Thinking Nutrition

Drink tea and carry on

• Dr Tim Crowe • Episode 12

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People all over the world have been drinking tea for centuries, and for good reason. A growing field of research has shown that tea has a role to play in reducing the risk of many chronic diseases. And on top of that, a good cup of tea has a real calming and relaxing effect – just what we need in these stressful times. So put the kettle on, sit back, and relax, as I explore the health benefits of tea. 

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People all over the world have been drinking tea for centuries, and for good reason. A growing field of research has shown that tea has a role to play in reducing the risk of many chronic diseases. And on top of that, a good cup of tea has a real calming and relaxing effect – just what we need in these stressful times. So put the kettle on, sit back, and relax, as I explore the health benefits of tea. 

After water, tea is the most popular beverage drunk in the world. Tea drinking has long been considered a healthy habit since ancient times. Now modern medical research is providing a scientific basis for this belief. While some brews provide more health advantages than others, there’s plenty of evidence that regularly drinking tea can have a lasting impact on your health. It is the beneficial bioactive compounds found in tea that are linked to reducing the risk of developing chronic diseases such as heart disease, diabetes, some forms of cancer and maybe even improve mental health.

Types of tea

In today’s podcast, when I talk about tea, I’m specifically referring to the main varieties that came from the plant Camellia sinensis. From this plant comes most of the popular brews including black tea, green tea, white tea and oolong tea. There is often a misconception that herbal tea is also tea. However, herbal tea is not made from the plant Camellia sinensis and instead is made from infusions of various herbs and spices like chamomile and peppermint, so I won’t be touching on those ones at all. I’ll just be sticking to traditional tea and its common varieties.

This popular little shrub is native to China and India and contains a unique mix of a family of bioactive compounds called polyphenols. Two, in particular, called catechins and theaflavins, are the main polyphenolic compounds in tea and are thought to be responsible for most of the physiological effects of tea. Then there is also some caffeine too. Not as much as coffee, but enough there to give some level of mental effect.

Black, green and oolong tea all come from the same plant, but each has its own unique flavour profiles because of how they are processed after picking. The more processed the tea leaves, usually the less their polyphenol content. Oolong and black teas are oxidised or fermented, so they have lower concentrations of polyphenols than green tea, but the levels are still considered very high.

For green tea, tea leaves are harvested then quickly heated and dried to prevent too much oxidation and fermentation from occurring that would otherwise turn the green leaves brown and alter their fresh-picked flavour.

For black tea, tea leaves are exposed to air to allow them to ferment for several hours by natural enzymes found in the leaf. This oxidation process turns leaves a deep brown colour and during this process, the flavour is intensified. The leaves are then left as such or are heated, dried and crushed. Black tea has the highest content of caffeine of the different tea varieties but is still much less than a regular cup of coffee.

Oolong tea is produced by partial oxidation of the tea leaf so is intermediate between the processes used to make green and black tea.

Health Benefits

Those polyphenols that I mentioned before are the stars of the show when it comes to the health benefits of tea. But first, a brief reading about tea and health on the Internet will bring up article after article banging on about the polyphenols being antioxidants and that explains their health benefits. Nutrition science though has well and truly moved on from using such simplistic language and concepts to describe how these thousands of polyphenols found in food work.

Focusing only on antioxidants or a single phytonutrient is like zeroing in on a section of a painting and seeing only the dots. You need to step back and see the bigger picture. It should instead be about polyphenols and their multitude of benefits and actions in the body such as:

  • Regulating cell growth and death
  • Slowing down cancer cell proliferation
  • Altering glucose responses and insulin sensitivity
  • Increasing activity of enzymes involved in removing harmful substances from the body
  • Decreasing inflammation

And just for fun to show you how complex it all is, our gut microbiota is also involved in the metabolism of polyphenols. So, a healthy gut microbiome may even help augment the function of polyphenols. See how simplistic it is to just talk about “tea being full of antioxidants and….health!”

