Immune booster. Detoxifying. Fat-burning. Prevents cancer.
These are just some of the claims you may see about herbal teas. And though herbal teas can sometimes benefit health, claims like these are often greatly exaggerated or based on scant evidence.
Herbal teas have been used for their medicinal properties for millennia. The Chinese used ginkgo tea to improve circulation; the ancient Egyptians drank a coriander tea to soothe irritated stomachs or inflamed urinary tracts. Moms from the Roman empire to present day might tell you to drink some chamomile tea if you have trouble sleeping.
There have not been many clinical trials done to really assess whether or not these teas do what our ancestors—or contemporary manufacturers—sometimes claim. Most of the claims you see on tea boxes, that the contents "encourage a healthy weight" or "support heart health" are supported by studies conducted on animals or in cell cultures isolated in petri dishes in a lab. In other words, they're not usually done on real humans, says Diane McKay, an assistant professor at the Friedman School of Nutrition Science and Policy at Tufts University.
From a regulatory standpoint, the wording of those claims has to be very specific. The US Food and Drug Administration (FDA) regulates herbal teas the way they do supplements (i.e., different from pharmaceuticals). That means the wording on the packaging can only be about the product's effect on the entire body and can't be directly associated with any health outcomes. The tea manufacturers can claim their product supports a healthy weight, but not that it causes drinkers to lose weight or that it can cure obesity, McKay notes.
Take, for example, mint tea. Cultural knowledge tells us that it can be used to treat indigestion; tea companies claim that it's a gentle laxative. "No one has tested the mint tea itself, but we do know from animal and cell culture studies that the oils from the mint plant do have this effect on the intestines that can help reduce muscle spasms," McKay says. So if you drink mint tea, you might reap some of those benefits—oil is present in the tea, but it's diluted with water so the effect would probably be milder. Plus, there's no guarantee that the herbs in your mint tea are what the retailers claim, or that the herbs were grown without pesticides or contaminants that could kill you before any stomach bug.
The results of nutrition studies are sometimes less cut-and-dry than are experiments to test new drugs. The gold standard to evaluate any substance—double-blind, placebo-controlled clinical trials—are often difficult to do with food and drink because the product you're testing is made of more chemicals and researchers sometimes don't know which one is the active ingredient. That makes it difficult to say that any positive health benefits are due to the product you're testing instead of something else in the diet.
Still, some of these studies have been done for herbal teas. In fact, McKay and her collaborators did one to see if hibiscus tea could lower blood pressure. They had 65 adults with hypertension drink three cups of hibiscus tea per day for six weeks. At the end of the experiment, many of the participants had lower blood pressure. They still don't know exactly what compound in the tea caused that effect, but they know the tea works. Similar results have been found for other teas—green tea is associated with a lower risk of colorectal or stomach cancer, for instance, and black tea may reduce the likelihood of heart disease (the design of these studies, like McKay's, only points to a correlation between tea consumption and health outcomes, not necessarily causation).
And just because there's limited scientific knowledge doesn't mean the claims passed down for generations are false. Researchers working in the fields of ethnobiology and ethnomedicine are looking with fresh eyes at these ancient treatments to see if they might have information about treatments that Western medicine has not yet made sense of.
So herbal teas aren't without value for your health. But it's clear that some tea manufacturers are making much more exaggerated claims than they should be. Online retailers pop up, claim their tea will cure what ails you, then get shut down by regulators.
McKay suggests taking health claims of teas with a grain of salt. Drinking herbal tea is a great way to hydrate if you're sick of water but, she says, "don't drink something that doesn't taste good because you hope it will do something it might not do." Oh, and anything that says it's a detox is automatically a lie. "If it sounds too good to be true," McKay says, "it probably is."