- Anthocyanins are natural compounds responsible for the red-orange and blue-violet colors in many fruits, vegetables, and tubers.
- Research has shown that anthocyanins have beneficial effects on the gut microbiome, energy metabolism, and inflammation.
- A recent overview of the literature highlights how purple vegetables and tubers may help prevent and manage type 2 diabetes by their action on energy metabolism, inflammation and gut microbiota.
- The research also indicates that acylated anthocyanins—which are found in vegetables such as red cabbage and purple sweet potato—may be superior to nonacylated anthocyanins—found in blackberries and blackcurrants—in terms of anti-diabetic properties.
According to the CDC, over 37 million Americans—about 1 in 10 individuals—have diabetes, and approximately 90-95% of them have type 2 diabetes.
Normally, a hormone called insulin moves glucose (sugar) from the blood into cells, where it’s used for energy. But in type 2 diabetes, the body doesn’t use or produce insulin properly, and glucose builds up in the blood instead of being used by cells.
If diabetes is not managed properly, it can cause many health problems over time, including cardiovascular disease, high blood pressure, nerve damage, eye damage and vision loss, kidney disease and foot problems.
While various factors can increase the risk of developing type 2 diabetes, including a family history of diabetes, research has shown that a diet rich in fruits and vegetables can delay or prevent the onset of diabetes and enhance the well-being of individuals with diabetes-related health problems.
The benefits associated with fruits and vegetables are attributed to their high concentrations of polyphenols. One particular class of polyphenols—anthocyanins—are responsible for giving red-orange to blue-violet colors in plants.
Studies in the United States and Finland have shown that consumption of anthocyanin-rich foods, particularly berries, reduces the risk of type 2 diabetes.
Many studies have been carried out to better understand the anti-diabetic properties of anthocyanins.
A new review article published in the Journal of Agricultural and Food Chemistry summarizes the effects of anthocyanins on the gut microbiome, energy metabolism and inflammation, with a special focus on acylated anthocyanins.
Acylated vs. nonacylated anthocyanins
Anthocyanins are divided into 2 categories based on their molecular structure: acylated and nonacylated.
Acylated anthocyanins have a chemical group called an “acyl group” (made up of a carbon atom and an oxygen atom double-bonded to each other, with a single bond to another carbon atom), which nonacylated anthocyanins lack.
Compared to nonacylated anthocyanins, acylated anthocyanins are more stable and more resistant to digestion. For this reason, they are not digested and absorbed in the stomach, and upper intestine, and they pass through to the colon, where they are degraded extensively by gut microorganisms.
Elderberry, blackberry, and blackcurrant mainly contain nonacylated anthocyanins, while acylated anthocyanins are found in red radish, purple corn, black carrot, red cabbage and purple sweet potato.
Studies on the two types of anthocyanins differ in design and analysis methods, making it difficult to draw clear conclusions about the differences in biological activity.
However, Dr. Baoru Yang, professor of food sciences at the University of Turku, and her coauthors on the review suggest that acylated anthocyanins may be superior to nonacylated anthocyanins in terms of anti-diabetic properties.
Dr. Taylor C. Wallace, adjunct professor in the Department of Nutrition and Food Studies at George Mason University and principal and CEO at the Think Healthy Group, told Medical News Today that it is important to consider other differences in the sources of acylated and nonacylated anthocyanins.
“[Acylated anthocyanin-rich] purple potatoes also contain a lot of resistant starch and other compounds that may contribute to their anti-diabetic effects. [Nonacylated anthocyanin-rich] berries contain a significant amount of natural sugar that may negate some of the anthocyanin’s anti-diabetic effects.”
— Dr. Taylor Wallace
Anthocyanins promote gut health
Researchers have studied the effects of various anthocyanins on bacteria living in the gut (or gut microbiome) using animal models.
In a mouse study, nonacylated anthocyanins from black rice increased the abundance of certain gut bacteria, including Akkermansia muciniphila. A. muciniphila has been shown to stimulate insulin secretion and lead to improved glucose metabolism in mice with type 2 diabetes.
