Dr Deborah Lee, Dr Fox Online Pharmacy, presents a new study that has found that drinking 4 cups of green tea or 2 cups of coffee per day reduces the risk of death in type-2 diabetes by 63%
It’s a sad fact, but having diabetes is likely to shorten your life expectancy. However, by managing your diabetes as best possible, you can minimise the long term effects of the disease and give yourself the best possible outcome. But have you thought about starting to drink green tea?
In 2020, a Japanese paper was published in the British Medical Journal (BMJ), which included data from the Fukuoka Diabetes Registry. This study is the first prospective study to examine the relationship between drinking green tea, or coffee, on mortality. The findings are remarkable.
- What did the study investigators do?
- What did they find?
- If you are diabetic, could drinking tea really lengthen your lifespan?
Read on and find out more.
Background to the study
Green tea has been reported to have a range of health benefits. This is thought to be due to the high polyphenol content of green tea leaves. The polyphenols in green tea are mostly epicatechins, specifically epigallocatechin gallate ECGC, which is the most abundant. These are plant substances that help protect plants from cellular damage. They are powerful antioxidants.
Green tea also contains the amino acid L-theanine, and caffeine, which is a stimulant drug.
Every day, our normal body cellular processes result in the production of electrically charged particles called reactive oxygen species (ROS). These are potentially dangerous molecules as they can damage DNA and initiate the production of cancer cells. This process is known as oxidative stress. This process underlies the development of many of the diseases we see today, such as cardiovascular disease (due to atherosclerosis), diabetes, dementia, and cancer.
Your body can protect itself from oxidative stress by eating a healthy diet. Many substances known as antioxidants are found in large quantities, particularly in brightly coloured fruit and vegetables. These antioxidants can neutralise ROS and prevent damage from oxidative stress. Green tea polyphenols, in particular ECGC, are potent antioxidants.
Coffee contains a mixture of caffeine, tannins, proteins, and oils. In a recent 2018 review, chronic coffee consumption was said to be linked to a lower risk of death from cardiovascular disease, and a reduced risk of stroke, and heart failure, as well as improvements in risk factors for cardiovascular disease, such as diabetes. Caffeine, along with many of the other constituents of coffee, is a potent antioxidant.
What did the study investigators do?
Between 2008 and 2010, 4923 patients with type-2 diabetes, who were registered with the Fukuoka Diabetes Registry were recruited for the study.
They all completed a dietary questionnaire asking about their green tea and coffee consumption. Basic measurements were taken such as height, weight, BMI, and blood pressure. Blood and urine tests were carried out. Patients were then followed up once a year, for 5 years, to check on their survival.
What did the study investigators find?
During the 5-year follow up period, 309 study participants died. A lower risk of death was strongly linked to increased daily consumption of green tea or coffee, compared to those who did not drink either beverage.
- 1 cup of green tea per day reduced the risk of death by 15%
- 2-3 cups of green tea per day reduced the risk of death by 27%
- 4 or more cups of green tea per day reduced the risk of death by 40%
- 1 cup of coffee per day reduced the risk of death by 12%
- 2 cups or more per day reduced the risk of death by 41%
When people drank both green tea and coffee, the risk of death showed the greatest reduction.
- 4 or more cups of green tea per day plus 2 or more cups of coffee reduced the risk of death by 63%.
Investigators positive comments on the study
The study investigators felt there were some good reasons to believe the findings of the study –
- The results of the study are in keeping with several other large research studies.
- The Ohsaki study – In this 2006 Japanese study, 40,530 fit and healthy, Japanese adults were followed up for 11 years. Over the study period, 4209 participants died.
A significant association was found between the consumption of green tea and all-cause mortality. The higher the green tea intake, the lower the risk of death. Regularly drinking more than 5 cups of green tea a day reduced the risk of death by 15% compared to non-green tea drinkers. The authors commented that the antioxidant effects of green tea polyphenols might underlie their findings.
- The Hisayama Study – In this 2019 study, 2,253 Japanese aged 40-79 without diabetes were followed up for 7 years. Over the course of the study, 282 people developed type-2 diabetes. The risk of developing diabetes was significantly reduced as levels of serum ethylamine increased. Ethylamine is a marker of L-theanine – found in large quantities in green tea. Although the authors noted that the effect of L-theanine on diabetes is unclear, they postulated that it might improve insulin resistance. Japanese typically drink potent green tea such as matcha green tea or gyokuro.
- The results are biologically plausible
There are biologically plausible reasons why consuming large quantities of green tea or coffee could be of benefit to people suffering from type-2 diabetes and could help prevent cardiovascular disease, and cancer.
Investigators negative comments on the study
The study investigators also felt there were some good reasons why the study findings should not necessarily be taken at face value –
- This is an observational study and cannot prove causation
Although this study has very interesting findings it is an observational study, meaning it is a study that has observed these findings in a specific population. It cannot prove causation. In other words, although deaths in those who were diabetic, green tea, and coffee drinkers, were reduced, it might not be due to the green tea or the coffee. It could be due to something else. The authors did not, for example, collect the socioeconomic data, and it may be that all the green tea and coffee drinkers had higher educational attainment, had better-paid occupations, and had better healthcare.
- The study relied on self-reported green tea and coffee drinking
Study participants were asked to recall how many cups of green tea they drank per day, and this may not have been accurate.
- There were only a small number of deaths
The statistics may have been skewed by the relatively small number of deaths.
- Japanese green tea may be different from green tea in the UK
Japanese often drink matcha green tea which is made by adding hot water to powdered tea leaves. The tea has a much higher antioxidant potential than standard green tea – in fact – it is said to have 137 times the potency!
If you are diabetic, could drinking green tea really lengthen your lifespan?
While there is no perfect remedy, it does appear that green tea and coffee contain specific ingredients which seem to be beneficial for health and these may confer particular health benefits to diabetics. However, this new study is only observational and cannot prove causation, so don’t go overboard!
Diabetes UK recommend tea, including green tea and black tea, but caution that to get the best benefits, you should not drink them with milk.
How to make green tea
To make green tea, use green tea bags, or tea leaves. Add hot, but not boiling water, and allow to steep for 2 minutes before drinking. Add a slice of lemon as this may aid digestion.
Can you drink too much green tea?
Take care not to overdo it if you decide to start drinking green tea. Excess green tea can cause nervousness, anxiety, nausea, and sleep problems. Drinking too much green tea may even be risky for people who already have severe cardiovascular disease. The effect of green tea in pregnancy and breastfeeding is not known, and it has been linked to miscarriage. Green tea could interfere with certain types of medication, for example, anticoagulants. Always check with your doctor if you are on long term medication before changing your lifestyle.
You should NOT drink more than 8 cups of green tea per day, maximum.
For those who do not like drinking green tea, you could consider green tea extract as a supplement.
We Brits love tea anyway! Is it such a hard swap to go for a green tea bag instead of a black one? Just considering the beverages we are drinking provides an opportunity to make better healthcare choices.
However, interesting as these findings are, it does not detract from the fact that careful management of blood sugars is the key to optimal diabetic management, both in the short and long term. If you do decide to go for green tea, you still need to follow your diabetic diet, check your blood sugars, and take your medication regularly.
For more information
- BMJ Open Diabetes Research and Care – Additive effects of green tea and coffee on all-cause mortality in patients with type 2 diabetes mellitus: the Fukuoka Diabetes Registry
- Diabetes UK – Tea and diabetes
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