Tea, in particular green tea, has a well-established health halo. We are regularly treated with magazine and newspaper articles about the many health-promoting properties of green tea, and as a result, many people believe green tea to be superior to black tea. A popular claim about green tea is that it may help to lower blood sugar levels and manage diabetes, but our recent work has shown that there is very little scientific evidence to support this claim.
Tea is the most consumed drink in the world, apart from water. The most popular tea plant is Camellia sinensis, and is used to make green, black, yellow, white and oolong tea. Green tea is touted to be rich in antioxidants, especially EGCG, to which many of its health-promoting properties are attributed, but all teas actually contain high levels of antioxidants. However, the quantities of different antioxidants vary between types of tea, and there is no indication that certain antioxidants should be favoured over others in our diet.
Surprising discoveries
Data from large population studies indicate that a lifetime of tea-drinking may prevent the development of diabetes (chronically high blood sugar and insulin resistance), but it is not clear whether tea can help to treat high blood sugar or diabetes. To answer this question, we examined the scientific literature of the past seven years to find randomised control trials (RCTs) where the effect of tea products on blood glucose levels and insulin resistance was tested.
RCTs are the gold standard of drug testing, as they compare the results of a treatment group with that of a closely-matched control group, to account for random differences that would have occurred over time. By examining these studies, we made a few surprising discoveries. Dozens of studies in laboratory animals have shown mostly positive results for a variety of teas from different plant species on high blood sugar and insulin resistance, but very few such studies have been performed in humans.
Black tea makes up nearly 80% of all tea consumed world-wide, but most human and animal studies focused on green tea, probably due to the perception that green tea is healthier than black tea. Only 14 RCTs testing the effect of tea on blood sugar regulation were performed across the world in the past seven years, totalling fewer than 1 000 study participants combined across all 14 studies.
Eleven of these RCTs tested green Camellia sinensis tea, while only two trials used black Camellia sinensis tea and one used Japanese olive tea. No other teas have been evaluated in RCTs for their effects on blood sugar. Only half of the RCTs that we reviewed showed a decrease in blood sugar levels and/or an improvement in insulin resistance, which could help to alleviate diabetes, but the other half did not show any improvement in blood sugar regulation with tea. Crucially, green tea was not more effective than black tea in relieving high blood sugar and insulin resistance, and in fact, black tea was found to reduce insulin resistance and blood cholesterol levels.
A zero-calorie drink
This is great news if you find green tea unpalatable, or if you simply enjoy drinking black tea and are reluctant to switch. In some of the RCTs, green tea consumption did assist with weight loss, but without lowering blood sugar. Surprisingly, the tea dosage or the duration of the trial did not appear to be related to positive results. It also did not make a difference whether tea drinks or capsules with purified compounds such as EGCG were used in the trials. Simply, in some trials, they worked; in others, they didn't.
Some magazine articles on the health-promoting effects of tea reason that the weight-loss effects and anti-diabetic properties of tea result from tea being a zero-calorie drink, as long as you do not add sugar, honey or milk. If consumed as such, tea can safely be included in calorie-controlled or sugar-controlled diets. However, the antioxidants in tea have their own health benefits, and luckily, these benefits do not suddenly disappear if you add milk or sugar. Unfortunately, given the weak scientific evidence currently available, we really cannot say if tea is effective as a treatment to reduce high blood sugar and to help manage diabetes.
Much more work, particularly clinical trials, is needed before we can answer this question with certainty. In the meantime, we may have to re-think the health halo of green tea with regards to body weight and blood sugar control, as your ordinary cuppa may already be good enough.
*Drs Hanl Sadie-Van Gijsen and Liske Kotze-Hrstmann are affiliated with the Centre for Cardio-metabolic Research in Africa (CARMA) at Stellenbosch University.
Image credit: Unsplash
Dr Hanl Sadie-Van Gijsen & Dr Liske Kotze-Hrstmann
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OPINION | Can tea really lower your blood sugar? - Health24
