Tea and Diabetes

Diabetes is caused by elevated blood glucose levels. According to the World Health Organization (WHO), global diabetes incidence is increasing and predicted to be the 7th leading cause of death by 2030. Diabetes exists in two forms, namely type 1 and type 2, with 90% of diabetics being afflicted with the latter. The hormone, insulin, controls blood glucose levels; type 1 is caused by lack of insulin production and type 2 is caused by a decline in the effectiveness of insulin.

Research has established that tea consumption could reduce the absorption of glucose from the digestive tract into the body whilst increasing the effectiveness of insulin. These mechanisms would assist in reducing the risk of type 2 diabetes. A review of the population studies assessing the relationship between tea consumption and diabetes risk, has revealed that tea consumption is associated with a reduction in the risk of type 2 diabetes, which is the most prevalent form of the disease.

Diabetes (Diabetes mellitus) is caused by elevated blood glucose levels. After a meal, as digested food is absorbed in to the blood, blood glucose levels increase, inducing the secretion of insulin. insulin is the hormone stimulating the liver and muscle cells to absorb glucose, thus reducing blood glucose levels. This mechanism ensures that blood glucose is maintained within certain limits in healthy individuals; a deficiency in this regulatory mechanism leads to Diabetes.

Diabetes has become a major global health issue. According to the WHO, the number of people afflicted has risen from 108 million in 1980, to 422 million in 2014. The global prevalence of diabetes among adults over 18 years of age has risen from 4.7% in 1980 to 8.5% in 2014. Diabetes is also a major contributory factor to blindness, kidney failure, heart disease, stroke and may even lead to lower limb amputation. In 2012, an estimated 1.5 million deaths were directly caused by diabetes and another 2.2 million deaths are attributed to high blood glucose. Almost half of all deaths attributable to high blood glucose occur before the age of 70.

Type I Diabetes

Type 1 diabetes (previously known as insulindependent/juvenile or childhood-onset diabetes) is caused by deficient insulin production. Its cause in most cases is not known and it is not preventable with current knowledge. In certain cases it is due to afflictions such as auto-immune diseases that destroy the insulin producing cells in the pancreas. However, by the time the symptoms appear, about 90% of the insulin producing cells are destroyed and it is too late for corrective measures. Symptoms include excessive excretion of urine (polyuria), thirst (polydipsia), constant hunger, weight loss, vision changes and fatigue. These symptoms may occur suddenly. Type I diabetes patients are only about 10% of the global diabetic community and require daily administration of insulin.

Type 2 Diabetes

Type 2 diabetes (formerly called non-insulindependent or adult-onset diabetes) results from the body’s inability to use insulin effectively. In most cases of type 2 diabetes, the normal level of insulin is produced. However, the glucose absorbing cells do not respond to insulin adequately to take up the required amount of glucose from blood, resulting in the elevation of blood glucose. In some cases insulin production is increased to compensate for the lack of response by the cells and the blood sugar levels would decline to normal levels. In some cases, either increased production is not possible, or the increased insulin production is unable to maintain constant blood glucose levels as the cells do not respond adequately to increased insulin levels.

This extremely heterogeneous disease, comprises approximately 90% of the total diabetes patients globally and in most cases is largely the result of excess body weight and physical inactivity. No single cause is adequate to explain the progression

from normal glucose tolerance to diabetes. Symptoms may be similar to those of type 1 diabetes, but are often less marked. As a result, the disease may be diagnosed several years after onset, once complications have already arisen. Until recently, Type 2 diabetes was seen only in adults. However, occurrence is increasing in children. This could be due to inappropriate diet and less physical activity resulting in obesity.1

Common Consequences of Diabetes

Over time, diabetes can damage organs in the body such as heart, blood vessels, eyes, kidneys, and nerves. Thus;

• Adults with diabetes have a 2-3-fold increased risk of heart attacks and strokes. • Combined with reduced blood flow, neuropathy (nerve damage) in the feet increases the chance of foot ulcers, infection and eventual need for limb amputation. • Diabetic retinopathy is an important cause of blindness, and occurs as a result of long-term accumulated damage to the small blood vessels in the retina of the eye. 2.6% of global blindness can be attributed to diabetes. • Diabetes is among the leading causes of kidney failure.

