The idea that South Africa’s iconic rooibos herbal tea can help you lose weight frequently pops up in publications.
For the most part, the idea is that if you swap 4 cups of coffee laden with sugar and dairy products for plain rooibos tea, the kilogrammes will melt away.
But spokesman Ernest du Toit also offered a medical explanation. “Rooibos’s unique bioflavonoid, aspalathin, helps to reduce stress hormones that trigger hunger and fat storage,” he was quoted as saying.
Has this been scientifically proven?
Bright colours of plants come from bioflavonoids
Bioflavonoids are a diverse family of phytonutrients, or plant chemicals, responsible for the bright colours of the plants they’re found in. Known for their general antioxidant and health promoting effects, they have stimulated a great deal of research interest.
To see whether bioflavonoids helps reduce levels of stress hormones, we specifically have to consider cortisol, the hormone perhaps most often associated with stress.
Cortisol is made by the adrenal glands under instruction from the brain. Excessive amounts of cortisol can suppress the immune system, increase blood sugar and promote overeating, leading to obesity.
On the other hand, too little cortisol can result in lethargy and weakness, or even dangerous drops in blood pressure in response to serious physiological stressors, such as severe infections or intense exercise.
As with most things in our bodies, the concentrations of stress hormones are carefully regulated and balance is key.
Cells, rats & humans studied
The Rooibos Council’s Margo Slabbert provided Africa Check with two studies from Cape Peninsula University of Technology as proof of their claim. But these did not shed light on how exactly rooibos may influence stress hormones.
However, citations on the Rooibos Council’s website showed that their funding has assisted Stellenbosch University to study the effect of rooibos and its flavonoids on stress hormones.
In 2014, by using unfermented rooibos leaf extracts on cell cultures as well as rats, the Stellenbosch group was able to demonstrate that rooibos leaf extract directly interferes with the production of the stress hormone cortisol. This happened by interfering with enzymes vital to both production and activation.
The human arm of these studies involved a small sample of 40 people (24 women and 16 men) drinking the equivalent of 6 cups of fermented rooibos daily for 6 weeks in 2014.
Unlike in the cellular studies, human blood tests did not reveal a reduction in the amount of circulating cortisol. Initial results seemed promising, as “inactive” hormone levels were found to be raised, yet the concentration of “active” hormone levels didn’t change significantly. (Note: Inactive hormone must be converted to active hormone before it can act on its cellular target.)
The authors speculated that rooibos perhaps acts in the same way in humans as it does in rats and cell cultures. The difference would be that the human brain simply responds to reduced levels of cortisol by increasing the drive for the adrenal glands to produce more. This would theoretically explain the raised inactive fraction in presence of an otherwise normal blood concentration of active fraction.
‘Sample size too small’
But drawing conclusions about the effect on humans from the animal and cellular studies is problematic, plant science researcher Kirsty Botha from the University of Pretoria told Africa Check.
“The study utilised fermented rooibos in the human population but unfermented in the lab trials,” she said. “The old saying goes: ‘More evidence is needed’, but there seems to be minimal to no toxicity, so in the meantime, go ahead.”
Pharmacologist Ashton Usher, also from the University of Pretoria, highlighted that while aspalathin was used on its own in cellular studies, the people studied took in a whole rooibos mix.
“To prove the exact claim with any certainty, aspalathin would also have to be shown to hold the same effect it does on cell cultures, when scaled up to human trials,” he said.
Finally, the “the sample size presented by this study is too small to truly evaluate whether rooibos may have any real world significant effects on cortisol levels,” Usher said.
Conclusion: The claim is unproven
Media publications frequently repeat a claim by South Africa’s Rooibos Council that rooibos tea has a specific ingredient – the bioflavonoid aspalathin – which helps to reduce stress hormones that trigger hunger and fat storage.
In cellular and animal studies, rooibos has been shown to interfere with the production of the stress hormone cortisol to a significant degree. Despite these findings, the evidence for the effect of both aspalathin, as well as whole rooibos, on humans remains lacking in practice, though promising in theory.
An argument for rooibos reducing “stress” in the broadest sense of the word can certainly be made, strongly so for its activity against free radicals, but the claim that it specifically reduces stress hormones in people is unproven.
Edited by Anim van Wyk
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