Methionine - an amino acid found in a range of fish, red meats, dairy and egg products - could raise the risk of memory loss if consumed in large quantities, according to researchers.
Study co-author Anuradha Kalani, of the University of Louisville, KY, and colleagues note that past studies have suggested excess methylation - a system involved in the regulation of gene expression, protein function and other biological processes - can cause memory loss by "silencing" genes that play a role in cognitive function.
This latest study builds on such findings, according to the team, suggesting that a diet high in methionine leads to increased methylation of netrins - a class of proteins involved in brain maintenance - which promotes memory loss.
For their study, Kalani and colleagues fed mice a diet high in methionine and low in folate, vitamin B6 and vitamin B12 for 6 weeks. The memory function of the mice was monitored weekly, as assessed through a test that measured their response to a fear stimulant.
Methionine is defined as an essential amino acid that is important for a number of cell functions in the body. It is commonly found in fish, red meats, dairy and egg products, including buffalo, halibut, cod and tuna.
Diet high in methionine reduced long-term memory in mice
Compared with a control group, after 4 weeks, the mice fed the high-methionine diet stopped responding to the stimulant that had previously induced fear, which suggests a loss in long-term memory.
The team hypothesized that excess methionine leads to hypermethylation of the netrin gene, effectively "silencing" it and causing memory loss. This theory was confirmed through injecting the brains of the mice with netrin genes, according to the researchers; they found it restored their long-term memory by around 50%.
What is more, they found that mice fed the high-methionine diet had reduced levels of a netrin receptor called DCC - the presence of which is eliminated in the development of colon cancer.
Commenting on their findings, the researchers say:
"Taken together, these results confirm that a diet containing high methionine, low folate and vitamin B6/B12 can induce defects in learning and memory. Furthermore, the data indicate that decrease in netrin level due to hypermethylation of its gene promoter can be associated with memory loss."
In future research, the team plans to investigate how diet can affect other genes - such as microRNA genes - to promote memory loss.
This is not the first study to associate diet with memory loss. In August 2013, Medical News Today reported on research from the University of California-Los Angeles suggesting that consuming too much red meat could raise the risk of Alzheimer's disease.
More recently, however, we reported on a study revealing how a new diet - called the Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diet - could reduce the risk of Alzheimer's by 35%, even with moderate adherence.