Acylated anthocyanins from various sources, such as purple sweet potato and Concord grape, have also been shown to affect the gut microbiome by promoting the growth of beneficial bacteria, inhibiting the growth of harmful bacteria, and increasing the production of short-chain fatty acids, which are beneficial for gut health and glycemic control.
Dr. Wallace told MNT that the majority of studies to date are not sufficiently sophisticated or validated “to really know what happens to anthocyanins in the GI tract.”
“Purified radio labeled non-acylated and acylated anthocyanins should be administered in humans to really know if there is a difference (this has not been done to my knowledge as it is very expensive),” he said.
Anthocyanins lower blood glucose levels
One of the pharmacological effects of anthocyanins is the inhibition of carbohydrate digestion enzymes, which leads to a reduction in blood glucose levels. Additionally, research has shown that anthocyanins activate glucose and lipid metabolism pathways in the liver and muscles, which also contribute to lowering blood glucose levels.
In one study, of diabetic mice allowed to feed freely for 2 weeks, in those supplemented with mulberry fruit extract containing nonacylated anthocyanins blood glucose levels were about 30% lower than non-supplemented mice at the end of the study.
Similarly, another study found that feeding acylated anthocyanin extracts from purple sweet potato for 4 weeks at a daily dose of 500 mg/kg body weight to diabetic mice led to a significant reduction in blood glucose and improvement in insulin insensitivity in the treatment groups.
The blood glucose-lowering effects of anthocyanins have been attributed to anthocyanin activation of AMPK (AMP-activating protein kinase) and PI3K/AKT (phosphoinositide 3 kinase/protein kinase B) pathways, which are crucial for glucose and lipid metabolism.
However, Dr. Wallace pointed out that most of the studies cited in the review used anthocyanin-rich extracts, rather than purified anthocyanins, so other polyphenols and flavonoids present in the extracts could have had synergistic effects.
Most flavonoids can also affect carbohydrate absorption by inhibiting carbohydrate-digesting enzymes and binding to sugars, thus preventing their absorption.
Anthocyanins lower inflammation
Eating carbohydrates or fat triggers a short-term inflammatory immune response. Normally, the inflammation goes away quickly, but if it doesn’t, it can become chronic. Chronic inflammation can harm the insulin-secreting cells in the pancreas, which can lead to obesity, insulin resistance, and type 2 diabetes.
In studies with diabetic mice, dietary supplementation of both nonacylated and acylated anthocyanins reduced inflammation. This reduction in inflammation reduces insulin resistance and improves glucose metabolism in diabetes.
Several studies have shown that anthocyanins exert an anti-inflammatory effect by inhibiting the NF-κB inflammation pathway. Other studies have shown that nonacylated anthocyanins activate the Nrf2 pathway, which helps produce antioxidant proteins to protect against inflammation or injury-induced oxidative damage.
Recent findings on the anti-diabetic effects of anthocyanins suggest that eating more anthocyanin-rich fruits and vegetables can be beneficial for the prevention and management of type 2 diabetes.
Although it is challenging to draw clear conclusions about the differences in biological activity between the two types of anthocyanins due to differences in study design and analysis methods across studies, Dr. Yang and coauthors conclude that acylated anthocyanins may have more potential benefits in regulating energy metabolism, inflammation, and gut microbiota in type 2 diabetes compared to nonacylated anthocyanins.
Other researchers, however, believe that it is too early to reach definitive conclusions.
“I’m not aware of any studies that directly compare purified acylated and non-acylated anthocyanins. I see this paper as ‘hypothesis generating,’ which would be published to justify a [g]rant to study this exact question. At this point I don’t think there is enough evidence to make any firm conclusions or public health statements,” said Dr. Wallace.
In their review of current research, Dr. Baoru Yang, professor of food sciences at the University of Turku, and her coauthors also acknowledge that “the gut microbiota-modulating effect of acylated anthocyanins deserves further study in different models as well as different physiological and pathological conditions.”
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