A breakdown product of the amino acid tryptophan, which is produced in the pineal gland, the gut and several other sites in the body. Until recently the only known function of melatonin was assumed to be its role in circadian regulation (sleep and waking cycles) - it is often taken by those suffering from jet lag to help adjust their cycles.

More recently melatonin has been found to act as a powerful antioxidant and free radical scavenger. Research has shown it to be five times better than glutathione at neutralizing the hydroxyl radical, and twice as effective as vitamin E in inactivating the peroxyl radical (see free radicals). Melatonin also stimulates the antioxidant enzyme glutathione peroxidase and inhibits the nitric oxide synthase enzyme which forms the free radical nitric oxide.

Experimental rats pretreated with melatonin are several times more likely to survive exposure to the toxic herbicide paraquat than untreated rats, which suggests that melatonin is highly protective against poisoning. It is described as a highly versatile antioxidant which, because it is both water- and fat-soluble, is not restricted to certain compartments of the cell but is present throughout it, in the membrane, cytosol and nucleus. Researchers believe that the most significant protective effect of melatonin is in the brain, which, due to its heavy consumption of oxygen, is particularly vulnerable to oxidative damage. Since diseases such as parkinsonism, motor neurone disease and Alzheimer's disease are linked with oxidative stress, an increased intake of melatonin may be valuable in helping to prevent them. Research has shown some success in using melatonin supplements against breast cancer.

A more recently discovered function of melatonin is its role in the immune system. If melatonin production is experimentally inhibited, a state of immunosuppression is produced which disappears when melatonin is restored. Melatonin is now being researched as a treatment for immune deficiency and against cancers.

Melatonin is not only found in the animal kingdom, it also occurs in small amounts in foods such as bananas, tomatoes and cereals, in amounts sufficient to influence physiological processes.

Menadione (see Vitamin K)

Menaquinone (see Vitamin K)