07/02/2022
A thorough analysis of the research, an overview of the literature, and suggestions for improved public policy to increase consumption of fruits and vegetables
Dietary risk factors associated with poor health in the United States are those considered to be low in fruits, vegetables, whole grains, nuts, and seeds and high in refined carbohydrates, added sugars, sodium, and certain saturated fats. Diets high in fruits and vegetables (F&V) are widely recommended in developed countries for their health-promoting properties; they have historically held a place in dietary guidance because of their vitamin, mineral, dietary fiber, and, more recently, dietary bioactive content. Most nutritional and global recommendations include consumption of at least 2 servings of fruits and 3 servings of vegetables per day for adults (WHO 2002; Nishida et al. 2004; Health Canada 2014; U.K. Government 2018; USDA 2018a). More than 100 countries worldwide have developed food-based dietary guidelines adapted to their nutrition situation, food availability, culinary cultures, and eating habits that encourage increased F&V consumption. However, F&V in their harvested consumed form have been shown to vary widely in nutritional contribution, density, and dietary bioactive content of a standard serving (Song et al. 2010; Hornick and Weiss 2011).
Eighty-eight percent of countries face a serious burden from two or three forms of malnutrition: acute and/or chronic undernutrition, micronutrient deficiencies, obesity, and diet-related diseases, including type 2 diabetes, cardiovascular diseases (CVDs), and certain types of cancer (FAO 2018). With noncommunicable disease (NCD) conditions accounting for nearly two-thirds of deaths worldwide, the emergence of chronic disease as the predominant challenge to global health is undisputed (Bauer et al. 2014). Twenty-three percent of the total global burden of disease is attributable to disorders in people aged ≥60 years; however, primary prevention in younger populations can significantly improve health in successive cohorts of older people. The leading global contributors to disease burden in older people are CVDs, cancers and malignant neoplasms, chronic respiratory diseases, musculoskeletal diseases, and neurological and mental disorders (Prince et al. 2015).
While dietary recommendations for F&V intake have many similarities between countries, many countries choose different strategies for separating F&V into subgroups based on the needs of their population. This approach makes sense from a menu-planning perspective but does not directly correspond with nutrient content (Slavin and Lloyd 2012). For instance, many F&V are excellent sources of vitamin C (e.g., citrus fruits, strawberries, green peppers, white potatoes) but are spread over many F&V categories (Slavin and Lloyd 2012). F&V categories in the 2015–2020 U.S. Dietary Guidelines for Americans (DGA) (USDA 2018a) are listed in Table 1. These categories are important because the DGA serve as the cornerstone of U.S. nutrition policy and direct the development and implementation of all nutrition education and assistance programs (e.g., MyPlate and the National School Lunch Program). No individual food or food group has all of the nutrients needed to support life. However, consuming a mixture of foods (i.e., dietary diversity) is optimal for achieving nutrient intakes required for growth, development, reproduction, and longevity. Thus, dietary guidance over time has consistently supported the principles of moderation and variety (Slavin and Lloyd 2012).
Determinants of food choice
What people eat is influenced by many complex factors that span from individual levels of influence to dimensions of their environment. According to the European Food Information Council (EUFIC), the key driver for eating is hunger; what we choose to eat is not determined solely by physiological or nutritional needs (EUFIC 2006).
Some of the other factors that influence food choice include the following:
Biological determinants such as hunger, appetite, and taste
Economic determinants such as cost, income, and availability
Physical determinants such as access, education, skills (e.g., cooking), and time
Social determinants such as culture, family, peers, and meal patterns
Psychological determinants such as mood, stress, and guilt
Attitudes, beliefs, and knowledge about food
The complexity of food choice is apparent, and the above list is not exhaustive (EUFIC 2006).
Behavioral strategies are needed to motivate and enhance the capacity of individuals to adopt and improve their lifestyle behaviors. Environmental change is also important because environmental context and conditions affect what and how people eat and what food choices are available (Hawkes et al. 2015). Children in particular have been shown to respond positively to an image of a figurehead regularly associated with a public health campaign’s message. A recent randomized controlled trial (RCT) of >1200 children aged >3 years found that viewing a photograph of Michelle Obama’s face relative to control conditions during Halloween “trick or treating” caused children to be 19% more likely to choose fruit over candy (Aronow, Karlan, and Pinson 2018). However, this study is limited since it did not allow for extrapolation of why Michelle Obama’s face influenced children’s dietary decision making. Other factors such as socioeconomic status and political leanings of the children’s parents may make these findings non-generalizable to the general population (Aronow, Karlan, and Pinson 2018). Use of rewards to encourage healthy eating in children remains controversial; however, a recent cluster-randomized trial (n = 422) of 4- to 6-year-old children found both tangible and social reward to be effective in increasing consumption of “disliked” vegetables, with effects maintained for 3 months (Cooke et al. 2011). On the contrary, most current school-based intervention efforts have not proven to be effective in reversing rates of childhood obesity and additional approaches to the problem are needed (Birch and Ventura 2009).
