Tea: the scientific story

I’ve written about coffee several times here, but until now, I’ve never addressed our other favourite hot drink.

That’s right. Tea.

 

I’m not planning to get into the long-lasting discussion about which is better, but I am going to talk about the scientific pros and cons of tea.

camellia_sinensis_flower_tea
Camellia sinesis, the tea plant. Sourced from Wikimedia Commons

Most studies on tea are done on black or green tea and some on white or oolong (herbal teas are not “proper” teas, by the definition of coming from the tea plant, Camellia sinensis).

 

 

 

All 4 types of tea contain polyphenols: chemical compounds including catechins, theaflavins, tannins and flavonoids. The main role of all the polyphenols is to act as antioxidants, reacting with harmful free radical compounds in your body that would otherwise go around destroying cells and structures.

 

The catechins have been the most studied and mostly strongly related to positive health benefits. They are present in highest concentrations in green tea, followed by black tea (yielding up to 10-15% of the mass of dry tea leaves and 30-40% in brewed green tea).

Being antioxidants, many of the studied benefits of tea are linked to their antioxidant properties. The catechins in particular have been linked to reducing oxidative stress and DNA damage, age-related oxidative stress and even reduction in cancer risk.

The most interesting thing about the reduction in cancer risk is that many studies simply draw a correlation between two things: ie. noting that tea drinkers have lower rates of cancer, without specifying a mechanism for how tea ingestion might affect cancerous cells (confused? Read more about correlation vs causation here). In contrast, the studies showing a reduced cancer risk associated with tea drinking actually provided a mechanism, showing that the main catechin compound in tea acts to initiate the normal cell death cycle (which goes out of control in cancerous growth, allowing cells to replicate and live indefinitely) and also prevents the breakdown of extracellular matrix that would otherwise enable tumorous growth. If you’re interested reading more about this fascinating topic, the paper is open-access and can be found here!

As for reducing oxidative stress, including age-related stress, that’s another good thing. If your cells are looking damaged from age-related stresses, and tea reduces that, then you effectively slow or reverse aspects of cell ageing. The only problem was that these remarkable results were seen in rats and were inconsistent in human experiments.

In another review, the researchers found that drinking green tea (containing 690 mg of catechins) per day, acted to reduce the concentration malondialdehyde-modified LDL, a compound strongly associated with weight gain/loss. Over a twelve-week study period, the people drinking the green tea lost weight, body fat and reduced their waist circumference. So that’s good news!

Other great benefits of tea include a reduction in risk of cardiovascular disease, and has been more loosely linked to improving oral health, bone density, UV protection (but not a substitute for sunscreen, unfortunately), and more effective immune function.

 

One of the biggest practical benefits is not necessarily caused by chemical reactions. Drinking tea after a stressful event was shown to reduce stress levels by 25% compared to those who didn’t get a cuppa. That’s pretty awesome news for all of the over-stressers out there, although the study didn’t compare other drinks, foods or activities to find which would be the best stress relief.

 

 

Negative effects

To offset the wonderful health benefits, there are a few caveats.

Firstly, don’t drink sugary tea. If you’re looking to reduce risks of cardiovascular disease or just improve your general health by drinking tea, don’t buy a sugar-filled iced tea, or add sugar and honey to yours at home. Sugar is linked to all sorts of negative health effects and you’d be better off just drinking water.

Second, if you’re iron deficient, it might be a good idea to limit your consumption. In individuals with healthy iron levels, tea will make little difference, but to people with already low iron, tea can prevent the absorption of necessary iron. A good rule of thumb is to avoid tea (or coffee) within a couple hours of meal times.

Third, tea does contain some caffeine, often around 40 mg per cup. So if you’re caffeine sensitive, you won’t want to drink too much and probably not in the evenings.

And lastly, don’t be excessive! It’s a good rule of thumb for almost anything in your diet, but there have been cases where people have drunk several litres of extremely strong tea every day which has resulted in osteoporosis and increased risk of prostrate cancer in men.

But sticking to a couple cups a day is more likely to do you good than harm!

 

 

 

For those with an interest in coffee, here’s more reading below:

Coffee or not? A discussion of the science

How Coffee is Made


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