Tea drinkers may be helping to keep their hearts healthy. Some research has shown a reduced risk of heart disease in people who regularly drink green or black tea. Although the findings have been mixed in terms of tea’s effect on lowering cholesterol and blood pressure and most of the research is observational. It seems that tea can reduce low-density lipoprotein, or LDL, which is the “bad” cholesterol that can build up in arteries while at the same time increasing the more beneficial HDL cholesterol which helps to remove excess cholesterol from the blood.

Several studies have shown that tea may favourably alter glucose metabolism and insulin signalling, which has spurred interest in the health effects of tea consumption on diabetes. Some studies suggest that the catechins in green tea may help to keep blood sugar in check, reducing the risk for developing type 2 diabetes. A 2014 meta-analysis of 15 published studies of more than 545,000 participants found, as seen with coffee, an inverse relationship between tea consumption and the risk of developing type 2 diabetes. For each additional two cups per day of tea drink, the risk of developing diabetes dropped by almost 5 percent. I’ll link to the study in the show notes. http://www.ncbi.nlm.nih.gov/pubmed/24331002 

Though there is lots of information online about tea as a cancer-fighting beverage, research has not proven that consuming tea helps to reduce the risk of cancer. Some studies suggest that tea drinkers have a lower risk for certain types of cancer, but other studies do not support these findings. At this time, it is unknown if tea drinking can reduce your risk of cancer. If we look at what the WCRF says about it there is only limited suggestive evidence that it may help lower the risk of bladder cancer, and crickets for most other forms of cancer.

And agreeing with the findings of the WCRF, a Cochrane systematic review that looked at associations between green tea and the risk of cancer which included 51 studies containing more than 1.6 million participants also found conflicting results and was not able to make any firm recommendation about tea helping to reduce the risk of cancer. I’ll link to the study in the show notes. http://onlinelibrary.wiley.com/doi/10.1002/14651858.CD005004.pub2/abstract 

Weight management

And next, we have the claims made about tea and weight loss. More specifically, green tea. Green tea is believed to be able to increase a person's energy output which could help with weight loss. Many of the commercial green tea weight loss preparations contain higher concentrations of ingredients (specifically catechins and caffeine) than the typical green tea beverage prepared from a tea bag and boiling water so it’s the supplements that are studied in clinical trials, rather than the beverage.

So, does popping a green tea extract pill help someone lose weight? Many clinical trials have been carried out with different findings from each of them. Combining all the research together was the goal of a Cochrane Review which is considered the upper echelon of evidence-based recommendations.

The review looked at 15 weight loss studies of around 3 months duration each, and three studies looking at weight maintenance. I’ll link to the study in the show notes. Each study involved some form of a green tea preparation given to one group with the results compared to a group receiving a placebo. All studies involved the participants being blinded to which group they were allocated to. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD008650.pub2/full

The clear conclusion was that weight loss in people who had taken a green tea supplement was not statistically different from the placebo group. The absolute amount of weight loss seen was very small and was not likely to be clinically important. Studies examining the effect of green tea preparations on weight maintenance did not show any benefit either compared to the use of a control preparation.

If green tea is a true weight loss miracle, then most of the miracle will be because a person believes it works and adjusts their eating and activity habits accordingly. That’s it.

Mental health

One recent health condition being associated with tea consumption is depression. Inflammation is part of the body’s response to stress and as such is thought to affect the production and function of neurotransmitters associated with depression symptoms. Tea, especially green tea, is thought to have a beneficial effect on the prevention and management of depression due to its anti-inflammatory properties. Green tea also contains large amounts of an amino acid called theanine which has been shown in laboratory studies to have a relaxing effect on the brain by increasing serotonin and dopamine production.