Emerging evidence suggests that interventions that reduce reward and attention region responsivity in the brain to food cues and increase inhibitory control may reduce overeating and excessive weight gain in adults (Stice and Burger 2019). Initial deficit in inhibitory control and bias for immediate reward have been suggested to contribute to overconsumption of high-calorie foods. A landmark pilot randomized trial in adults demonstrated for the first time a reversal of obesity-related abnormalities as well as mental “reward system” responsivity to food cues with a 6-month behavioral intervention (Deckersbach et al. 2014). Compared to controls, participants (n = 13) achieved significant weight loss and had increased activation of the reward system for low-calorie food images, as well as large shifts in relative activation favoring low-calorie vs. high-calorie foods (Deckersbach et al. 2014), providing the first demonstration of a positive shift toward healthy vs. unhealthy food cues in a behavioral intervention.
Because food choice differs according to life stage and the power of one factor may vary between individuals or groups of people, single interventions to modify food choice behavior will not suit all population or subpopulation groups (EUFIC 2006). Recent evidence suggests that the interaction between food preferences and the environment in which those preferences are learned, expressed, and reassessed has a central role in determining food choice (Hawkes et al. 2015). Potential mechanisms to help individuals alter their food choices to include more F&V, among other healthy options, include enabling an environment for learning of healthy preferences, overcoming barriers to the expression of healthy preferences, encouraging people to reassess existing unhealthy preferences at the point of purchase, and stimulating a food-systems response (Hawkes et al. 2015). Unfortunately, in the United States, the ready availability and low cost of energy-dense and nutrient-deplete foods provides an eating environment that fosters food preferences inconsistent with dietary guidelines. Food scientists and the food industry have the ability to play a critical role in shifting individuals toward more healthy dietary patterns through developing new innovative flavorful products and improving both local and global supply chains to increase availability and decrease cost of F&V.
Current gaps in fruit and vegetable intake
According to a report from the Produce for Better Health Foundation (2015), recent F&V consumption losses are tied to two common behaviors: a decline in dinner side dishes for vegetables, and reduced consumption of fruit juice at breakfast (Produce for Better Health Foundation). Staples such as orange juice, lettuce/salad, corn, and green beans have led the declines. Fewer side salad dishes also reduce the consumption of salad-related produce, such as tomatoes and cucumbers. The report shows that vegetable intakes have long been affected by shifts occurring at the dinner table. As Americans look for more convenient dinner occasions (e.g., ready-to-eat or frozen main dishes), vegetable consumption has decreased because there are fewer side dishes at dinner. As public health bodies continue to encourage decreased consumption of “sugar-sweetened” beverages, consumers often include 100% juice in this mix (Produce for Better Health Foundation 2015).
Since publication of the 2010 DGA, adequate fruit intake has remained low or stable and vegetable intake has declined, particularly among children of all ages, adolescents, and adult males (DGAC 2015). Across the U.S. population, only about 15% of individuals meet daily fruit intake recommendations (DGAC 2015; USDA 2018a). Children aged 1–3 years and 4–8 years differ from the rest of the population in that many do meet recommended intakes for total fruit, which is 1 cup and 1–1.5 cups per day, respectively. Among older children, adolescents, and adults of all ages, few consume the recommended daily amounts of fruit, which range from 1.5 to 2 cups for older children and adolescents to 1.5 to 2.5 cups for adults. Almost 90% of all fruit intakes come from single fruits, fruit salads, or fruit juices. The most commonly consumed fruits are apples, bananas, watermelon, grapes, strawberries, oranges, peaches, cantaloupe, pears, blueberries, raisins, and pineapple. Commonly consumed fruit juices are orange juice, apple juice, and grape juice.
