Parkinson’s disease is the second most common neurodegenerative disease, but its aetiology is not fully understood. Parkinson’s disease is characterised by degeneration of the dopaminergic neurones in the substantia nigra of the midbrain. Treatments for involve administration of synthetic L-dopa, which crosses the blood-brain barrier and is converted to dopamine in the damaged neurones, temporarily restoring dopaminergic activity. The nutritional sciences are interested in dietary and lifestyle components that modify the risk of the disease, and evidence suggests that both smoking and coffee consumption are protective in humans. Studies involving animals have shown a protective effect for tea against Parkinson’s disease. The evidence that tea is protective of Parkinson’s disease in humans is based mainly on observational data, with a number of studies having been carried out on different continents. Researchers have used this data in a meta-analysis to demonstrate the benefits of tea consumption in humans1.
Using 12 previous observational studies carried out between 1981 and 2003 over 3 continents, and involving 2215 cases of Parkinson’s disease and 145,578 controls, the authors found a pooled odds ratio of 0.8 for tea consumption. This was therefore suggestive of a significant protective effect for tea against Parkinson’s disease. Within these results, data from China showed that tea consumption produced a pooled odds ratio of 0.73. Therefore, a stronger protective effect for tea consumption amongst the Chinese population was present when compared to the pooled results as a whole. The authors reported that Parkinson’s diseses was much rarer in Asian countries compared to North American and Europe, which may suggest genetic or environmental confounding variables. One possibility is that the green tea commonly consumed in Asian countries contains higher concentrations of polyphenols when compared to the black tea consumed in the North American and European countries.
The beneficial effects of nicotine and caffeine on Parkinson’s disease are interesting because they provide further insights into the possible aetiology. Caffeine is able to block adenosine A2A receptors, which may remove inhibition of dopaminergic neurones, stimulating the dopaminergic system by increasing dopamine release. Further, the flavonoids present in green tea may have additional protective effect against the development of Parkinson’s disease. Green tea contains a number of flavan-3-ols (catechins) that may act as in vivo antioxidants and may chelate pro-oxidant metals such as iron. Therefore green tea catechins may protect the dopaminergic neurones from oxidative stress and decrease lipid peroxidation in cell membranes. The polyphenols and caffeine may therefore have synergistic protective effects against Parkinson’s disease. That smoking is protective, and yet increases oxidative stress, suggests that it is the caffeine that is the primary protective effect in tea.
RdB
