by Bianca Garilli, ND
Polyphenols are naturally occurring secondary metabolites found in plants. In nature they protect against ultraviolet radiation and play a role in defending against pathogens. Over 8,000 polyphenolic compounds have been identified from a large variety of plant species.1 Based on the structure and number of phenol rings they contain, polyphenols are classified into 4 main groups: phenolic acids, flavonoids, stilbenes, and lignans.1
Phenolic acids are found extensively throughout the plant kingdom, playing a role in plant growth, reproduction, and defense against environmental stressors and pathogens. They have powerful antioxidant activities, making them useful commercially as a preservative and clinically for their anti-inflammatory, neuroprotective, anticancer, antidiabetic, cardioprotective, antimicrobial, antioxidant, and hepatoprotective properties.2 Common food sources of phenolic acids include a wide variety of nuts and fruits such as raspberries, grapes, strawberries, walnuts, cranberries, and black currants.2 Some of the better known phenolic acid compounds are gallic acid (found in red fruits, black radish, onions, and tea), salicylic acid (present in many fruits, vegetables, and grains), ellagic acid (found in many fruits and nuts including pomegranate and walnuts), and caffeic acid (present in a wide variety of fruits and vegetables, as well as coffee).1,3
Stilbenes are found only at low levels in the human diet, as their polyphenolic activities center around antifungal properties in plants. One well known stilbene consumed by humans, however, is resveratrol, a potent antioxidant sourced from grapes and red wine.1
Lignans are often used clinically and nutritionally for their phytoestrogenic properties. Linseed, better known as flaxseed, is a rich source of dietary lignin precursors; these precursors are metabolized into their active forms through the actions of human intestinal bacteria.1,5
Flavonoids are the most studied group of polyphenols, with more than 4,000 compounds identified.1 Flavonoids’ claim to fame include giving vibrant color to flowers, fruits, and leaves;1 clinically they are used for their antioxidant activity and free radical scavenging capacity, in addition to cardioprotective, hepatoprotective, anti-inflammatory, and anticancer properties. Flavonoids may also potentially offer antiviral properties.4 Flavonoids can be further classified into subclasses: flavonols, flavones, flavanones, flavanols, anthocyanins, and isoflavones.1 Quercetin (found in onions, apples, and tea) and catechins (contained in most fruits, legumes, and tea) are just two common flavonoids frequently used in the medicinal and nutritional fields.4
Polyphenols Impact Risk and Physiology of Type 2 Diabetes (T2D)
Ubiquitous throughout the plant world, polyphenols provide an excellent preventative and therapeutic resource for a wide array of chronic disease processes. For example, individuals with T2D benefit from the antidiabetic effects of polyphenols1 most likely due to their positive influence on impaired glucose metabolism, the hallmark characteristic of this disease.6 A study published in the American Journal of Clinical Nutrition conducted a systematic review (including 18 prospective epidemiologic studies) to analyze the relationship between polyphenol dietary exposure in association with T2D incidence. Analyses revealed significant inverse associations between total polyphenol intake (and specifically for flavonoids, flavonols, flavan-3-ols, catechins, anthocyanidins, isoflavones, daidzein, genistein, and stilbenes) and T2D incidence.6
Outcomes from this study underscore the importance of consuming a diet rich in polyphenols to reduce risk of T2D, with a particular emphasis on flavonoids, according to the authors.6 Anthocyanins, one of the subclasses of flavonoids, have been studied for their influence on specific cardiometabolic markers including blood pressure, blood glucose, and lipid profile7, all of which may impact risk of T2D. A published summary of evidence from 19 randomized clinical trials (RCTs) examining the effects of anthocyanin supplementation in adults on specific cardiometabolic markers found the following:7
- No significant effect on weight, waist circumference, BMI, systolic nor diastolic BP, HbA1c, fasting glucose, fasting insulin
- Significant, positive effects on HOMA-IR (surrogate measure of insulin resistance and β-cell function), total cholesterol, and LDL cholesterol (observed with ≥300 mg/day anthocyanin supplementation interventions lasting >12 weeks)
The dose and duration of the anthocyanin RCTs varied considerably, so future investigations should further vet the therapeutic dosing and time period for anthocyanin supplementation as it relates to T2D prevention and treatment.7
In summary, various epidemiological studies and meta-analyses indicate a therapeutic benefit, particularly for T2D, to long-term adherence to nutritional profiles rich in plant polyphenols, including the large class of flavonoids. Benefits of this dietary direction potentially extend to a variety of chronic disease states such as cancers, cardiovascular disease, osteoporosis, and neurodegenerative diseases.1 Habitually consuming a wide variety of plant foods ensures consumption of various subclasses of polyphenols including phenolic acids, flavonoids, lignans, and stilbenes; each confer specific bioactive properties and potential health benefits. Diets high in flavonoid containing foods, particularly anthocyanins (the compounds responsible for the blue, red, or purple pigments found in plants, especially flowers, fruits, and tubers) improve insulin resistance and cholesterol levels.7-8 So, color your plate!
- Pandley KB et al. Plant polyphenols as dietary antioxidants in human health and disease. Oxid Med Cell Longev. 2009;2(5):270–278.
- Venkata S et al. Therapeutic potential of dietary phenolic acids. Adv Pharmacol Sci. 2015:823539.
- Watson R et al. Polyphenols in human health and disease. Academic Press, 2013.
- Kumar S et al. Chemistry and biological activities of flavonoids: an overview. Sci World J. 2013;162750.
- OSU. Linus Pauling Institute. Lignans. https://lpi.oregonstate.edu/mic/dietary-factors/phytochemicals/lignans. Accessed August 21, 2018.
- Rienks J et al. Polyphenol exposure and risk of type 2 diabetes: dose-response meta-analyses and systematic review of prospective cohort studies. Am J Clin Nutr. 2018;108(1):49-61.
- Daneshzad E et al. Effect of anthocyanin supplementation on cardio-metabolic biomarkers: a systematic review and meta-analysis of randomized controlled trials. Clin Nutr. 2018;S0261-5614(18):31198-1.
- Khoo HE et al. Anthocyanidins and anthocyanins: colored pigments as food, pharmaceutical ingredients, and the potential health benefits. Food Nutr Res. 2017;61(1):1361779.
Bianca Garilli, ND
Dr. Garilli is a former US Marine turned Naturopathic Doctor (ND). She works in private practice in Northern California as well as running a consulting company working with leaders in the natural and functional medicine world such as the Institute for Functional Medicine and Metagenics. She is passionate about optimizing health and wellness in individuals, families, companies and communities- one lifestyle change at a time. Dr. Garilli has been on staff at the University of California Irvine, Susan Samueli Center for Integrative Medicine and is faculty at Hawthorn University. She is the creator of the Veterans for Health Initiative and is the current President of the Children’s Heart Foundation, CA Chapter.