Effect of Tea Consumption on Diabetes

A large number of research studies have been carried out, to ascertain whether tea consumption could reduce blood glucose levels and the mechanism through which this is effected. Population studies have been carried out using large number of individuals with diabetes and healthy individuals with similar life styles as controls, to assess the impact of tea consumption on diabetes. Additionally, the effect of tea consumption on obesity and weight loss has also been studied as obesity is one of the major contributory factors in diabetes. Inferences from these research studies are discussed below.

Effect of Tea on Blood Glucose Levels

After a meal blood glucose level is increased, as glucose in digested food is absorbed into blood. Although increase of postprandial (occurring after a meal) glucose within certain limits is normal, consistent, dramatic increases of postprandial glucose, due to inappropriate meals and quantities, will increase the risk of diabetes. Therefore, a number of studies have been carried out to find the effect of tea on postprandial blood glucose levels. A review of results of these studies reveal that tea flavonoids have the ability to reduce postprandial blood glucose levels.2

Mechanistic Studies on Tea and Diabetes

Many studies have been carried out to find the mechanisms involved in the reduction of blood glucose levels and diabetic risk by tea consumption. These studies have revealed that tea extends beneficial effects through several different mechanisms.

Effect of Tea on Absorption of Glucose from the Digestive Tract

Molecules in our diet, required for the production of energy and the formation of body structures, are absorbed from the digestive tract and circulated through blood to the body part where it is required. However, most of the large molecules in our diet cannot be absorbed directly into the blood stream from the digestive tract. The process of digestion breaks them into smaller molecules, which are then absorbed into the blood stream and delivered to the required places. Most of the large carbohydrates in the diet such as starch cannot be directly absorbed. Therefore, large carbohydrate molecules are broken into glucose in the digestion, to facilitate absorption.

Enzymes such as Alpha-amylase and BetaD-glucosidase are involved in the digestion of carbohydrates helping to break large carbohydrates into glucose to facilitate absorption. Many studies, carried out to find the effect of tea on the digestive enzymes, have revealed that tea flavonoids possess the ability to inhibit Alphaamylase and Beta-D-glucosidase, thus contributing to reduce glucose absorption into blood. This mechanism would be important in controlling blood sugar levels after meals in individuals, who consume carbohydrate rich meals. Anti-diabetic drug Acarbose also functions through the same mechanism.3,4

Tea on the Effectiveness of Insulin

As described above, in type 2 diabetes, inadequate response to insulin by glucose absorbing cells results in high blood glucose levels. Several research studies have shown that tea flavonoids have the ability to improve the insulin sensitivity of the cells and, thereby, increase the glucose absorption into the cells. Studies carried out to find the mechanisms of action of tea, using animal models, have demonstrated that both black tea and green tea could increase the glucose uptake by muscle cells and adipocytes (fat storing cells).

Tea increases the binding of insulin to the cells and also enhances the activity of glucose transporters in the cells.5, 6, 7 Seventeen research studies on humans, to find the effect of tea on insulin sensitivity, reviewed recently, have indicated that tea consumption had resulted in reduced fasting blood glucose and glycated hemoglobin levels (Hb A1C, glycated hemoglobin indicates the average blood glucose levels in the past three months). Simultaneously, the fasting insulin levels were also reduced indicating improved insulin activity in the presence of tea.8

Inflammation and Diabetes

Inflammation is the body’s natural response to injury, pathogens and toxins and the first step by the immune system to protect the body from any invader or injury (See chapter 5 for details on inflammation). When the threat is removed, the body reverts back to the normal state. However, under some circumstances the immune system could be continuously stimulated, resulting in chronic inflammation. Chronic inflammation could be due to many reasons and obesity is an example.