The U.S. population also consumes too few vegetables; only 10% and 15% of boys and girls aged 1–3 years, respectively, consume the recommended 1 cup of vegetables per day. For children aged 4–8 years, less than 5% consume the recommended 1.5–2 cups of vegetables per day. Vegetable consumption is lowest among boys aged 9–13 years and girls aged 14–18 years, as less than 1% of this population consumes the recommended 2–2.5 cups per day. Intakes increase slightly with age during the adult years but still remain low. Overall, nearly 90% of the U.S. population does not meet daily vegetable intake recommendations (DGAC 2015; USDA 2018a). White potatoes, the most commonly consumed single discrete vegetable also commonly categorized as “staple foods” instead of as a vegetable, make up about 80% of all starchy vegetable consumption (USDA 2015) and 21% of all vegetable consumption. Mean gram intake of all potatoes and more energy dense French fries by U.S. adults is 110 ± 1.9 g and 72.1 ± 2.5 g, respectively (Freedman and Keast 2012), and potatoes are among the most affordable vegetable options (Drewnowski and Rehm 2013). Tomatoes account for 18% of all vegetable consumption (note: tomatoes are grouped as a vegetable by USDA), while lettuce and onions are the only other vegetables that make up more than 5% each of total vegetable group consumption (USDA 2015).
It is worth noting that consumption patterns for some specific F&V follow a socioeconomic gradient. For example, whereas whole fruit consumption tends to be associated with higher education and incomes, the consumption of 100% fruit juice is higher among groups of lower socioeconomic status. Whereas the consumption of white potatoes is associated with lower incomes, the consumption of salad leafy greens is much higher among higher income groups. A recent paper modeled the frequency of salad consumption using residential property values as a predictor of dietary behavior (Drewnowski, Buszkiewicz, and Aggarwal 2019). These disparities in FV consumption patterns can be explained in part by differences in affordability and cost. Nutrient density profiling methods have been used to assess affordability of different F&V (Drewnowski and Rehm 2013).
Contribution to nutrient intakes
F&V prepared without added fat or sugar have low energy density and are an excellent source of nutrients such as (but not limited to) vitamin C, carotenoids, potassium, and dietary fiber (DGAC 2015; USDA 2018a). Thus, increasing consumption of F&V improves diet quality (DGAC 2015) and can lead to a decrease in total kilocalorie consumption, particularly when F&V are promoted as alternatives for foods higher in saturated fat and added sugar. According to the 2015 DGA, nutrients of concern in the American diet include potassium, dietary fiber, calcium, and vitamin D (USDA 2018a). F&V vary widely in their nutrient content; however, they remain a top contributor to both potassium and dietary fiber intakes. In 2009–2010, the average dietary potassium intake of the U.S. population aged ≥2 years was 2640 mg per day. F&V contribute ∼20% of total potassium intakes in the United States (Bailey et al. 2016). F&V intake accounts for 28% of the U.S. population’s dietary fiber intake (Hoy and Goldman 2014).
On any given day, 20% of Americans consume a salad; this has been shown to contribute to intakes of nutrients that tend to be lower than recommendations, including dietary fiber and vitamins A, C, and E (USDA 2018b). Canned F&V have also been associated with higher intakes of select nutrients such as dietary fiber, total sugar, choline, and potassium, as well as a higher-quality diet in U.S. adults. Child consumers of canned F&V also eat more protein, vitamin A, calcium, and magnesium (Freedman and Fulgoni 2016). Intake of nutrients of concern is significantly higher among those National Health and Nutrition Examination Survey (NHANES) participants aged >1 year who consume frozen F&V, whereas sodium intakes are lower (Storey and Anderson 2018). Vegetable consumption in any form has been shown to increase intakes of potassium, dietary fiber, calcium, magnesium, iron, vitamin C, and folate among most subpopulations, including women of childbearing age (Storey and Anderson 2016). However, geo-localized disparities in food consumption patterns are apparent and a new study demonstrates associations between higher residential property value with higher salad and lower soda intakes (Drewnowski, Buszkiewicz, and Aggarwal 2019).
Dietary bioactive compounds in fruits and vegetables: beyond basic nutrition
Nutrition and health experts around the world recognize the importance of F&V in providing essential micronutrients and fiber to the diet; however, produce is also the primary source of dietary bioactive compounds, endogenous protective substances that often contribute the vibrant colors present in F&V. The U.S. National Institutes of Health (NIH) has defined dietary bioactive compounds as “compounds that are constituents in foods and dietary supplements, other than those needed to meet basic human nutritional needs, which are responsible for changes in health status” (NIH Office of Dietary Supplements 2018). Current public health recommendations to increase the consumption of F&V are largely driven by the need to meet essential nutrient intakes from foods; however, policy recommendations (e.g., the 2015–2020 DGA) and groups (e.g., the National Fruit and Vegetable Alliance) also note the contribution of dietary bioactives to health promotion (DGAC 2015; Wallace et al. 2015; USDA 2018a). Consumers are now choosing F&V not only for their content of vitamins, minerals and fiber, but also for their concentration of dietary bioactives.
