Before I launch into today’s post, I just want to give a big hello to those visiting from the blog Swirl, Sip, Snark, who recently wrote a great mini-review of this blog! Of course, hello to both my new and returning visitors!
I’ve been neglecting the medicinal side of academics as of late, so this post features current research related to just that.
Type 2 diabetes has been on the rise throughout the United States and the rest of the world, in large part as a result of the increase incidence of obesity. With the increasing frequency of Type 2 diabetes, there has also been an increase in discovering ways in which to treat the disease. One approach recently has been to increase phytochemicals in the diets of those diagnosed with Type 2 diabetes, which would improve glycemic control, and also reduce postprandial hyperglycemia, an early symptom of the disease. Basically, postprandial hyperglycemia occurs when pancreatic β cells no longer secrete the required level of insulin for normal function, which if not treated, can sometimes lead to an irreversible state of diabetes.
This postprandial hyperglycemia is a function of the amount of starch (the body’s primary source of glucose) digested by the body, and the rate in which this digestion occurs. To reduce this postprandial hyperglycemia, current research has examined ways in which to reduce or slow the digestion and absorption of carbohydrates, specifically by inhibiting starch hydrolyzing enzymes in the digestive tract. Hydrolysis occurs in mammals by the enzymatic actions of α-amylase, which creates by-products that are further hydrolyzed by α-glucosidase, which ultimately creates and releases glucose into the blood stream. By slowing this process, theoretically one would reduce or eliminate postprandial hyperglycemia, which would ultimately result in the reduction or elimination of Type 2 diabetes.
Some phytochemicals that are gaining interest in helping to decrease the rate of postprandial hyperglycemia and Type 2 diabetes are polyphenols and other antioxidants that have been shown to have a multitude of health benefits. It has been well documented that red wine, and particularly the grapes that red wine is created from, contains high levels of polyphenols and antioxidants that have been shown to have numerous health benefits. Grape pomace, a by-product of the winemaking process, which contains the skins and seeds of grapes, is extremely rich in these polyphenols and antioxidants, and should therefore be considered for use as potential health aide. Currently, grape pomace (a.k.a. grape marc or grape residues) is used for many purposes; including, but not limited to use as fertilizers, animal feed, and even potential meat preservers.
These polyphenols and antioxidants in the skins and seeds of grapes may play a role in the reduction of postprandial hyperglycemia and Type 2 diabetes when ingested, however, very little research has been done to investigate this possibility. The study reviewed today, which was published in 2010 in the journal Nutrition and Metabolism, sought to examine the potential benefits of grape pomace on Type 2 diabetes, by comparing pomace made from red grapes, white grapes, and apples as a type of control that is also rich in antioxidants, but may have different bioactivities. Extracts created from these different sources of pomace were examined for their total phenolic contents, flavonoids, free-radical scavenger activities, and their inhibitory effects on α-glucosidase.
Red grape pomace extracts were created from Cabernet Franc grapes, and white grape pomace extracts were created from Chardonnay grapes. Grapes were sources from a vineyard near Blackstone, Virginia, USA. Apple pomace extracts were created from the National fruit product company, INC, in Winchester, Virginia, USA. The following were measured for each extract: total phenolic content, total flavonoid content, oxygen radical absorbance capacity, DPPH radical scavenging activity, and α-glucosidase inhibition (in vitro and in vivo).
In vitro α-glucosidase inhibition assays involved using rat intestinal acetone powder, which was used to create mammalian α-glucosidase. This solution was mixed with the different extracts and maintained for 75 minutes, in order to determine in vitro inhibition of α-glucosidase. Acarbose was used as a positive control (a chemical known to inhibit α-glucosidase).
For the in vivo animal experiments, wild-type B6 mice were initially fed a rodent feed containing 60% carbohydrates, 23% protein, and 17% fat. Animals were housed and experiments performed at Virginia Tech, Blacksburg, VA, USA. For those mice in the diabetes treatment group, diabetes was induced with STZ dissolved in a 10mM sodium citrate buffer. After 5 to 7 days, those mice with a fasting blood glucose level of 126mg/dL were considered to have diabetes, and were then randomly assigned to two different treatment groups. The two treatment groups for the in vivo study were the control group, which received only water by oral gavage, and the red grape pomace extract group, which received the red grape pomace extract solution via oral gavage.
After 30 minutes of either the water or red grape pomace extract solution treatment, mice were challenged with 0.2mL of a potato starch suspension, which was administered by oral gavage. Ultimately, this in vivo experiment would determine if red grape pomace extract had any effect on the glycemic response of diabetic mice, following a potato starch challenge. Blood samples were collected from the mice at 0, 30, 60, and 120 minutes following the starch challenge, and blood glucose levels were measured.
