A protein building block found in many foods plays a key role in the spread of deadly breast cancer, research has shown.
Foods rich in the amino acid asparagine include dairy products, beef, poultry, eggs, fish, asparagus, beans, nuts, seeds, soy and whole grains.
The discovery by British and US scientists could pave the way to new treatments based on suppressing the molecule.
Using a drug and special diet to cut levels of asparagine greatly reduced the spread of an especially deadly form of breast cancer in mice, scientists found.
The animals had triple-negative breast cancer, which grows and spreads faster than other types and is notoriously hard to treat.
Lead scientist Professor Greg Hannon, from the Cancer Research UK Cambridge Institute, said: "Our work has pinpointed one of the key mechanisms that promotes the ability of breast cancer cells to spread.
"When the availability of asparagine was reduced, we saw little impact on the primary tumour in the breast, but tumour cells had reduced capacity for metastases (spread) in other parts of the body.
"This finding adds vital information to our understanding of how we can stop cancer spreading - the main reason patients die from their disease.
"In the future, restricting this amino acid through a controlled diet plan or by other means could be an additional part of treatment for some patients with breast and other cancers."
The researchers also found an asparagine link when they examined data from breast cancer patients.
Breast cancer cells with higher levels of the amino acid were more likely to spread.
In several other cancer types, increased asparagine levels were associated with reduced survival.
An early trial in which healthy patients consume a low-asparagine diet is now under consideration. The next step would involve a clinical trial with cancer patients.
The study, part-funded by Cancer Research UK, is published in the journal Nature.
The charity's head nurse Martin Ledwick said: "Research like this is crucial to help develop better treatments for breast cancer patients.
"At the moment, there is no evidence that restricting certain foods can help fight cancer, so it's important for patients to speak to their doctor before making any changes to their diet while having treatment."