Examples of bioactives present in F&V are illustrated in Table 2. Upward of 5000 dietary bioactive compounds have been identified to date (Gonzalez-Gallego, V. Garcia-Mediavilla, S. Sanchez-Campos, and Tunon 2010; Liu 2013; Casas, Estruch, and Sacanella 2018), many of which now have a substantial body of evidence supporting their benefits on human health. Lupton et al. (2014) recognized the importance of dietary bioactives and proposed criteria for establishing recommended dietary intakes based on these putative health outcomes. In a few instances, the dietary reference intake framework for essential nutrients has been applied in using nonessential food components, such as carotenoids, dietary fiber, and certain trace elements, in the United States and Canada (IOM 1998, 2001, 2005a). The Chinese Nutrition Society (2018) has published intake recommendations for at least 19 nonessential bioactives (Table 3). However, the issue that arises when examining mostly observational evidence is the inability to distinguish between the effects of a single bioactive ingredient or class of compounds and other nutritional constituents (Ellwood et al. 2014).
Of the dietary bioactive classes present in F&V, polyphenols and carotenoids have been the most widely studied. There is consistent evidence from prospective cohort studies that high intakes of flavonoids are associated with a reduced risk of CVD and a reduced incidence of CVD, coronary heart disease (CHD), and all-cause mortality in men and women (Kim and Je 2017). Among the flavonoids, anthocyanins appear to have a significant impact on glucose metabolism (Li et al. 2017), as well as total and low-density lipoprotein cholesterol levels, particularly among people with hyperlipidemia (Wallace, Slavin, and Frankenfeld 2016), across RCTs. Although they are often considered a simple biomarker of F&V intake, carotenoids possess important anti-inflammatory and antioxidant properties independent of their pro–vitamin A activity. Interestingly, a dose-response relationship has been found between blood concentrations of α- and β-carotene as well as total carotenoids and risk of lung cancer (Abar et al. 2016). Lutein, a yellow xanthophyll carotenoid found in spinach and other colorful F&V, is strongly correlated with visual performance, a reduced risk of age-related macular degeneration (AMD), and enhanced cognition (Johnson 2014; Renzi-Hammond et al. 2017; Mewborn et al. 2018a; Mewborn et al. 2018b). A recent expert review of the literature demonstrates that lutein meets the criteria outlined by Lupton et al. (2014) and suggests that this evidence is sufficient to propose recommendations for reference intake values (Ranard et al. 2017).
Emerging biomarkers of dietary intake
A biomarker is a measurable substance that is taken to reflect some underlying physiological state, whether normal or pathological. In theory, almost any measurement that reflects a change in biochemical processes, structures or functions can be used as a biomarker (Cross et al. 2017). Biomarkers can be used to assess dietary intakes (exposure), biological responses, or responses to a behavior or nutrition intervention (effect); can serve as predictors of a clinical endpoint or disease outcome (intermediate biomarker); and/or can measure predisposition to a disease or response to treatment (genetic susceptibility) (Cross et al. 2017).
The use of biomarkers to characterize and monitor dietary exposure to F&V is not a new concept. Researchers in the early 1990s first described plasma carotenoids as being useful biomarkers of F&V intake (Campbell, Gross, G. A. Grandits, and Potter 1994). More recently, resonance Raman spectroscopic evaluation of skin carotenoids has been utilized as a biomarker of carotenoid status (Mayne et al. 2013). In addition, the urinary metabolite 3,3′-diindolylmethane (DIM) has been shown to discriminate between volunteers fed high and low doses of Brassica vegetables, and DIM has been suggested to be a reliable biomarker of glucobrassicin exposure and indole-3-carbinol uptake (Fujioka et al. 2016).
The current pace of biomarker discovery and its applications is greater than ever before, in part due to the rapid development of “omics” technologies and data collection. Some single-omics analyses have provided valuable data, mostly in the field of nutrigenomics. Thus, several gene-diet interactions in determining both intermediate (plasma lipids, etc.) and final cardiovascular phenotypes (stroke, myocardial infarction, etc.) have been reported (Fitó et al. 2016). In order to support this development, there is a need to develop ontologies for food, nutrition, and diet-related health areas. There is also a need to classify biomarkers in such a way that systematically attempts to validate them and develop them into trusted research and clinical tools according to standardized criteria based on their intended use (Dragsted et al. 2017).
