Polyphenol-rich plant foods

Polyphenols are a diverse group of naturally occurring compounds found in plant-based foods, such as fruits, vegetables, whole grains, nuts, and legumes—including soy products, coffee, tea, cocoa, red wine, herbs, and spices [73, 74]. There are thousands of different polyphenols, which can be categorized into subclasses, including flavonoids (found in fruits, vegetables, tea, coffee, cocoa, and soy), lignans (found in seeds, grains, and vegetables), phenolic acids (found in coffee, nuts, and fruits), and resveratrol (found in grapes and wine). Because of their antioxidant, anti-inflammatory, and anticarcinogenic properties, polyphenol-rich foods are considered an important component of healthy dietary patterns that promote overall health and well-being. In addition to their cardiometabolic benefits, such as improved blood lipid profiles and reduced blood pressure, polyphenols have the potential to improve cognitive function and lower the risk of neurodegenerative diseases [75, 76]. Furthermore, they act as prebiotics, promoting the growth of beneficial gut bacteria and maintaining a healthy gut microbiome [74]. Experimental studies indicate that polyphenols exhibit antiaging properties by influencing various hallmarks of aging, including inflammation, oxidative stress, epigenetic alterations, and protein homeostasis [77]. Below we highlight epidemiologic and clinical trial evidence on the health benefits of several polyphenol-rich foods, such as coffee, tea, extra-virgin olive oil, blueberries, avocados, and culinary herbs and spices, as an exhaustive review of all major foods high in polyphenols is beyond the scope of this paper. It is worth noting that foods rich in polyphenols also contain other beneficial nutrients and compounds, such as minerals, vitamins, and carotenoids. Carotenoids, which are phytochemicals with antioxidant effects that give plant foods their vibrant colors, have been associated with lower risk of chronic diseases [78, 79]. Therefore, the health benefits of these foods may result from a combination of many components, not just polyphenols alone.

Among polyphenol-rich foods, coffee stands out as a major source of polyphenols in many populations. Chlorogenic acids are the most abundant polyphenols in coffee, possessing strong anti-inflammatory and antioxidant properties. Numerous epidemiologic studies have examined the associations between coffee consumption and risk of chronic diseases [80]. Consistent evidence has shown that moderate coffee consumption (3–5 standard cups per day) is associated with reduced risk of developing type 2 diabetes, CVD, and some cancers, such as liver, endometrial, and colorectal cancer. Moderate coffee consumption is also associated with lower risk of premature death in diverse populations [81, 82]. In addition, coffee consumption has been associated with a reduced risk of cognitive disorders [83] and Parkinson’s disease [84]. Moreover, there is consistent evidence that coffee consumption is associated with a reduced risk of depression [85] and suicide [86]. The health benefits of coffee are likely due to various bioactive compounds including polyphenols, although caffeine—a central nervous system stimulant—also plays a role in improving cognitive function and reducing risk of neurodegenerative diseases. It should be noted that individuals’ response to coffee can vary depending on genetics, and social and behavioral factors [80].

Regular consumption of tea has been associated with myriad health benefits, including lower risk of type 2 diabetes, CVD, and mortality [87, 88], although the overall evidence is less consistent and robust compared to that for coffee. Green tea has received a great deal of attention because of its high contents of catechins and other polyphenols with potent antioxidant effects. Green tea polyphenols—particularly epigallocatechin gallate—have been shown to improve blood lipid profiles, reduce blood pressure, and enhance vascular function [89, 90]. These polyphenols have also been associated with improved cognitive function and lower risk of neurodegenerative diseases [91].

Extra virgin olive oil (EVOO) is obtained from the mechanical pressing of olives without the use of heat or chemicals. Compared to regular olive oil, EVOO contains higher amounts of polyphenols such as oleuropein and hydroxytyrosol, which contribute to its color and flavor as well as potential health benefits [92, 93]. High olive oil consumption is a hallmark of the traditional Mediterranean diet (MedDiet), which has been consistently associated with lower risk of chronic diseases and mortality [94]. In the PREDIMED trial, participants who followed a MedDiet supplemented with EVOO or mixed nuts had significantly reduced risk of CVD including strokes, heart attacks, and cardiovascular deaths compared to the control group [95]. The intervention group supplemented with EVOO also had significantly reduced risk of type 2 diabetes [96] and breast cancer [97]. Despite relatively low consumption of olive oil compared to Spanish or other European populations, participants in the NHS and HPFS who consumed four or more tablespoons of olive oil per week had significantly lower risk of developing type 2 diabetes [98], CVD [99], and mortality [100].

