A key vitamin found in meat, fish and milk may help protect the brain as it ages, researchers said.
Vitamin B12 could help stop the brain shrinking - possibly preventing memory loss in older people and dementia.
A study of 107 people aged 61 to 87 found that those with lower vitamin B12 levels in their blood were six times more likely to experience brain shrinkage compared with those who had higher levels of the vitamin.
Anna Vogiatzoglou, from the department of physiology, anatomy and genetics at Oxford University, which led the study, said: "Many factors that affect brain health are thought to be out of our control, but this study suggests that simply adjusting our diets to consume more vitamin B12 through eating meat, fish, fortified cereals or milk may be something we can easily adjust to prevent brain shrinkage and so perhaps save our memory.
"Research shows that vitamin B12 deficiency is a public health problem, especially among the elderly, so more vitamin B12 intake could help reverse this problem."
She said more research was needed into whether taking a B12 supplement would result in less shrinkage.
In the study, published in the journal Neurology, brain volume loss was measured every year for five years.
None of the people enrolled in the study were suffering memory loss at the start of the study and none had a vitamin B12 deficiency.
The participants were given yearly physical examinations, MRI scans of their brains, tests to check their cognitive and memory skills, and blood tests to determine their levels of vitamin B12.
The results showed that the decrease in brain volume was greater among those with lower vitamin B12 levels.
The authors concluded: "Low vitamin B12 status should be further investigated as a modifiable cause of brain atrophy and of likely subsequent cognitive impairment in the elderly."
Ms Vogiatzoglou said previous research on vitamin B12 had shown mixed results and few of the studies had used brain scans of elderly people.
"We tested for vitamin B12 levels in a unique, more accurate way by looking at two certain markers for it in the blood," she said.
The researchers were from the Oxford Project to Investigate Memory and Ageing (OPTIMA).
Its director, Professor David Smith, said: "This study adds another dimension to our understanding of the effects of B vitamins on the brain. The rate of shrinkage of the brain as we age may be partly influenced by what we eat.
"We are following this study up with a clinical trial of B vitamins in the elderly to see if taking these vitamins, including B12, can slow the shrinkage of the brain."
The trial will report next year.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, which helped fund the research, said: "This study suggests that consuming more vitamin B12 through eating meat, fish, fortified cereals or milk as part of a balanced diet might help protect the brain. Liver and shellfish are particularly rich sources of B12.
"Vitamin B12 deficiency is a common problem among elderly people in the UK and has been linked to declining memory and dementia.
"Seven hundred thousand people live with dementia in the UK, and more research like this is urgently needed if we are to tackle this cruel condition."
Dr Susanne Sorensen, head of research at the Alzheimer's Society, said brain shrinkage was usually associated with the development of dementia.
"As vitamin B may be given as a food supplement, it may be useful to include tests of vitamin B levels in the general assessment of health of older individuals.
"This is another example of why it is crucial for people to lead a healthy lifestyle with a balanced diet rich in B vitamins and antioxidants.
"The best way to reduce your risk of developing dementia is to keep active, eat a balanced diet, don't smoke and visit your GP to get your blood pressure and cholesterol checked."