Fatty tissues in the body could send signals to activate the immune system and therefore obesity could lead to chronic inflammation, a risk factor for many non-communicable diseases including diabetes.

Chronic inflammation could, also, result in the reduction of insulin sensitivity and be the main cause of diabetes complications. Therefore, attenuation of inflammation by tea consumption also contributes to reduce the complications of diabetes.

Obesity

Both overweight and obese individuals have a very high risk of developing type 2 diabetes. It has been described as the best single predictor of type 2 diabetes as it is the most important risk factor for diabetes. Almost 90% of people living with type 2 diabetes are either overweight or obese. Individuals who are overweight, or obese, have added pressure on their body’s ability to use insulin to effectively control blood sugar levels, and are therefore more likely to develop diabetes.

In overweight or obese people the cells are stressed as there are more nutrients than the process can handle. This sends a signal to decrease the intake of nutrients into the cell, which works against insulin, whose role is to increase the glucose uptake by the cells. Therefore, overweight and obese people have a higher risk of developing type 2 diabetes. Further, as fatty tissues constantly send signals to increase the inflammation in the body, overweight and obese people have a higher risk of developing diabetic complications. According to WHO, diabetes could be controlled in a large proportion of diabetes patients merely by controlling the body weight.1

Many research studies have shown that regular tea consumption contributes to weight loss and therefore could contribute to prevention of obesity9. (See chapter 13 for details on weight loss by tea) Therefore, regular tea consumption could indirectly contribute to reducing the risk of type 2 diabetes by preventing obesity.

Population Studies

In 2009, 18 research studies carried out up to 2009, on beverage consumption and diabetes incidence, were systematically reviewed. There were 457,922 participants in these studies from diverse populations, from different parts of the world, such as Singapore, Puerto Rico, United Kingdom, Finland, United States, Japan, Netherlands and Sweden. The conclusion was that individuals who drank more than 3 to 4 cups of tea per day had an approximate one-fifth lower risk of diabetes than those who did not consume tea.10

Another review had been carried out using thirteen research studies which were performed in Japan, United Kingdom, Finland, Sweden, Germany and Brazil. The objective was to ascertain whether polyphenol rich sources, have an effect on the increase of blood glucose after meals and the insulin levels. Findings reveal that although blood glucose and insulin responses differed depending on the polyphenol-carbohydrate combination,

overall, polyphenol sources were shown to reduce the peak and early-phase blood glucose and maintain the blood glucose in the later stages of digestion. Polyphenols were also able to sustain the insulin response. Conclusion of the review was that polyphenols in food and beverages have a beneficial effect on reducing the risk of type 2 diabetes and black tea was one of the sources of polyphenols.2

Yang and co-workers had carried out a systematic review on the research studies on tea consumption and type 2 diabetes. A total of 545,517 had participated in these studies. Conclusion of the study was that consumption of tea reduced the risk of type 2 diabetes. Individuals who consumed two cups of tea per day had a lower risk than those who do not consume tea. Further, those who consumed more than four cups of tea had 15% lower risk.11

A similar systematic review had also established that tea consumption reduces the risk of type 2 diabetes, observing that the consumption of three or more cups of tea was associated with a lower risk. The review considered twelve research studies, six having been conducted in the Unites States and three each in Europe and Asia.12

Conclusion

Regular consumption of tea could have an effect on type 2 diabetes (90% of the diabetes is of this type) through several mechanisms. Research reveals that tea consumption could reduce the glucose absorption into the body from the digestive tract, especially from carbohydrate rich food. Tea flavonoids also have the ability to increase the sensitivity of insulin, the hormone which controls blood glucose levels. Further, tea consumption could have indirect effects on diabetes through reduction of inflammation and obesity. These two are the main risk factors for diabetes and also contribute to increases in diabetic complications.