Methods
Literature search and selection criteria
We followed standardized procedures for performing an umbrella review as previously described by Aromataris et al. (2015). We systematically searched the PubMed database from inception until May 21, 2019 using the search strategy outlined in Table 4. We included all systematic reviews with or without meta-analyses of human studies with a health-related outcome (e.g., CVD, cancer, mortality, etc). English language restrictions were applied. One investigator (DDW) performed the primary title and abstract screening. The full texts of all potentially eligible articles were retrieved, and consensus between two investigators (DDW and TCW) determined the final eligibility of each manuscript.
Data extraction
Two investigators (DDW and TCW) extracted the following information for each article: 1) first-author last name, 2) year of publication, 3) country in which the study was conducted, 4) outcome variable (e.g., bladder cancer), 5) number of studies for F&V and F&V included in the systematic review, 6) outcome comparison (e.g., high vs. low or dose-response), 7) the most-adjusted, study-specific estimates for the health outcome (e.g., relative risk [RR], odds ratio [OR], hazard ratio [HR], or incident risk ratio), along with the 95% confidence interval (CI). We included all systematic reviews, regardless of whether multiple ones were available for the same health outcome. We did not conduct any data analyses due to heterogeneity between systematic reviews and the overall large quantity of available data.
Effect of fruit and vegetable intakes on all-cause mortality
NCDs are the leading cause of death worldwide, primarily from CVD and cancers (GBD 2013 Mortality and Causes of Death Collaborators 2015). In 2016, NCDs were responsible for 41 million (71%) of the 57 million deaths that occurred globally (Riley, Gouda, and Cowan 2017). During the past 50 years, lifestyle factors have been identified as modifiable factors associated with premature death. Despite often unclear direct biological mechanisms, epidemiological risk factors can change the probability of death and can serve as important public health indicators. Several dated meta-analyses have shown that obtaining recommended intakes of F&V, among other food groups such as whole grains, nuts, and fish, is one of the most important factors associated with a lower risk of all-cause mortality (Wang et al. 2014b; Aune et al. 2016; Mayhew et al. 2016; Zhao et al. 2016). The most recent systematic review and meta-analysis found that with increasing intake (for each daily serving), the risk of all-cause mortality decreased for fruits (RR, 0.94; 95% CI, 0.92–0.97), vegetables (RR, 0.96; 95% CI, 0.95–0.98), whole grains (RR, 0.92; 95% CI, 0.89–0.95), nuts (RR, 0.76; 95% CI, 0.69–0.84), and fish (RR, 0.93; 95% CI, 0.88–0.98) (Schwingshackl et al. 2017c). Optimal consumption of risk-decreasing foods (i.e., fruits, vegetables, whole grains, nuts, and fish) resulted in a 56% reduction in all-cause mortality (Schwingshackl et al. 2017c). However, these investigated food groups are often only part of healthy dietary patterns. High adherence to the HEI and Dietary Approaches to Stop Hypertension (DASH) diets has been associated with a 22% lower risk of all-cause mortality (Schwingshackl and Hoffmann 2015). In the United States, morbidity and chronic disability now account for nearly half of the health burden. Diet has been suggested to be a more important factor associated with disease burden in the United States than both physical activity and BMI. Data from the Global Burden of Disease (GDB) database indicate that the most important dietary risks in the U.S. population are diets low in fruits, vegetables, nuts, and seeds and high in sodium, processed meats, and saturated fats (Murray 2013).
Effects of fruit and vegetable intake on health outcomes
The literature search strategy identified 401 articles, of which 96 systematic reviews were identified after completion of the title/abstract screening that assessed F&V intake on various disease outcomes (Tables 5–7). All 96 studies were included for data extraction after full-text review. Most systematic reviews contained meta-analyses that assessed high vs. low intake of F&V on disease outcomes. Fewer systematic reviews assessed whether a dose-response relationship existed between the amount of F&V consumed and specific health outcomes. Various types of cancer and cardiovascular outcomes were the most frequently assessed disease outcome, with other disease states such as type-2 diabetes, AMD and osteoporosis, among others, being less common. As anticipated for assessment of disease outcomes, the majority of systematic reviews included observational data and human interventions studies were scarce. Results of the individual systematic reviews are discussed in the corresponding disease state sections of this manuscript. The review was focused on the major global causes of morbidity and mortality: CVD, cancers, infectious diseases, musculoskeletal diseases, and other important health topics. F&V intakes with regard to contributors to chronic disease including weight status, inflammation, and immunity are also discussed.
