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Black tea is a popular beverage and is generally considered good for your overall health. In fact, black tea is protective against diabetes, one of the main risk factors for kidney disease.
However, results have varied in studies of people who already have kidney disease. The exact impact of drinking black tea on kidney function and the formation of stones is affected by a lot of variables, including age and sex, baseline kidney function, presence of comorbidities such as diabetes and hypertension, and differences in the tea itself due to different extracts and methods of brewing.
Caffeine, the stimulant that is a primary ingredient in some teas and also in coffee, can have both positive and negative effects.
Studies¹ have shown that caffeine can protect the kidneys as it has a diuretic effect and may improve the estimated glomerular filtration rate (eGFR), especially in adults over 45. Higher coffee intake (1 to 3 cups a day) was found to be associated with a lower risk of incident acute kidney disease or injury.
However, caffeine has a dose-related effect on blood pressure, with higher doses causing an increase in both systolic and diastolic blood pressure in some studies. As hypertension is both a risk factor for and a symptom of kidney disease, excessive caffeine consumption could potentially worsen kidney disease.
Studies² have also reported a further decline in kidney function in overweight individuals with metabolic syndrome who consumed more caffeinated drinks. You should talk to your doctor or nutritionist about how much caffeine you should consume in accordance with your health condition and any medications you might be taking.
Green and black tea also contain a flavonoid called catechins,³ which is known for significant antioxidant and free radical scavenging activity. Many of the health benefits of tea are likely associated with this chemical, and it may counteract some of the negative effects associated with other components in tea.
Another class of chemicals found in tea is oxalates, which are also found in potatoes, star fruit, spinach, and leafy greens. Black tea has a high concentration of soluble oxalates, about 4.68 to 5.11mg/g of tea. These oxalates bind to the calcium, forming crystals that turn into kidney stones, and thus consumption of tea might potentially increase your risk of kidney stones.
These oxalates also have low bioavailability, meaning that little of them is absorbed, so you will have to consume a lot of tea to risk oxalate toxicity and kidney damage. In a reported case study,⁴ one person managed to do so, but he consumed 16 cups of iced black tea daily, which is 3 to 10 times higher than the recommended tea consumption.
Finally, potassium is found in black tea and coffee. Potassium has cardiovascular protective properties, and higher consumption of potassium is associated with lower blood pressure. However, consuming too much potassium can be dangerous if you have kidney disease.
Your kidneys may be unable to filter all of it out, which causes hyperkalemia, a condition in which the potassium levels in your blood become too high.
Most doctors advise against consuming too much potassium, particularly in cases of kidney disease. However, some recent studies have shown that dietary potassium can also be beneficial as it helps reduce sodium levels.
Considering that most people eat a modern diet that is too high in salt and too low in potassium, slightly increasing potassium levels if you also consume a lot of sodium can be beneficial.
Both green tea and black tea are made from the leaves of the Camellia sinensis plant. Black tea is oxidized, while green tea is not. Therefore, green tea has similar health benefits — and potential issues — to black tea.
However, it is lower in caffeine and soluble oxalates, which may make it a healthier option for those with kidney disease. Green tea has been shown not to have an impact on kidney function. It has also appeared to have a protective effect on the kidneys.
Green tea is also high in antioxidants and has been shown to help with weight loss, reducing obesity and associated risks. In other words, green tea is not only safe for people with kidney disease, but it also may be beneficial for overall health.
Another alternative is white tea, which has a similar profile and a more delicate flavor that some people prefer, although it is often more expensive.
With herbal teas, it is important to understand the contents of the tea and what impact they might have. First, always check the potassium content of herbal teas, especially blends. Many herbal blends contain citrus, which can have a lot of potassium.
If you are on a low-potassium diet, limit or avoid herbal teas rich in potassium. The following teas are considered safe for people with kidney disease:
Mint (peppermint or spearmint)
Chamomile (Be aware that chamomile tea can make some people sleepy; it can be a good choice to drink late at night.)
Orange blossom tea
Rooibos or "red" tea
Astragalus (which may potentially be beneficial in chronic kidney disease)
Hibiscus (which has been reported⁵ as being protective against kidney disease)
However, you should ensure that any herbs you take do not interact with your medication. In general, sticking to one or two cups of herbal teas a day could yield some benefits without containing too many ingredients that are potentially harmful in high doses.
Some teas are specifically not recommended for kidney disease. The following herbs may affect your blood pressure or potassium levels:
If you have diabetes, you should be careful with elderberry, ginger, and licorice root, as they can affect blood sugar. While this can be helpful in diabetes management, if you are prone to going hypo (seriously low levels of blood sugar) or are already on medication, you should limit your consumption of these herbs.
However, you can consume products that contain deglycyrrhizinated licorice (DGL), which has been modified not to affect blood sugar.
Also, if you are taking antihypertensive medications and have your blood pressure under control, you should be careful with astragalus and hibiscus, as these can lower blood pressure further and potentially cause hypotension. While dandelion can affect blood pressure and potassium, some studies⁶ in rats find it potentially protective against kidney toxicity.
Any tea you drink should be consumed in moderation. Also, always make sure to choose high-quality products, as herbal teas are not regulated, and the dosage and ingredient quality can vary greatly. Cross-contamination is also a risk with lower-quality herbal teas.
Talk to your doctor about any potential medication interactions, and it's also a good idea to discuss your tea consumption with your dietitian.
In general, it is safe for people with chronic kidney disease to drink tea, coffee, and herbal teas in small to moderate amounts. Some herbal teas may be beneficial for people with kidney disease, while others may be potentially dangerous, especially in high amounts, as they could be toxic to the kidneys, contribute to forming kidney stones, or impact blood pressure or potassium levels.
You should talk to your dietitian about your tea consumption and get specific recommendations, and when drinking herbal teas, always make sure to consume small amounts to avoid any potential side effects. Also, consult your physician to ensure they will not interact with your medications.
Renal and cardiovascular effects of caffeine: A dose-response study | Portland Press