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Grape Extracts for Type 2 Diabetes Treatment through Specific Inhibition of Alpha-Glucosidase and Antioxidant Protection
Enzymatic digestion of carbohydrates
Achievement/Results
NSF MILES-IGERT Trainee Shelly Hogan under the mentorship of Professor Kevin Zhou in the department of Food Science and Technology at Virginia Tech identified novel bioactive compounds from extracts derived from a native red wine grape cultivated in the state of Virginia called Norton (Vitis aestivalis). This grape research project was important because novel and naturally derived bioactive agents with dual antioxidant and antidiabetes properties may be used to treat chronic diseases associated with oxidative stress such as type 2 diabetes and obesity, both of which are conditions that have reached epidemic proportions.
The Zhou Functional Foods and Nutraceutical Laboratory has extracted candidate compounds for both in vitro and in vivo experiments designed to clarify the potential of grape extracts in regards to managing oxidative stress and blood glucose levels. Much of the in vitro worked involved collaboration with faculty, students, and staff across multiple disciplines ranging from the food science and technology, chemistry, biochemistry, human health and nutrition, agriculture, and small animal science. Over the past year, the Zhou Lab has identified novel fruit derived compounds from grapes with dual bioactive properties that may be useful in preventing or treating type 2 diabetes by reducing glucose and free radical stress. For example, antioxidant rich grape skin extracts were tested for their ability to inhibit a key glucose releasing enzyme called alpha-glucosidase. This enzyme can breakdown dietary carbohydrates into glucose in the small intestine. This is significant because of the fact that if this metabolic enzyme is inhibited then there is a decrease or delay of glucose absorption into the blood stream and thus lower blood sugar levels (Figure 1).
Thus, the main objective of this grape research project was to investigate this popular and naturally rich antioxidant fruit. In particular, grape extracts which are comprised of a variety of plant or phyto-derived compounds with antioxidant properties were acquired by common extraction techniques and analyzed in our laboratory. This research is unique because very limited research has been conducted on grape extracts and natural therapies for type 2 diabetes prevention or treatment.
To begin this project, red Norton wine grape (Vitis aestivalis) and pomace extracts were evaluated and measured to have notable phenolic content and antioxidant properties. Next, Zhou’s laboratory identified that this particular grape which is native to the state of Virginia can potently inhibit alpha-glucosidase. In particular, after screening over 300 different extract samples from food, herbs, and plant protein sources, this particular grape extract (GE) was found to be the only sample showing potent inhibition on alpha-glucosidase activity (Figure 2). Additional enzyme inhibitory and enzyme kinetic assays were conducted using different sources of Norton grape extracts. For example, both grape skin extracts (GSE) and grape pomace extracts (GPE) were tested and both found to have in vitro yeast and mammalian alpha-glucosidase inhibitory activity. In particular, the GSE was 32-times more potent at inhibiting yeast alpha-glucosidase than acarbose, a commercially available oral anti-diabetes drug. Additionally, from the use of highly sophisticated chromatography equipment it was determined that three specific phyto-compounds in the GSE (i.e. resveratrol, ellagic acid, and catechin) had strong inhibitory activity on yeast alpha-glucosidase. Their research is now focused on identifying the specific combination or cocktail of grape derived compounds with strong inhibitory qualities. Once identified and tested to be active inhibitors then a more pure extract can be acquired which may have even more inhibitory activity on this specific enzyme, similar to that of commercially available pharmaceutical drugs.
Based on the in vitro results which indicated an inhibitory activity of GE on alpha-glucosidase the Zhou Lab wanted to see if GE could lower blood glucose levels in vivo or in a live animal model. Thus, to determine if GE can reduce or delay the absorption of glucose in a biological system they used mice with diabetes. These mice were injected with a toxic chemical which specifically damages the insulin producing alpha-pancreatic cells and thus results in mice with diabetes. It was determined that after these mice had been administered GE (at a dosage of 400 mg/kg bodyweight) followed by a high carbohydrate meal (2 g/kg bodyweight of maltose), they had 35% lower blood sugar levels compared to the control group (Figure 3). The acute animal study results helped to establish a potential benefit of GE on blood glucose levels following a meal in vivo.
Next the Zhou Lab wanted to determine the long term effect GE has on blood glucose levels in mice. To address this question, a 12-week study was designed to determine whether dietary supplementation of GE improves fasting blood sugar levels as well as other important type 2 diabetes biomarkers such as body weight, insulin levels, oxidative and inflammatory stress. After 12-weeks of eating a high fat diet incorporated with GE (dosage of approximately 250 mg of GE/kg bodyweight per mouse per day), mice in the GE group were found to have lower fasting blood glucose levels (119.3 mg/dL) compared to the high fat group (144.6 mg/dL). The 12-week supplementation of GE was also found to be associated with moderate improvements in oxidative stress and inflammatory status compared to the high fat fed group. However, there was an increase in body and liver weight which was not expected. Thus, this pilot study indicated a potential therapeutic role of grape extract supplementation for improving blood glucose levels and as a countermeasure against oxidative stress and corollary inflammation associated type 2 diabetes independent of weight gain.
Address Goals
General background of the importance of nutraceutical compound discovery: Pursuing this area of research is important because it is well acknowledged that a healthy diet and physical activity are effective means of reducing the risk of developing type 2 diabetes. However these may not be realistic strategies as evidenced by the steady rise in obesity and the low numbers of individuals who meet physical activity recommendations. Addressing the need for preventing and treating type 2 diabetes has gained national attention not only in the US but worldwide because it is a serious chronic health condition which has reached epidemic proportions. Just as concerning is the number of people with pre-diabetes. In particular, the Center for Disease Control (CDC) has estimated that over 54 million North Americans have pre-diabetes or very poorly regulated blood glucose levels. Eventually, unmanaged type 2 diabetes can lead to a suite of devastating conditions including cardiovascular disease, blindness, kidney failure, and nerve damage. Biologically, type 2 diabetes is a progressive and gradually debilitating disease in which the body is no longer able to metabolize glucose efficiently resulting in hyperglycemia or high blood glucose levels. In additional to high blood glucose levels there are high levels of reactive molecules which can also damage biological tissue, organs, and even DNA.
In the field of diabetes and obesity research, there is a growing consensus that type 2 diabetes is associated with not only glucose stress but also the generation of radical stress. Thus, great interest and scientific investments are currently being devoted to the management of this epidemic by addressing both blood glucose levels and oxidative stress. A primary strategy for reducing oxidative stress is by improving antioxidant status. A growing body of epidemiological studies has associated the consumption of antioxidant rich grapes, wine, and grape juice with a wide variety of health-promoting effects. The beneficial effects of grape and grape-derived products are believed to be related to specific chemical compounds. For example, compounds with antioxidant properties are found in high concentrations in grapes compared to other types of fruits. Some of these plant or phyto compounds include phenolic acids, polyphenols and flavonoids all of which have been shown to scavenge highly reactive molecules such as superoxide, peroxyl, and hydroxyl radicals. As such, grapes are thought to be a rich source of phyto-compounds with antioxidant properties that may be helpful in combating diseases associated with oxidative stress such as type 2 diabetes.