In a subsample of the PREDIMED trial, the participants assigned to the MedDiet group supplemented with either EVOO or mixed nuts experienced significantly improved cognitive function compared to the control group [101]. These benefits are likely due to both the antioxidant and anti-inflammatory effects of polyphenols in the intervention diets as well as the indirect effects through improved vascular function by the interventions with their healthy fats.

Blueberries are rich in polyphenols, especially anthocyanins. Prospective analyses using data from NHS and HPFS have shown that higher consumption of anthocyanins or blueberries was associated with lower risk of type 2 diabetes [102], CVD [103, 104], and all-cause mortality [105]. In the longest duration RCT to date among participants with metabolic syndrome, 1 cup [150 g] of blueberries/d for 6 months led to clinically significant improvements in endothelial function, systemic arterial stiffness, and HDL-cholesterol levels, compared to the control group [106].

In animal studies, supplementation with blueberries led to improvements in cognitive and motor behaviors as well as in learning and memory [107]. Small RCTs in humans have shown that blueberry supplementation improves cognitive performance in children and older adults [108]. In the NHS and HPFS, higher consumption of total flavonoids and subclasses, including flavones, flavanones, and anthocyanins, was associated with lower odds of subjective cognitive decline [SCD] [109]. In this study, many flavonoid-rich foods, including berries, oranges, grapefruits, citrus juices, apples/pears, celery, peppers, and bananas, were significantly associated with lower likelihood of SCD.

Avocados are a nutrient-rich fruit that contain a variety of beneficial components, such as fiber, monounsaturated fat, potassium, and magnesium, and bioactive compounds including polyphenols, especially flavonoids [110]. Small RCTs have shown that diets that incorporated avocados significantly decreased LDL cholesterol levels [111]. In the NHS and HPFS, [112] higher avocado intake (≥2 servings/week) was associated with a 16% lower risk of CVD (HR, 0.84; 95% CI, 0.75–0.95) and a 21% lower risk of CHD (HR, 0.79; 95% CI, 0.68–0.91). No significant associations were observed for stroke. In substitution analyses, replacing half a serving/day of margarine, butter, egg, yogurt, cheese, or processed meats with the equivalent amount of avocado was associated with a 16% to 22% lower risk of CVD.

Herbs and spices are widely used in global cuisines, contributing not only to diversity of flavor but also potential health benefits due to their rich content of bioactive compounds, including polyphenols [113]. Notable examples include turmeric in Indian dishes; basil, rosemary, and oregano in Mediterranean cuisine; and ginger, garlic, and cloves in East Asian cuisines. Experimental studies have indicated that many herbs and spices have antimicrobial, antioxidant, and anti-tumorigenic properties [113]. Beyond its culinary use, turmeric has historically been used in Chinese and Indian traditional medicine for treating various diseases and conditions. Curcumin—a polyphenol and the active component of turmeric—has been extensively studied in recent decades, demonstrating its potential antioxidant, anti-inflammatory, anti-diabetes, and anticancer effects [114]. A systematic review and meta-analysis of 64 RCTs found that turmeric/curcumin supplementation was effective in improving blood levels of TC, TG, LDL-c, and HDL-c [115]. Turmeric/curcumin supplementation has also been shown to improve markers of liver function [116]. Moreover, curcumin supplementation significantly increased serum brain-derived neurotrophic factor levels [117], which might have positive effects on cognitive function, learning, and memory. Despite these encouraging results, the quality of these RCTs is generally low due to limited sample sizes, short durations, and varied supplement preparations. Recently, a randomized, crossover, controlled feeding study found that the addition of a relatively high dosage of mixed herbs and spices to a standard US-style diet significantly improved 24-h blood pressure but did not affect other cardiometabolic risk factors after four weeks, compared with lower dosages among adults at elevated risk of cardiometabolic diseases [118]. In epidemiologic studies, the low consumption levels of herbs and spices and lack of specific biomarkers of their intakes make it challenging to examine their associations with long-term outcomes. Nevertheless, incorporating turmeric and other herbs and spices into diets can offer promising approaches for enhancing health. Going forward, more rigorous research is needed to standardize spice and herb extracts and preparations, obtain more accurate information about the chemical profiles or active ingredients, develop specific biomarkers for these compounds, and better understand underlying molecular pathways [113].

Taken together, the health benefits of polyphenol-rich foods, including coffee, tea, EVOO, blueberries, and avocados, are well supported by the convergence of evidence from observational cohort studies, small-scale RCTs, and mechanistic studies. In addition, emerging evidence suggests potential health benefits associated with culinary herbs and spices prevalent in global cuisines. Although further research is needed to understand the biological mechanisms underlying these effects, these findings highlight the importance of including a variety of polyphenol-rich foods in a healthy dietary pattern to promote healthy aging and longevity.