Population studies on tea consumption and risk of diabetes also reveal that tea consumption could reduce the risk of type 2 diabetes.

• Diabetes is caused by elevated blood glucose levels which is mainly caused by either lack of the hormone called insulin, which control the blood sugar levels, or due to reduced effectiveness of insulin, which controls the blood sugar. Diabetes caused due to lack of insulin is called type 1 diabetes and diabetes due to decreased effectiveness of insulin is called type 2 diabetes. Approximately 90% of the diabetics suffer from type 2 diabetes. • Tea consumption could reduce the risk of diabetes through several mechanisms. Major mechanisms are the reduction of absorption

of glucose from the digestive tract and increasing the effectiveness of insulin. • Regular tea consumption could reduce inflammation and obesity, two main risk factors for diabetes, thus indirectly reducing the risk of diabetes. • Population studies, investigating the association between tea consumption and diabetes risk, indicate that consumption of more than 3 cups of tea per day could reduce the risk of diabetes incidence.

REFERENCES

1. Diabetes, Fact Sheet, World Health Organization, available at; www.who.int/mediacentre/factsheets/fs312/en/ accessed on 26th

October 2016 2. S. Coe and L. Ryan, 2016, Impact of polyphenol rich sources on acute postprandial glycaemia; A systematic review, Journal of

Nutritional Science, 5, 1-11 3. Peluso I and Serafini M, 2016, Antioxidants from black and green tea: from dietary modulation of oxidative stress to pharmacological mechanisms, British Journal of Pharmacology, doi: 10.1111/bph.13649. 4. Williamson G, 2013, Possible effects of dietary polyphenols on sugar absorption and digestion, Molecular Nutrition and Food

Research, 57(1), 48-57. 5. Anderson RA, Polansky MM., 2002, Tea enhances insulin activity. Journal of Agricultural and Food Chemistry, 50, 7182–7186. 6. Wu LY, Juan CC, Ho LT, Hsu YP, Hwang LS., 2004, Effect of green tea supplementation on insulin sensitivity in Sprague-Dawley rats.

Journal of Agricultural and Food Chemistry, 52, 643–648. 7. Wu LY, Juan CC, Hwang LS, Hsu YP, Ho PH, Ho LT., 2004, Green tea supplementation ameliorates insulin resistance and increases glucose transporter IV content in a fructose-fed rat model, European Journal of Nutrition, 43, 116–24. 8. Liu K, Zhou R, Wang B, Chen K, Shi LY, Zhu JD, Mi MT, 2013, Effect of green tea on glucose control and insulin sensitivity: a metaanalysis of 17 randomized controlled trials, American Journal of Clinical Nutrition, 98(2), 340-348. 9. Kimberly A. Grove and Joshua D. Lambert, 2010, Laboratory, Epidemiological, and human intervention studies show that tea (Camellia sinensis) may be useful in the prevention of obesity, Journal of Nutrition, 140,446–453. 10. Rachel Huxley, Crystal Man Ying Lee, Federica Barzi, Leif Timmermeister, Sebastien Czernichow, Vlado Perkovic, Diederick E.

Grobbee, David Batty and Mark Woodward, 2009, Coffee, Decaffeinated Coffee, and Tea Consumption in relation to Incident of Type 2 Diabetes Mellitus, Archives of Internal Medicine, 169(22), 2053-2063 11. Wan-Shui Yang, Wei-Ye Wang, Wen-Yan Fan, Qin Deng and Xin Wang, 2014, Tea consumption and risk of type 2 diabetes: a doseresponse meta-analysis of cohort studies, British Journal of Nutrition, 111, 1329–1339 12. Jian Yang, Qun-Xia Mao, Hong-Xia Xu, Xu Ma and Chun-Yu Zeng, 2014, Tea consumption and risk of type 2 diabetes mellitus: a systematic review and meta-analysis update, British Medical Journal, Available at, dx.doi.org/10.1136/bmjopen-2014-005632, accessed on 25th October 2016.