Tea and Diabetes: Insights from Recent Scientific Studies

Diabetes is on the increase worldwide, and in France too. While many scientific advances have been made to better understand and manage the disease, some people are interested in tea in addition to medical advice. Is tea anti-diabetic? Does it reduce symptoms or prevent the onset of the disease?

The importance of controlling blood sugar levels

Glucose is one of our body's energy sources. Our cells convert sugar into ATP, an energy molecule that is essential to our body's functioning. (1) Glycemia is the level of sugar in the blood. On an empty stomach, it is estimated at 1mg/L of blood, and varies throughout the day. Hyperglycemia (>1.26 mg/L fasting) and hypoglycemia (<0.6mg/L fasting) have serious long-term consequences. Hypoglycemia prevents the body from functioning properly, as it runs out of energy. Hyperglycemia can lead to cardiovascular disease (stroke, myocardial infarction), nephropathy, retinopathy, etc. (2)

Blood sugar levels are controlled by two organs: the liver and the pancreas. In the event of hyperglycemia, the beta cells of the pancreas detect excess sugars and release a hormone called insulin into the bloodstream. Insulin binds to the surface of our cells and activates glucose transporters (GLUT2). Blood sugar enters the cells, and blood glucose levels fall. Glucose will also be stored in muscles, fat cells and the liver to meet future blood sugar drops between meals and during the night. Alpha cells in the pancreas and liver detect this drop and secrete a hormone called glucagon. This hormone binds to the surface of the cells of the sugar storage organs, releasing sugar into the bloodstream (3)

What is diabetes? 

Diabetes affects 463 million people worldwide and 4.5 million in France: 4 times more than 40 years ago (4). There are 3 forms of diabetes:

• Type 1 diabetes accounts for 10% of all diabetics. It is a deficiency in insulin secretion, resulting in continuous hyperglycemia. It is an autoimmune disease, meaning that the body's own immune cells (mainly cytotoxic T lymphocytes) no longer recognize the beta cells of the pancreas as belonging to the body, and destroy them. It is a disease that can be genetic and can appear at any age (preferably young, before puberty and in men) (5)(6)

• Type 2 diabetes accounts for the vast majority of diabetics. It is a disease caused by the cells' insensitivity to insulin. The cells no longer store sugar correctly: blood sugar levels remain high. In an attempt to compensate for this insensitivity, the beta cells of the pancreas produce more and more insulin, until they can no longer keep up. The onset of diabetes 2 is favored by obesity, a diet rich in sugar and a sedentary lifestyle. Other causes include genetics, old age, pregnancy (which disappears after birth) and the action of certain drugs (7)

• Type 3 diabetes affects the brain. It is still poorly defined, but is thought to be associated with a reduction in the number of insulin receptors in the brain, which prevents neurons from absorbing sugar and therefore from functioning. Other causes are associated, such as insufficient insulin secretion, or a disturbance in sugar metabolism in the brain (8)

The first evidence of tea's effects on diabetes: from cell to human?

Type 1 diabetes

Tea is thought to have two beneficial effects on type 1 diabetes: its hypoglycemic effect and its ability to prevent beta-cell destruction. Studies have focused on EGCG, the tea polyphenol with the highest antioxidant potential.

While the hypoglycemic effects of EGCG have been demonstrated in cell cultures, a few studies in non-obese diabetic mice have shown that daily consumption of pure EGCG lowers blood sugar levels. In Fu's 2011 study (9), diabetic mice fed pure EGCG for 31 weeks had blood sugar levels that did not rise for 16 weeks, compared with diabetic mice fed water. The blood glucose levels of untreated diabetic mice quadrupled after 31 weeks, i.e. by 50% compared with EGCG mice.

The hypoglycemic effect of EGCG was shown to reduce the incidence of diabetes 1 (25% vs. over 60%) and mortality (90% of mice alive vs. 40%). What's more, glycated hemoglobin levels (the protein in red blood cells that has bound sugar) fell by 2%, and insulin levels increased fivefold 2 hours after glucose consumption and after EGCG consumption.

But does EGCG prevent the destruction of pancreatic beta cells?

The results are more mixed. In Fu's study, EGCG had no protective effect on pancreatic beta cells in diabetic mice. In 2011, Zhang and colleagues showed that EGCG protects mouse beta-cell cultures from death induced by the presence of inflammatory molecules, by reducing the appearance of biological markers that induce cell death (10)

EGCG appears to attenuate hyperglycemia and slow disease onset in cellular and animal models. But in the absence of human data, and in view of the contradictions found in the various models, it is not possible to conclude that tea has a clear protective effect on diabetes 1.

Diabetes 2

Tea has been studied for its hypoglycemic properties. Similar to diabetes 1, cellular and animal models prove this hypoglycemic effect, but under highly experimental conditions. In contrast to diabetes 1, there are numerous studies on daily tea consumption in humans for the prevention of the disease. Three scientific reviews have compiled studies in humans: in 2014 by Yang et al, Yu et al in 2017 and Meng et al in 2019. 

These researchers came to the same conclusion: contradictory results, whatever the color of tea. Some studies show protective effects against the onset of disease and for the attenuation of hyperglycemia (Yang et al. even showed that these effects would appear from 3 cups / day). Other studies show that tea does not slow down the onset of the disease. 

What's the problem?

Compared to the studies listed and the conclusions of the three publications, there are many reasons that could explain the differences in results:

• The form of the tea. Most of the time, tea extracts and pure molecules are prescribed. Half of the publications described by Yu et al. are based on the consumption of tea extracts or EGCG. If each dose and formulation differs, results may be inconsistent.

• Rigorous methodology. Yang et al. compiled 500 publications, but only 12 were selected. These publications specified: infusion of tea leaves, doses (cup/day), type of tea chosen, details of tea preparation. In the publication by Meng et al. the vast majority of publications did not specify the quantities of tea drunk (cup/day).

• Consideration of parameters other than tea. Differences were observed in relation to gender (more beneficial effects in men) and ethnicity (mainly between Asians, Americans and Europeans).

As with diabetes 1, polyphenols have been shown to reduce glycemia and slow the onset of the disease in cell and animal models, as well as in certain individuals. However, the compilation of studies on tea and diabetes 2 in humans does not allow us to conclude that tea has a clear protective effect against the onset of the disease.

Could tea's anti-diabetic effects be a pipe dream? Sciences Thé won't lie to you: there are too many contradictions and too many experimental conditions to be able to speak of anti-diabetic effects. Even for the prevention of diabetes 2. The studies conducted are still in their infancy (most are less than 15 years old), and we'll need a little more time to harmonize methods.