In vitro study of grape and apple extracts
- Total phenols and total flavonoids:
o The red grape pomace extract had the highest levels of both (followed by white grape pomace extract and then the apple extract, which had the lowest levels).
o The red grape pomace extract had significantly higher levels of phenols and flavonoids than the apple extract.
o Flavonoids made up 72% and 66% of the total phenols in the red grape and white grape pomace extracts, respectively, while they only accounted for 50% of the total phenols in the apple extracts (suggestive of marked differences in phenolic profiles between grapes and apples).
- Oxygen Radical Absorbance Capacity (ORAC) and DPPH radical scavenging activity
o The red grape pomace extract had the highest ORAC, compared to the white grape and the apple extracts.
o The grape pomace extracts tended to have higher DPPH radical scavenging activity than the apple extract, but this was not statistically significant.
- In vitro α-glucosidase inhibition experiment
o Both the red and white grape pomace extracts significantly inhibited α-glucosidase, whereas the apple extract showed no effect (suggests that grape pomace may contain specific α-glucosidase inhibitory compounds that other extracts do not).
o Both the red and white grape pomace extracts showed strong inhibition of α-glucosidase than even the commercial extract (acarbose).
o At a dose of 1.5mg/mL, the red grape pomace extract inhibited α-glucosidase activity by 47%, which was significantly higher than the white grape pomace extract, which inhibited α-glucosidase activity by 39%. The apple extract did not inhibit α-glucosidase.
o There appeared to be a dose-dependent inhibition of α-glucosidase by the red grape pomace extract, between the concentrations of 0-2.5mg/mL. The inhibition of α-glucosidase by the red grape pomace extract at the highest concentration of 2.5mg/mL was 64%.
- In vivo postprandial hyperglycemia inhibition study in mice
o The administration of red grape pomace extract (dose of 400mg/kg body weight) successfully suppressed postprandial hyperglycemia in STZ-induced diabetic mice.
o The red grape pomace extract treatment group had postprandial hyperglycemia significantly reduced by 35% compared to the control group that did not receive any pomace extract.
What does this all mean?
The results of this study show great promise for those suffering from postprandial hyperglycemia and Type 2 diabetes. The in vitro studies showed that the grape pomace extract were highly successful in inhibiting α-glucosidase, whereas apple extract did not have such an effect. This indicates that grape pomace extracts may contain certain compounds that are specific for reducing or inhibiting α-glucosidase, where other extracts may not.
Currently Acarbose is used for treatment of the symptoms of diabetes. However, this treatment is known for creating adverse gastrointestinal side effects, which are caused by the inhibition of α-amylase (in addition to the desired inhibition of α-glucosidase). By inhibiting α-amylase, the body is less able to digest carbohydrates, thus leaving a very uncomfortable accumulation of undigested carbohydrates in the large intestine. However, this study determined (results not shown) that grape pomace extracts do not inhibit α-amylase, and only target α-glucosidase, thereby grape pomace extracts would be a more ideal treatment for Type 2 diabetes as a result of the decreased side effects.
In the in vivo study, the results clearly indicate that administration of red grape pomace extract had a significant inhibitory effect on α-glucosidase. Specifically, according to the authors, the results suggest that red grape pomace extracts can potentially act in an anti-diabetic manner, resulting from the suppression of postprandial hyperglycemia through the inhibition of α-glucosidase.
Of course, these studies were performed on mice, and not, humans, thereby one must use caution in extrapolating these results to human patients. However, these results do show great promise that grape pomace may be a significant therapy for treating postprandial hyperglycemia and Type 2 diabetes, and justify further research on the subject (eventually using human study subjects). This article was the first of its kind to look at using grape pomace extracts for this type of medicinal purpose, and based on the very positive results of this study, hopefully it will not be the last!
Grape pomace isn’t just for compost or fertilizer anymore!
I’d love to hear what you all think! Please feel free to leave your comments below!
Source: Hogan, S., Zhang, L., Li, J., Sun, S., Canning, C., and Zhou, K. 2010. Antioxidant rich grape pomace extract suppresses postprandial hyperglycemia in diabetic mice by specifically inhibiting alpha-glucosidase. Nutrition and Metabolism 7: 71
I am not a health professional, nor do I pretend to be. Please consult your doctor before altering your alcohol consumption habits. Do not consume alcohol if you are under the age of 21. Do not drink and drive. Enjoy responsibly!