Let food be thy medicine and medicine thy food.

Who hasn’t heard this famed quote from Hippocrates exalting the medicinal benefits of a nutritious diet? Yes, pharmaceuticals have extended humanity’s medical arsenal far beyond what past societies could ever have imagined, but a wholesome diet remains crucial to healthy living. Many of today’s drugs were inspired by chemicals that are naturally found in herbs, fruits, and other plants. The vast plant kingdom contains thousands of different compounds, many of which interact with the body in similar ways that contemporary drugs do.

Take turmeric, for example--an east Asian spice that studies suggest reduces blood sugar levels,

Though the scientific community has long recognized turmeric’s positive influence on blood sugar levels, none were certain how the spice achieved its effects. In 2007, University of Manchester researchers published an article proposing an answer to the mystery. They explored how curcumin, turmeric’s primary bioactive ingredient, influences electric activity in pancreatic beta cells, the cells that produce insulin. Their experiments proved that turmeric directly stimulated the pancreas to increase the production of insulin, a hormone that helps regulate blood sugar levels¹.

In this way, turmeric mimics the biological effects of glimepiride, one of the most popular antidiabetic drugs in the world. Glimepiride too reduces blood sugar levels by increasing pancreatic insulin production.

While opinions differ on whether a diet centered around blood sugar-modulating foods can entirely manage hyperglycemia and replace conventional medicine, none dispute that certain plants can help reduce high blood sugar and complement prescription regimes.

Green tea is one such plant. Inexpensive and accessible to most, green tea is rich in polyphenols that reduce blood sugar. Polyphenols are a diverse category of chemical compounds found in plants. They're responsible for the majority of plants’ antioxidative effects, for the curative properties plants possess that guard against digestive, cardiovascular, oncological, and neurodegenerative conditions.

Green tea contains a polyphenol called epigallocatechin gallate (EGCG), which supports high insulin levels by reducing the production of an enzyme called glutamate dehydrogenase (GDH). The body produces this enzyme as a response to low blood sugar because it minimizes insulin levels in the body, allowing blood sugar levels to rise. This is obviously not a desirable effect for those already suffering from high blood sugar. The EGCG in green tea counteracts this effect by preventing GDH from fulfilling its role in the insulin reduction process².

Green tea is not the sole member of the tea family that can lower blood sugar levels. Pu-erh, an aged tea with deep roots in Chinese culture, has been shown to have even more significant impacts on blood sugar reduction than green tea. Complex carbohydrates in pu-erh tea upregulate the expression of GLUT-1 and GLUT-4, specialized proteins that bind to sugar molecules and transport them out of the bloodstream, thus lowering blood sugar³. However, this is not the only way pu-erh tea achieves its hypoglycemic effects. It also helps keep glucose levels low by preventing complex sugars from breaking down into glucose in the first place.

How? When we consume carbohydrate-filled foods like potatoes and corn, these carbohydrates eventually break down into smaller glucose molecules, raising blood sugar levels and causing weight gain. This contributes to why many health-conscious individuals avoid carbohydrates in addition to processed sugars and foods high in healthy fats. Two enzymes are responsible for breaking complex sugars down into glucose: a-amylase and a-glucosidase⁴. The former breaks down complex sugars in the mouth and pancreas, while the latter continues breaking them down into glucose within the small intestine. Pu-erh tea inhibits this process.

In 2015, researchers isolated tea polysaccharides from pu-erh tea and studied how they affected the behavior of a-amylase and a-glucosidase. When only the enzymes and carbohydrates were present, the enzymes successfully reduced the carbohydrates into simple glucose molecules⁵. However, when researchers added the tea polysaccharides to the mixture, the enzymes could no longer break the carbohydrates down into glucose efficiently. (Pu-erh’s effects on a-glucosidase activity was significant, almost completely halted, but a-amylase’s function was only mildly affected. However, this was still sufficient for minimizing glucose production.)

To ensure that these observations produced outside the body held true within a biological system, the researchers tested to see whether pu-erh tea could control blood glucose levels in living mice. They found that not only was it effective in lowering blood sugar in mice, but that it was more effective than acarbose--a popular conventional drug for hyperglycemia.