So, what does the research say? A 2016 meta-analysis of 11 studies with almost 23,000 participants found that for every three cups of tea consumed per day, the relative risk of depression decreased 37 percent. I’ll link to the study in the show notes http://www.ncbi.nlm.nih.gov/pubmed/25657295 

So, regular drinkers of green tea appear to have lower rates of depression. More definitive research is needed to be able to state with some conviction that the link is a direct one. Until such research findings are in, sit back and enjoy a relaxing cup of tea or two.

Miscellaneous

A question I’ve been asked before is if putting milk in your tea will block the absorption of all those beneficial polyphenols. While it was thought in the past that it would do this, current expert opinion is that the effect is negligible.

At the end of all of this, there are lots of health benefits to drinking tea, but to be honest, when I look at the total research related to tea and compare that to the quantity and quality of to do with coffee, I would have to say that coffee is in front here. Catch my earlier podcast about the surprising health benefits of coffee to get the low down on this. 

But it is all a moot point in comparing tea to coffee, they’re both great beverages. If you enjoy coffee, keep drinking it. And if you enjoy tea, keep drinking it, and carry on.

Research Wrap Up

Now on to my research wrap up segment where I profile a study that has grabbed my attention during the week. And this is to do with one of my favourite foods – blueberries.

Blueberries are much more than a deliciously sweet fruit. Medical science is looking into the health benefits that this powerhouse fruit can give. Blueberries are high in plant chemicals called anthocyanins. These anthocyanins are a type of flavonoid found naturally in several foods that have antioxidant and anti-inflammatory properties. They are also the pigments that give red, purple, and blue plants their rich colouring with the colour changing depending on the pH of the food.

Observational research points to regular blueberry eaters having lower rates of heart disease and type 2 diabetes. Such research though cannot conclusively prove that blueberries on their own are responsible for the health benefit. To date though, no longer-term clinical trials have been done to see if eating blueberries can reduce the risk of metabolic disease.

Filling the research gap, British researchers have carried out a 6-month randomised-controlled trial to see if eating blueberries had any effect on metabolic syndrome. Metabolic syndrome is a collection of symptoms including high blood pressure, elevated blood sugar, excess fat around the abdomen and abnormal levels of blood lipids. Someone with metabolic syndrome has a very high risk of developing heart disease, stroke and diabetes.

A group of 115 volunteers with an average age of 62 years took part in the study. All the people had metabolic syndrome. Each person was randomly assigned to eat either 1 cup (that’s 150 grams) or half a cup (or 75 grams) of blueberries each day. A third group was given a purple-coloured placebo food that had none of the anthocyanins found in blueberries. I’ll link to the study in the show notes. https://www.ncbi.nlm.nih.gov/pubmed/31136659

After 6 months, it was the group of people eating a cup of blueberries each day that saw a clear health benefit. There was a significant improvement in their blood vascular health with better blood flow and less arterial ‘stiffness’. Arterial stiffness describes the rigidity of the arterial wall and is linked to the development of heart disease. Also, in the study, those eating the blueberries saw their ‘good’ HDL-cholesterol levels go up, especially in people not taking statin medications that are used to lower cholesterol. The favourable changes in metabolic health would be expected to translate into a fall in cardiovascular disease risk by between 11 and 15 percent.

Blue is the new black in the world of nutrition research, and this latest research into delicious blueberries adds more evidence for the health benefits they can offer. You can enjoy your blueberries fresh, but for convenience and when they are out of season, you can’t go past having frozen blueberries on hand. And as a bonus for choosing frozen, because they are processed and frozen soon after picking, the nutrient losses are small with research showing that drying and freezing blueberries has no impact on the antioxidant activity of anthocyanin extracts. 

So that’s it for today’s show. You can find the show notes either in the app you’re listening to this podcast on if it supports it, or else head over to my webpage www.thinkingnutrition.com.au and click on the podcast section to find this episode to read the show notes.

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I’m Tim Crowe and you’ve been listening to Thinking Nutrition. 

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