Blackwood 
Most individuals associate the critical hormone melatonin with the promotion of sleep and regulation of the sleep-wake cycle but less is known about its regulatory roles in other functions that promote anti-inflammatory, antioxidant, immune-enhancing, and metabolic benefits.
Produced primarily by the pineal gland in the brain, melatonin’s synthesis and release is regulated by light cues i.e. melatonin starts to be released at night with diminishing light, peaking between 2-4 am in the morning, and declines with the emerging morning light each day. Other sites of synthesis include the gastrointestinal tract, retina, bone marrow, and immune cells which is thought to contribute to its broader actions and benefits.
Dietary sources of melatonin
What we have come to know is that melatonin is also naturally present in low concentrations across most plant and animal tissues where it functions in similar regulatory roles as in humans. Since this discovery, dietary sources of melatonin have been further explored to determine if they can raise melatonin concentrations in human plasma and whether this can contribute meaningfully to health outcomes.
In evaluating the melatonin content of a breadth of different foods of plant and animal origins, the highest levels in animal foods were found within fish and eggs whilst in plant foods, nuts especially pistachios and walnuts, black and red varieties of rice, tart cherries, grapes, mushrooms, and tomatoes also contained reasonable levels. With beverages, red wine and coffee from roasted coffee beans also contained an appreciable amount of naturally present melatonin.
Influence on plasma melatonin
There are several documented examples of how natural dietary sources of melatonin can lead to raised plasma levels in both animals and humans despite relatively low quantifiable levels. In rats fed walnuts, a four-fold increase in baseline melatonin levels was observed whilst in another rat study, consumption of germinated kidney beans significantly altered the melatonin levels in plasma and the excretion of its by-product metabolite in urine (another indicator of altered blood plasma levels of melatonin).
Similar data was observed in human studies. In a study of three groups of healthy volunteers separated by age range, an increase in urinary excretion of the by-product metabolite was observed in all groups after consumption of 200ml of grape juice twice a day (indicating raised plasma levels). Another small human study found volunteers who consumed tropical fruit and their juice (banana, orange, and pineapple), significantly increased their plasma melatonin concentration 2 hours after consumption.
The potential role of co-factors
Vitamins from the B complex group and minerals such as magnesium and zinc are also present in many of these foods and are essential co-factors for melatonin synthesis. Whether the observed results from these studies were directly related to the naturally present melatonin or to the nutritional co-factors required for the body’s own endogenous production are yet to be elucidated.
With the gradual decline of endogenous melatonin production with age, there may be value in the future production of melatonin supplements from dietary sources to support some of its key functions. As melatonin is rapidly metabolised and eliminated, intake from dietary sources is not considered to be unsafe.
Summary
Whether dietary sources of melatonin in supplemental form can have a meaningful effect on human health requires further research. What these studies do represent is the diversity of natural active constituents including melatonin present in whole foods, their bioavailability in humans, and promising potential for future product development and clinical applications.
References
- Meng, X et al. Dietary Sources and Bioactivities of Melatonin. Nutrients 2017, 9, 367; doi:10.3390/nu9040367
- Perez-Llamas, F et al. Development of a Phytomelatonin-Rich Extract from Cultured Plants with Excellent Biochemical and Functional Properties as an Alternative to Synthetic Melatonin. Antioxidants 2020, 9, 158; doi:10.3390/antiox9020158
- Salehi, B et al. Melatonin in Medicinal and Food Plants: Occurrence, Bioavailability, and Health Potential for Humans. Cells 2019, 8, 681; doi:10.3390/cells8070681
- Arnao, M; Hernandez-Ruiz, J. The Potential of Phytomelatonin as a Nutraceutical. Molecules 2018, 23, 238; doi:10.3390/molecules23010238
- Bonomini, F et al. Dietary Melatonin Supplementation Could Be a Promising Preventing/Therapeutic Approach for a Variety of Liver Diseases. Nutrients. 2018, 10, 1135; doi:10.3390/nu10091135
