There is insufficient evidence to support a relationship between diet and the prevention of dementia, health officials have said.
The comments from Public Health England (PHE) come after scientific advisers reviewed the evidence on nutrition and cognitive impairment and dementia in adults.
The Scientific Advisory Committee on Nutrition (SACN), which advises PHE and other UK government organisations on nutrition, examined studies on diets, nutrients such as omega-3 fatty acids and caffeine.
Following the review, PHE said that the report broadly supports existing advice to eat healthily - as shown in the Eatwell Guide which informs on how to achieve a healthy balanced diet.
Eating a healthy diet can help people to achieve lifestyle factors linked to a lower risk of dementia such as maintaining a healthy weight and blood pressure.
But PHE said that overall, there is not enough evidence to support a relationship between prevention of dementia and diet.
The SACN report concluded that that adherence to a Mediterranean dietary pattern is associated with a reduced risk of mild cognitive impairment and dementia, including Alzheimer's disease.
But the authors stressed that most evidence behind this finding comes from observational studies - which means that no firm conclusions can be drawn about cause and effect - and they called for more evidence to establish the link.
Meanwhile, they found that there is insufficient evidence to draw any conclusions on the association between "healthy" dietary patterns, other than Mediterranean diets, and risk of cognitive impairment.
And there is not enough evidence to confirm a link between taking individual nutrients - including B vitamins, vitamins C and E and omega-3 fatty acids - and risk of dementia.
The authors also found that the evidence is "limited" and indicates that there is no association between caffeine intake and cognition over the longer term.
Dr Alison Tedstone, chief nutritionist at Public Health England, said: "This report broadly supports existing advice to eat a healthy diet as depicted in the Eatwell Guide.
"However, the report indicates that, overall, there isn't currently enough evidence to support a relationship between diet and the prevention of dementia."
NHS advice to reduce the risk of dementia is to eat a healthy diet, maintain a healthy weight, exercise regularly, do not drink too much alcohol, stop smoking and maintaining a healthy blood pressure.