Japanese Study: Mulberry Tea Cuts Blood Sugar Spikes After Meals

Japanese Researchers Find Mulberry Tea May Help Tame Post-Meal Blood Sugar Spikes

A study from Japan published in Nutrients (2025) suggests that a traditional tea blend made from mulberry leaves and water chestnut may help reduce dangerous blood sugar spikes after eating — a finding with real relevance for American patients managing diabetes or prediabetes.

Researchers at Saga University's Graduate School of Medical Science tested a product called Kanzaki MW tea, a blend of Morus alba (white mulberry) leaves and Trapa japonica (Japanese water chestnut) leaves and husks. The tea is rich in plant compounds called polyphenols and a naturally occurring sugar-blocking compound called 1-deoxynojirimycin (DNJ), which works similarly to certain diabetes medications by slowing the breakdown of carbohydrates in the gut.

What Makes This Study Unique

Unlike most US supplement studies that rely on single-dose testing or self-reported blood sugar readings, this Japanese trial used continuous glucose monitoring (CGM) — the same wearable sensor technology increasingly popular among American diabetics — to capture blood sugar changes every 15 minutes over a full two weeks. This approach provides a far more complete picture of daily glucose patterns than standard finger-stick tests.

Japan also has a long cultural and regulatory history with functional food research, meaning this study reflects a level of rigor applied to traditional remedies that is still relatively uncommon in Western supplement research.

What the Study Found

The trial enrolled 31 healthy adults in a randomized, double-blind, placebo-controlled crossover design — considered a gold standard study format. Participants drank either MW tea or a placebo before meals for two weeks, then switched groups after a one-week break.

• Blood sugar variability dropped significantly in the MW tea group, as measured by the coefficient of variation (CV) — a key marker of how wildly blood sugar swings throughout the day.
• Post-meal blood sugar levels were lower during the first hour after a standardized meal, as measured by the area under the glucose curve.
• 1,5-anhydroglucitol (1,5-AG), a blood marker that rises when blood sugar control improves (the opposite of HbA1c behavior), increased significantly — providing an independent signal of better glycemic control.
• No safety concerns emerged: there were no increases in low blood sugar (hypoglycemia) episodes, and liver, kidney, and inflammation markers remained normal.

How This Translates for US Patients

Post-meal blood sugar spikes — often defined in US guidelines as glucose exceeding 180 mg/dL (10 mmol/L) two hours after eating — are increasingly recognized as a cardiovascular risk even in people not yet diagnosed with diabetes. The American Diabetes Association (ADA) does acknowledge post-meal glucose management as important, though current US guidelines focus primarily on HbA1c and fasting glucose targets. This Japanese research adds to a growing international body of evidence supporting post-meal glucose control as a standalone goal.

Why This Matters for US Patients

Millions of Americans with prediabetes or type 2 diabetes struggle specifically with after-meal glucose spikes. A natural, well-tolerated tea that demonstrably reduces this variability — backed by CGM data and a rigorous crossover trial — represents a meaningful option worth discussing with your healthcare provider. While this was a small, short-term study in healthy Japanese adults and results may differ in people with established diabetes, the safety profile was reassuring.

If you're already tracking your post-meal numbers and looking for evidence-based nutritional tools to support your routine, MDS Diabetes at mdsdiabetes.com offers resources and products designed to complement lifestyle strategies for blood sugar management. Explore our supplement guidance section to see how ingredients like mulberry leaf fit into a broader diabetes care plan.

Full Citation

Shinkawa, Yasuda, Nishida (2025). 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. Nutrients. PMID: 40732933. DOI: 10.3390/nu17142308. Country of Origin: Japan — Department of Medical Science, Graduate School of Medical Science, Saga University, Saga, Japan.