Also hailing from Asia, the banaba plant is well-known for its hypoglycemic effects on human blood sugar levels. While the tannins found in banaba plants have been shown to act like insulin within the body, research has better documented the plant’s high composition of corosolic acid for its role in lowering blood sugar⁶.  

Unlike the herbal solutions described above, corosolic acid’s ability to reduce hyperglycemia has been known for over 70 years. In 1940, researcher F. Garcia found that feeding rabbits banaba leaves reduced their blood sugar 4-6 hours after consumption. The hypoglycemic effect was somewhat proportional to the rabbit's glucose levels; rabbits with the most extreme cases of hyperglycemia experienced the greatest effects from banaba leaves.

Humans who consume banaba extract experience its sugar-modulating effects much faster than rabbits do. Trials show that banaba leaves have discernible hypoglycemic effects in human subjects within one hour of extract consumption⁸.

Japan's Kyoto University enrolled 15 women and 16 men in a study exploring how corosolic acid affected fasting plasma glucose (FPG) levels⁹. A person’s FPG level is standardly used to determine whether someone is hyperglycemic and/or suffering from diabetes. Only subjects with FPG levels ranging between 110mg/dL - 140mg/dL participated in this study. After abstaining from food overnight, participants were given either a placebo or a supplement containing 10mg corosolic acid. Researchers recorded each participant’s blood sugar level at half-hour intervals after consuming their assigned substance.

Within one hour, researchers observed that subjects who had consumed corosolic acid had lower blood sugar levels than those who had taken the placebo product. There was also a measurable increase of insulin in their blood, suggesting that corosolic acid may reduce blood sugar by employing similar pathways as turmeric and glimepiride. 

References

1. Best, L., Elliott, A. C., & Brown, P. D. (2007). Curcumin induces electrical activity in rat pancreatic β-cells by activating the volume-regulated anion channel. Biochemical Pharmacology, 73(11), 1768–1775.doi:10.1016/j.bcp.2007.02.006
2. Li, C., Allen, A., Kwagh, J., Doliba, N. M., Qin, W., Najafi, H., … Smith, T. J. (2006). Green Tea Polyphenols Modulate Insulin Secretion by Inhibiting Glutamate Dehydrogenase. Journal of Biological Chemistry, 281(15), 10214–10221.doi:10.1074/jbc.m512792200
3. Kramer D, Shapiro R, Adler A, Bush E, Rondinone CM. Insulin-sensitizing effect of rosiglitazone (BRL-49653) by regulation of glucose transporters in muscle and fat of Zucker rats. Metabolism: clinical and experimental. 2001;50:1294–300.
4. Wu, X., Ding, H., Hu, X., Pan, J., Liao, Y., Gong, D., & Zhang, G. (2018). Exploring inhibitory mechanism of gallocatechin gallate on a -amylase and a -glucosidase relevant to postprandial hyperglycemia. Journal of Functional Foods, 48, 200–209.doi:10.1016/j.jff.2018.07.022
5. Deng, Y.-T., Lin-Shiau, S.-Y., Shyur, L.-F., & Lin, J.-K. (2015). Pu-erh tea polysaccharides decrease blood sugar by inhibition of α-glucosidase activity in vitro and in mice. Food & Function, 6(5), 1539–1546. doi:10.1039/c4fo01025f
6. Klein G, Kim J, Himmeldirk K, Cao Y, Chen X. Antidiabetes and anti-obesity activity of Lagerstroemia speciosa . Evidence-based Complementary and Alternative Medicine. 2007;4(4):401–407.
7. Garcia F. On the hypoglycemic effect of a decoction of Lagerstroemia speciosa leaves (banaba) administered orally. Philippine Medical Association. 1940;20:193–201.
8. Stohs, S. J., Miller, H., & Kaats, G. R. (2011). A Review of the Efficacy and Safety of Banaba (Lagerstroemia speciosa L.) and Corosolic Acid. Phytotherapy Research, n/a–n/a.doi:10.1002/ptr.3664
9. Fukushima M, Matsuyama F, Ueda N, et al. 2006. Effects of corosolic acid on post-challenge plasma glucose levels. Diab Res Clin Pract 73: 174–177.