Melatonin is a hormone that is naturally produced in humans by the pineal gland. It is most commonly associated with inducing and regulating sleep, since it is produced during the hours of darkness. In the US, melatonin supplements are commonly used by people whose sleep patterns are irregular, and those doing shift work or suffering from jet lag.
The new research was carried out at the University of Texas Health Science Center and published in the September issue of Nutrition.
The researchers set out to investigate whether melatonin is present in walnuts, and whether eating walnuts has an effect on levels of melatonin and antioxidants in the blood.
First, they extracted melatonin from walnuts and quantified it using HPLC (high-performance liquid chromatography). The amount of melatonin in the nuts was found to be between 2.5 and 4.5 nanograms per gram.
The researchers then fed either walnuts or regular chow to rats, after having restricted their diet prior to the study. Their serum melatonin levels and 'total antioxidant power' were measured.
The walnuts were seen to increase blood melatonin concentrations threefold in the rats that ate them, compared with those on the regular chow. Serum antioxidant power also increased.
Lead researcher Russel Reiter said: "Our studies demonstrate that walnuts contain melatonin, that it is absorbed when it is eaten, and that it improves our ability to resist oxidative stress caused by toxic molecules called free radicals."
Although it is not clear how many walnuts humans would need to eat in order to benefit, in theory they might be expected to reduce the incidence of cancer, delay or make less severe neurodegenerative diseases of aging, including Parkinsonism, Alzheimer's disease and reduce the severity of cardiovascular disease, said Reiter.
Melatonin produced by the human body decreases with age. The study raises the question as to whether there may be a link between declining melatonin and the development of free-radical-related diseases in later life.
The omega-3 content of walnuts has already been shown to help reduce the risk of heart disease. Reiter maintains that the synergistic effect of omega-3, melatonin and other nutrients give walnuts the edge over a supplement containing just one element.
"I think the value of the walnut is the composite of what it contains," he said. He is planning further research on the interplay between melatonin and omega-3.
Reiter's research was supported by a grant from the California Walnut Industry.