Tea Health: Nutrition and Functional Foods
If history suggests tea was first and foremost medicinal, what can science tell us about the curative powers of tea?
Investigating the ancient and modern claims of the health benefits of tea inevitably crosses into the marshy ground of nutritional science; an area of research where speculation trumps evidence. Finding and proving connections between diet and disease is both complex and contentious. It is especially difficut when trying to calculate the overall benefits of food and drink beyond basic nutrition and simple pleasure. Widely-held concepts of 'health-boosting' or cancer-combatant 'superfoods' are misleading: clinically speaking, "there are no good or bad foods, and no universally good or bad diets." 1
Diet and Disease
This is, of course, all within reason. One need only spend a few weeks on a traditional Mediterranean diet to know that it probably does more good than harm.2 Indeed, early research into the relationship between food and disease estimated that up to 30% of all cancers were preventable by dietary and lifestyle changes. One of these was a landmark report submitted to American Congress by two scientists from Oxford University who explored avoidable risks to cancers.3 Though their main finding was related to the role of smoking in cancer rates, they also highlighted the impact of diet. They estimated that around 35% of cancers could be linked to diet, but they were uncertain to what extent (some conditions were said to be between 10-90% preventable by diet). When this paper was published in Nature the then editor famously included a footnote imploring readers not to "take the accompanying article as a sign that the consumption of large quantities of carrots [as a source of beta-carotene] is necessarily protective against cancer." 4 The editor's cautiousness and researchers' uncertainty was far-sighted. Later studies would disprove the link between carotene and lower risk of cancer, while subsequent research into fruit and vegetable intake on cancer outcomes has not shown any protective effect.
One of the more reliable conclusions to draw from decades of nutritional research is the more confident the claim, the less likely it is to be true. The role of antioxidants - especially relevant to tea - reveals the problem of confusing nutrition with medicine.
The role of 'antioxidants'
Antioxidants have absorbed a great deal of research, but it is a generally misunderstood term: there are no antioxidants, only things that act as antioxidants.5 Why is this important? Because compounds that act as positively as antioxidants can, at higher levels, also act negatively as pro-oxidants; to simply say they are healthy is misleading.
The main benefit of antioxidants is said to be their ability to reduce the threat of 'free radicals.' These 'free radicals' are molecules whose hunger for electrons can cause considerable damage to cells. Accumulative damage to cells or 'stress' has been linked to specific illnesses such as cancer, cardiovascular disease, and even the process of ageing. Molecules that act as antioxidants are said to donate electrons to distract free radicals from attacking vulnerable parts of the cell. In this way, antioxidants appear strengthen the body's response to free radicals, lowering cellular stress and minimising damage.
It is reasonable to assume, therefore, that eating foods rich in antioxidants, or compounds that act in this way, would strengthen the body's resistance to free radicals and, over time, prevent disease. However, scientific research has failed to find proof that eating or drinking antioxidants leads to improved health. (In fact, used as supplements it is possible that consuming antioxidants increases the risk of cancer.) In any case, the health-boosting power of antioxidants remains an attractive theory, apparently grounded in science, but ultimately unproven in practical research. This is a failure partly born out of the reality gap between theory and practice, but also from the dubious claims of nutritional science: strictly speaking, there aren't any "proven health benefits" of anything we eat or drink.6
Diet: a complex mosaic
And here we return to the heart of the matter: the complicated connection between diet and disease. Isolating one set of molecules out of a cast of millions and declaring they can solve profound questions such as cancer or the ageing process is simply, and literally, too good to be true. The relationship between eating (or drinking) and health is complex; all diets have unforeseen consequences: the 'good' constituents of a food may be cancelled out by the 'bad.' Trying to tally up the total good of any food is an impossible task. Diets, amongst other things, are a mismash of personal preferences, spending power and cultural habits. How the body responds to this diet is unpredictable. The cryptic nature of this relationship is made more puzzling by the knowledge that each person responds differently to the same food. What is healthy for one person, may have no effect on another.7
A balanced diet
With all this in mind, is it possible to prove the benefits of any food or beverage? We would need to unearth an unknowable amount of information, beginning with a person's total diet, details about their lifestyle, perhaps even their own genetic code, and gut microbiota.Once this pandora's box of mitigating factors is opened it's hard to avoid the conclusion that the more we research, the less we know. Indeed, even modern clinicians are instructed that "[even] after decades of research, we still do not know who should eat what, or when." 8
It is clear that plausible nutritional advice is far less life-changing (or life-saving) than some of the more outlandish claims. We should be wary of terms such as 'proven benefits,' particularly when, in the case of food and drink, what we put in isn't necessarily what we get out. But there is also the thornier issue related to the nutrition industry which, by cloaking dietary advice in scientific language, obscures the simple truth that a healthy diet is little more than common sense: more fruits and vegetables, less fat and sugar, but, ultimately, everything in moderation.
Dietary advice cannot rescue us from sedentary lifestyles, but neither can you outrun a bad diet: in truth, both are sensible ways to improve life quality, but they cannot be confused with medicinal prescriptions.
The uncertain and complicated relationship between food and health provides the backdrop for any discussion on the health value of tea. Understanding the fallibility of nutritional advice is a remedy against the more potent claims attributed to tea drinking. There is more credible and plausible research into the potential health properties of tea, and it is on this much firmer ground that the true medicinal or healthful nature of tea can be examined.
Notes
1, 6, 8. Oxford Handbook of Clinical Medicine, 7th ed, Longmore, Murray et al. 2007
2. Recent research investigated the impact of a 'Mediterranean' diet on health: for more information see: SMILES Trial, https://bmcmedicine.biomedcentral.com/articles/10.1186/s12916-017-0791-y
3. Sir Richard Doll, one of the authors of this report, was the first scientist to link lung cancer to smoking. Also: William J. Blot, Robert E. Tarone "Doll and Peto's Quantitative Estimates of Cancer Risks: Holding Generally True for 35 Years"
4. Quoted from Bad Science, Ben Goldacre, 2009, p104
5. For more information on antioxidants and role in health see: https://www.hsph.harvard.edu/nutritionsource/antioxidants/
7. Spector, Tim, The Diet Myth, 2015
