Current evidence for the health benefits of garlic comes primarily from laboratory and population studies. What do these studies show - and how does this compare with findings from clinical trials in humans?

Laboratory studies have shown that the antioxidant polyphenols in green tea inhibit the growth of a number of cancer cells, including breast cancer, lung cancer, prostate cancer and stomach cancer - and some of these results have been replicated in animal studies. However, what works in the test tube or in mice may not always work in humans, so it is important to have evidence from research among people.

Population studies tend to suggest that green tea has a number of health benefits. For example there are lower cancer rates in Japan, where green tea is frequently drunk. In 2011 the Cancer Research UK website reported that some studies in Asian countries have found that green tea could reduce the risk of many cancers including cancers of the breast, prostate, mouth, foodpipe, stomach and bowel. The National Cancer Institute in the USA reports that more than 50 epidemiologic studies of the association between tea consumption and cancer risk have been published since 2006. Whilst it concludes the results have often been inconsistent, it notes some have linked tea consumption to reduced risks of cancers of the colon, breast, ovary, prostate, and lung. Population studies also suggest green tea’s antioxidant properties may help reduce liver disorders, help with weight loss and prevent coronary heart disease (although the U.S. Food and Drug Administration (FDA) rejected a petition from tea makers to allow tea labels to claim that green tea reduces the risk of heart disease). A 2003 article in the Journal of Nutrition (Rietveld and Wiseman) reported that population studies suggest the consumption of both green and black tea might bring positive health benefits.

One hypothesis reported was the high levels of flavanoids in tea which could protect cells and tissues from oxidative damage by free radicals – with green tea containing 3 to 10 times more catechins than black tea (polyphenols which laboratory studies have shown could block the growth of cancers).

However, Cancer Research UK, the National Cancer Institute and Rietveld and Wiseman recognize the limitations of research findings to date and conclude that more evidence is needed from large scale studies and clinical trials.

One of the difficulties with population studies is that many variables can be at work – including individual genetic differences, other aspects of diet and lifestyle (from alcohol and tobacco consumption to levels of physical activity) and, in the case of a natural substance like tea, different types of tea, tea preparation, tea consumption, the bioavailability of tea compounds (the amounts that can be absorbed by the body) – and even how hot the tea is (with some studies suggesting scalding hot tea increases the risk of cancer of the throat).

Clinical studies so far have suggested a mixed picture. The University of Maryland Medical Centre’s review suggests:

Clinical studies have suggested some positive results as regards helping prevent bladder cancer among women and improve the survival rate of male bladder cancer patients; helping prevent breast cancer among women aged under 50; as well as being likely to be beneficial as regards preventing pancreatic cancer and skin cancer and (Mayo Clinic research reported in 2010 ) reducing the number of Lymphatic leukaemia cells.

However clinical studies have been inconclusive in some other respects, for example as regards colorectal cancer, cancer of the throat, lung cancer and stomach cancer – with some clinical studies suggesting green tea was beneficial and others suggesting it was a risk.

As regards ovarian cancer and oral cancer in each case one clinical study suggested a beneficial effect but one or more others suggested no effect.

As regards prostate cancer a large clinical study in Southeast China found that the risk of prostate cancer declined with increasing frequency, duration and quantity of green tea consumption. However, both green and black tea extracts also stimulated genes that caused cells to be less sensitive to chemotherapy drugs ie not advisable for prostate cancer patients undergoing chemotherapy.

NB The National Cancer Institute also reports that a number of clinical trials have investigated the role of tea and tea polyphenols in cancer prevention – but that few trials have examined the effects of tea or tea polyphenols on cancer incidence or mortality.

On a positive note the University of Maryland Medical Center’s review of research findings suggests green tea may help reduce inflammation associated with Crohn's disease and ulcerative colitis, two types of IBD; that green tea has been used traditionally to control blood sugar in the body and a few small clinical studies have corroborated this; that drinking green tea has been found effective in a small clinical study for dental caries, or tooth decay; that green tea may benefit arthritis by reducing inflammation and slowing cartilage breakdown; and that green tea may also be effective in preventing symptoms of colds and influenza (as also reported in Japanese research published in 2011 by Matsumoto et al).

Overall the Center advises that studies show that drinking green tea is associated with reduced risk of most causes of mortality.

Health Warning- Any substance which is powerful enough to have a clinical effect could potentially have an adverse effect in certain circumstances and green tea is no exception.

The University of Maryland Medical Center advises that people with heart problems, kidney disorders, stomach ulcers, and psychological disorders (particularly anxiety) should avoid green tea – as should pregnant and breastfeeding women.

It advises that excessive amounts of green tea over prolonged periods can result in a range of adverse effects, including irritability, insomnia, heart palpitations, dizziness, and (in the case of caffeine overdose) nausea, vomiting, diarrhea, headaches, and loss of appetite.

Having said this, there is little evidence of health risks for adults consuming moderate amounts of caffeine (about 300 to 400 mg per day). A review by Health Canada concluded that moderate caffeine intakes of up to 400 mg per day were not associated with adverse effects in healthy adults.Green teas contain from 24 to 39 mg per 8 fl oz.

The Center also advises you should not drink green tea or take green tea extract without first talking to your health care provider if you are taking the following medications: adenosine; beta-lactam antibiotics; benzodiazepines (such as diazepam and lorazepam); beta-blockers, propranolol, and metoprolol; blood thinning medications (including warfarin and aspirin); chemotherapy (for prostate cancer in particular); clozapine; ephedrine; lithium; monoamine oxidase inhibitors (MAOIs) such as phenelzine and tranylcypromine; oral contraceptives; and phenylpropanolamine (an ingredient used in many over-the-counter and prescription cough and cold medications and weight loss products).

On the evidence available so far it seems reasonable to conclude that:

  • More large scale clinical trials are needed, to ascertain how much reliance can be placed on the laboratory, animal, epidemiological and small scale clinical trails conducted so far.
  • The health warnings above should be heeded if you are in one of the categories at potential risk.
  • For healthy adults green tea, provided it is not taken in excess, appears to have some potential health benefits.  

Published 22/05/2011, Review date August 2014