Drinking tea: a risk for your kidneys? Scientific deciphering

6 mars

When I started writing on this blog, I often heard that tea in excess led to the formation of chronic kidney stones. But with its reputation as a “miracle” drink and its ability to hydrate the body, what exactly is the truth?

The kidney: two organs that do us good.

The kidney is a major organ of the body, responsible for many of its vital functions. The kidney acts as a filter for our blood. But that's not all! The kidney is capable of acting on our brain and vascular network to control the sensation of thirst and blood pressure respectively.

The kidney is like a kidney bean... only more complex! There are what we call nephrons, a set of tubes connected to blood vessels. It is in these nephrons, and in particular in a part of the tubes called the renal glomerulus, that filtration takes place. The filtration capacity of the kidneys is not unlimited, and certain molecules such as large proteins (hemoglobin, creatine, etc.) or excess lipids (free fatty acids, cholesterol) can accumulate in the glomerular filter, leading to the formation of clots known as kidney stones.

Contrary to popular belief, you need to drink a lot to “dilute” the flow of blood arriving at the filter, and thus limit the accumulation of molecules on the filter (although one well-known consequence is more frequent toilet stops).

Is caffeine a cause of kidney stone formation?

For many years, caffeine has been singled out as an inductor of calcium kidney stones. Indeed, beverages high in caffeine, such as tea, contain oxalate, one of the ingredients leading to stone formation.

The reality in the cup and in the body is far more complex. In 2007, Bolignano et al. retrieved clinical data from 6 studies (1985-2004) of healthy patients and those with kidney disease. They found that caffeine did not increase the risk of developing kidney stones, regardless of the patient's state of health (although the studies did not take into account age or the use of diuretics or analgesics, for example, which are factors favouring the development of stones).

What's more, if we look at the data on tea, it does not appear to have a significant protective effect on calcium stone formation, which raises the question of whether tea has a beneficial effect on the kidneys.

Tea and kidneys: what do we know over the last 5 years?

If we select a dozen publications including the terms “Tea” and “Kidney” ... we quickly realize the diversity of results. In 2017, Biao et al. conducted a study on 8807 people with undifferentiated lifestyles (sedentary lifestyle, diet, etc.) and showed that daily tea consumption (regardless of frequency) increased the occurrence of calcium crystals in the kidneys.

However, in 2014, Van Hasselt et al. demonstrated that daily consumption of green, black and oolong teas in 12428 patients did not impair renal filtration functions.

Finally, in 2019, Shu et al. combined the results of two studies (58054 men and 69166 women) on weekly green tea consumption (at least three times a week for more than six months) and found that tea would have a protective role on the appearance of kidney stones.

To understand why the results in the literature are so contrasting, the respective authors of these publications looked at the patients in their studies (age, sex, lifestyle, etc.). For example, Biao et al. found that the tea-drinking group contained more people with chronic hypertension than non-drinkers. High blood pressure is known to promote the formation of kidney stones, which could also indicate that tea has not only a negative but also a protective effect on the development of this pathology.

The general state of health of patients is therefore important to take into account in studies on tea and kidneys. Indeed, a 2017 in vivo study in rats by Xie et al. seems to demonstrate that the probability of contracting kidney pathology linked to a diet rich in fatty acids would decrease after consuming green tea extracts. It is interesting to note in this study that the effects of green tea extracts are similar to those of an adapted diet in the context of kidney pathologies. This would suggest that tea may also have a protective role in relation to renal pathology.

Apart from kidney stones, studies of tea's chemical compounds, such as catechins/EGCG, on the prevention of other kidney pathologies (nephrotoxicity linked to anti-cancer treatments, renal fibrosis, etc.) are still limited to in vitro and in vivo studies on cells and animals, but they seem to indicate a protective effect.

It would seem that tea and kidneys have a rather complicated history. Between caffeine-induced stone formation and protective effects, the reality is more complex and would seem to depend on the pathology and underlying health status of the patient. Apart from studies on stones, there is as yet little data on the protective effect of tea on other kidney pathologies.