The reported benefits could be linked to the 1-deoxynojirimycin (DNJ) content of mulberry leaves, explained researchers from Japan’s Saga University and Nishikyushu University. DNJ is a compound which plays a role in suppressing post-meal blood glucose elevation.
The husk of water chestnut is also rich in polyphenols, which has been shown to inhibit carbohydrate-digesting enzymes and reduce postprandial glycemic response in both mice and humans, said the researchers.
Writing in Nutrients, they said that the results of the five week study supported the efficacy of using mulberry and water chestnut tea in glycemic management.
Study method
A total of 31 non-diabetic adults completed the study, where they were randomized to take either the mulberry and water chestnut tea or placebo drink before each meal for two weeks.
On the third week, all participants underwent a one-week washout period.
After which, they switched the assigned beverage for the remaining two weeks of the study.
The mulberry and water chestnut tea used for this study was produced locally in Kanzaki City, Saga Prefecture.
Each three-gram sachet of the tea contained 2.85 g of mulberry leaves and 0.15 g of water chestnut husks. The active ingredients included 5.10 mg of DNJ and 87.3 mg of polyphenols.
The continuous glucose monitoring (CGM) tool FreeStyle Libre was used to measure changes in pre and post-meal blood sugar levels.
Area under the curve (AUC) of glucose level changes was measured to assess the impact of mulberry and water chestnut tea on pre and post-meal blood sugar levels.
Blood samples were also taken to measure HbA1c, insulin, and 1,5-anhydroglucitol (1,5-AG) etc.
Findings showed that participants who took mulberry and water chestnut tea had significantly lowered blood sugar levels as compared to those taking placebo.
This was based on comparing the coefficient of variation (CV) of continuous glucose levels in participants during the periods taking mulberry and water chestnut tea and placebo.
The larger the CV, the larger the variability in continuous glucose monitoring readings, which also means a bigger spike in blood glucose levels.
“The primary outcome, the CV of glucose levels measured using CGM, was significantly lower during the MW (mulberry and water chestnut) tea period than during the placebo period (p= 0.0006).
“Additionally, the physical activity and sleep data did not differ significantly between the MW tea and placebo groups. These results support the efficacy of MW tea in glycemic management,” said the researchers.
Spike suppressed at the 1st hour
Findings also showed that as compared to the placebo, the effect of mulberry leaf and water chestnut tea in suppressing blood glucose levels was significant in the first hour post meal, but not for the subsequent hours.
This was based on measuring the three-hour AUC of glucose level changes, where the mean AUC at one hour post meal was 1145 for the group taking mulberry and water chestnut tea and 1289 for the placebo group.
“A significant difference was observed in the AUC at 1 h, which was lower in the MW tea group compared to the placebo group (p= 0.02). No differences were observed at 2 h and 3 h,“ said the researchers.
This suggested that peak glucose level was delayed following mulberry and water chestnut tea consumption as compared with the placebo.
“These findings suggest that MW tea may primarily suppress early postprandial glucose spikes and that pre-meal consumption is particularly effective,” said the researchers.
Increase in 1,5-AG
Another notable finding was the significant increase in 1,5-anhydroglucitol (1,5-AG) levels when the participants were taking mulberry leaf and water chestnut tea as compared to when they were taking placebo.
This is a sugar alcohol that competes with glucose for renal reabsorption and is sometimes used as a short-term marker of glycemic control.
It exists in lower levels in the blood when post-meal blood sugar increase persists.
In other words, higher levels of 1,5-AG is preferred, as this suggests a lower blood sugar spike.
During the period taking mulberry and water chestnut tea, the participants’ mean mean 1,5-AG levels were 1.234 μg/mL.
This was significantly higher than the period when they were taking placebo (1.100 μg/mL).
“When postprandial hyperglycemia persists, glucose saturates renal tubular reabsorption, which, in turn, inhibits 1,5-AG reabsorption and reduces its blood concentration.
“Therefore, the observed increase in 1,5-AG levels during the MW tea period suggests that MW tea suppresses postprandial hyperglycemia and consequently reduces urinary glucose excretion.”
In addition, total cholesterol levels had decreased during the period taking mulberry and water chestnut tea, but increased during the placebo period. This difference between the two periods was statistically significant.
There was however, no significant changes between the two groups in terms of fasting glucose levels and CRP levels - a marker of inflammation.
This suggests that anti-inflammatory effects of mulberry and water chestnut tea were not evident and the researchers said that further investigation would be needed to explore this pathway.
Source: Nutrients, 2025, 17(14), 2308; doi: 10.3390/nu17142308, “Evaluation of the Postprandial-Hyperglycemia-Suppressing Effects and Safety of Short-Term Intake of Mulberry Leaf and Water Chestnut Tea: A Randomized Double-Blind Placebo-Controlled Crossover Trial”, Authors: Y. Shinkawa, et al.
