice or a supplement to restore levels of vitamin D2.