Green Tea and Hypertension: Benefits and Harm
Green tea: processing and bioactive compounds
Tea or Camellia sinensis is the most popular beverage in the world and is considered healthy due to the presence of antioxidants and other bioactive molecules with known benefits to the human body. Green tea is obtained when the harvested tea leaves are withered and heated either using steam (Japanese method) or by firing in a pan (Chinese method). This process halts oxidation of the leaves and helps in retaining the green color and delicate fresh aroma.
Interaction of bioactive compounds in the human body
Both, black and green tea contain small amounts of xanthine alkaloids as well as caffeine (1 -5%). Green tea also contains high amounts of phenolic substances or tannins (5 - 27%), which contain catechin (flavonol) and gallic acid, but the concentration of these compounds is higher in green teas when compared to black teas.
Flavanols are present as the most abundant class of flavonoids in tea and have potent anti-oxidative properties that help in counteracting the damage from free radicals and other reactive oxygen species. The principal Flavanol monomers called catechins include epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG) .
Green tea also contains a central nervous system stimulant caffeine. Consumption of moderate amounts of caffeine increases alertness and reduces fatigue. However, higher doses of caffeine can result in anxiety and impaired sleep . Caffeine content in tea varies considerably and is influenced by brewing method (tea content used, brewing time) and young tea leaves have a higher caffeine content in comparison to older leaves. A study conducted by the US department of agriculture found that the caffeine content in different varieties of green teas varies considerably, ranging from 10-20mg per gram of dried tea leaves .
Green tea leaves contain many different amino acids and brewing the tea extracts the soluble amino acids. L-theanine, which is a derivative of glutamic acid, accounts for almost 60% of the soluble amino acids content in brewed tea. L-Theanine has an effect that counteracts the negative effects of caffeine and gives a relaxed feeling while attenuating an increase in blood pressure in high stress response individuals .
Tea plants accumulate fluoride, which is a trace mineral required for human health. Fluoride levels in green tea are generally comparable to those recommended for the prevention of dental caries (cavities).
Health benefits on the cardiovascular system
1. Decreased blood pressure
Several independent scientific studies as well as dose-response relationship studies indicate a pronounced cardiovascular health benefits of regular consumption of 5-6 or more cups of green tea per day . Moreover, interventional studies using similar amounts of green tea, containing 200-300 mg of EGCG, have demonstrated positive effects in maintaining cardiovascular and metabolic health.
2. Improvements in cholesterol profile: Decreased LDL and increased HDL
Increased consumption of green tea is associated with decreased total cholesterol and triglyceride levels in the blood and inversely related to the risk of coronary heart disease. Scientific studies indicate that green tea consumption has greater health benefits when compared to black tea, attributed to a higher antioxidant capacity .
3. Effects on fasting blood glucose
Green tea consumption significantly reduces the fasting glucose levels and glycated hemoglobin A1c (Hb A1c) concentrations, as has been shown by a meta-analysis of 17 clinical trials comprising of a total of 1133 subjects.
Potential risks in people with pre-existing conditions
1. Pre-existing heart diseases
Patients with pre-existing heart conditions taking ß- blockers (medication against high blood pressure) need to exercise caution while consuming green tea as studies have shown that the bioactive components of green tea interact with the ß- blockers and inhibit the uptake of these medications, severely affecting the efficacy of these medications .
Regular consumption of green tea has been shown to increase the absorption of calcium. However, excessive consumption of green tea can increase the levels of caffeine and tannins to a level that reduces the absorption of calcium in the body. This effect can be minimized if tea is consumed between meals and tea consumption should not be paired with the intake of calcium sources.
3. Anxiety disorders
Caffeine content in green tea can potentially interact with the medications used to treat anxiety and mood related disorders and can result in high blood pressure. Caution must be exercised in case one is on medications.
Dosing and interaction with medications
In general, people without any pre-existing conditions can safely drink 3-6 cups of green tea per day. It is suggested to brew the teas no longer than 2-3 minutes to minimize the leaching of bitter tannins into the infusion.
However, one must be cautious while on medications related to pre-existing conditions. It is also advisable to consult the doctor and inform of tea drinking habits to the physician. In most cases, caffeine in green tea interacts with the medications to alter their effect or absorption and may decrease the efficacy of medication, putting the patient at risk. It is therefore advisable to check with your doctor before taking any green tea supplements or drinking excessive amounts of green tea.
See our infographics to learn more about How different drinks impact on heart