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The definition of what constitutes a healthy diet is continually shifting to reflect the evolving understanding of the roles that different foods, essential nutrients, and other food components play in health and disease. A large and growing body of evidence supports that intake of certain types of nutrients, specific food groups, or overarching dietary patterns positively influences health and promotes the prevention of common non-communicable diseases (NCDs). Greater consumption of health-promoting foods and limited intake of unhealthier options are intrinsic to the eating habits of certain regional diets such as the Mediterranean diet or have been constructed as part of dietary patterns designed to reduce disease risk, such as the Dietary Approaches to Stop Hypertension (DASH) or Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) diets. In comparison with a more traditional Western diet, these healthier alternatives are higher in plant-based foods, including fresh fruits and vegetables, whole grains, legumes, seeds, and nuts and lower in animal-based foods, particularly fatty and processed meats. To better understand the current concept of a “healthy diet,” this review describes the features and supporting clinical and epidemiologic data for diets that have been shown to prevent disease and/or positively influence health. In total, evidence from epidemiological studies and clinical trials indicates that these types of dietary patterns reduce risks of NCDs including cardiovascular disease and cancer.
Keywords: healthy dietary patterns, non-communicable diseases, macronutrients, micronutrients, non-essential nutrients, plant-based diets
Non-communicable diseases (NCDs) such as cardiovascular disease, cancer, chronic respiratory diseases, diabetes, obesity, and cognitive impairment are among the leading causes of death and disability throughout the world, affecting populations in developed as well as developing countries [1]. Although there are established genetic and environmental contributors to NCD risk, modifiable lifestyle-related factors play a large role at the individual level [2,3,4]. Dietary choices, for example, contribute to the risk for developing hypertension, hypercholesterolemia, overweight/obesity, and inflammation, which in turn increase the risk for diseases that are associated with significant morbidity and mortality, including cardiovascular disease, diabetes, and cancer [5]. Indeed, the marked rise in chronic NCDs has a causal link to global dietary patterns that are becoming increasingly Westernized [6], being characterized by high levels of fatty and processed meats, saturated fats, refined grains, salt, and sugars but lacking in fresh fruits and vegetables.
In recognition of the importance of the diet as a determinant of disease risk, the World Health Organization (WHO) Global Action Plan for the Prevention and Control of Noncommunicable Diseases includes strategies for addressing unhealthy diet patterns among its initiatives directed at reducing behavioral risk factors; the other components comprise physical inactivity, tobacco use, and harmful alcohol use [1]. Dietary changes recommended by WHO include balancing energy intake, limiting saturated and trans fats and shifting toward consumption of unsaturated fats, increasing intake of fruits and vegetables, and limiting the intake of sugar and salt. Many of these dietary targets naturally occur in regional diets such as the Mediterranean diet [7] or are included as part of evidence-based diets designed to reduce disease risk, such as the Dietary Approaches to Stop Hypertension (DASH) [8] or Mediterranean-DASH Intervention for Neurodegenerative Delay (MIND) [9] diets. To better understand the current concept of a “healthy diet”, this narrative review describes the features and supporting clinical and epidemiologic data for diets that align with the general WHO guidance and have been shown to prevent disease and/or positively influence health.
A healthy diet is one in which macronutrients are consumed in appropriate proportions to support energetic and physiologic needs without excess intake while also providing sufficient micronutrients and hydration to meet the physiologic needs of the body [10]. Macronutrients (i.e., carbohydrates, proteins, and fats) provide the energy necessary for the cellular processes required for daily functioning [11]. Micronutrients (i.e., vitamins and minerals) are required in comparatively small amounts for normal growth, development, metabolism, and physiologic functioning [12,13].
Carbohydrates are the primary source of energy in the diet and are found in the greatest abundance in grains, fruits, legumes, and vegetables [14]. In terms of deriving a health benefit, whole grains are preferred over processed grains, the latter having been stripped of germ and bran during the milling process, resulting in lower amounts of fiber and micronutrients [15]. Meta-analyses of prospective cohort studies have linked increased whole-grain intake to a reduced risk of coronary heart disease, stroke, cardiovascular disease, and cancer, as well as to the decreased risk of mortality due to any cause, cardiovascular disease, cancer, respiratory disease, diabetes, and infectious disease [15,16,17]. Fresh fruits and vegetables supply energy as well as dietary fiber, which promotes the feeling of satiety and has positive effects on gastrointestinal function, cholesterol levels, and glycemic control [18]. In addition, fresh fruits and vegetables are key sources of phytochemicals (e.g., polyphenols, phytosterols, carotenoids), which are bioactive compounds believed to confer many of the health benefits associated with fruit and vegetable consumption [19]. The mechanistic effects of these various phytochemicals are unclear but include their antioxidative properties, as well as their role in regulating nuclear transcription factors, fat metabolism, and inflammatory mediators. For example, flavonoids have been shown to increase insulin secretion and reduce insulin resistance, suggesting that these phytochemicals provide some benefits in obesity and diabetes [20]. Additionally, polyphenols interact with gastrointestinal microbiota in a bi-directional manner by enhancing gut bacteria and being metabolized by these bacteria to form more bioactive compounds [20]. Fruit and vegetable intake has been shown to inversely correlate with the risk of NCDs, including hypertension [21], cardiovascular disease [22,23], chronic obstructive pulmonary disease [24], lung cancer [25], and metabolic syndrome [26].
Dietary proteins provide a source of energy as well as amino acids, including those that the human body requires but cannot produce on its own (i.e., essential amino acids). Dietary proteins are derived from both animal (meat, dairy, fish, and eggs) and plant (legumes, soya products, grains, nuts, and seeds) sources, with the former considered a richer source due to the array of amino acids, high digestibility, and greater bioavailability [27]. However, animal-based sources of protein contain saturated fatty acids, which have been linked to cardiovascular disease, dyslipidemia, and certain cancers. Although the mechanisms are unclear, red meat, and processed meat in particular, have been associated with an increased risk of colorectal cancer [28,29]. Animal-derived proteins also increase the dietary acid load, tipping the body’s acid-base balance toward acidosis [30,31]. The increased metabolic acid load has been linked to insulin resistance, impaired glucose homeostasis, and the development of urinary calcium stones [30,31].
Adequate dietary protein intake is important for maintaining lean body mass throughout the life span. In older adults, protein plays an important role in preventing age-related loss of skeletal muscle mass [32], preserving bone mass, and reducing fracture risk [33]. For older individuals not obtaining adequate protein from their diets, supplementation with amino acids can improve strength and functional status [34].
Fats (or lipids) are the primary structural components of cellular membranes and are also sources of cellular energy [35]. Dietary fats fall into 4 categories: monounsaturated fats, polyunsaturated fats, saturated fats, and trans fats. The fat content of food is generally an admixture of these different types [35]. Unsaturated fats are found in a variety of foods, including fish, many plant-derived oils, nuts, and seeds, whereas animal products (and some plant-derived oils) contribute a larger proportion of saturated fats [35,36]. Trans fats found in foods are predominantly the result of processing vegetable oils but are also present in small quantities in animal products (i.e., ruminant trans fats from cows, sheep, and goats) [35,36]. Among the types of dietary fats, unsaturated fats are associated with reduced cardiovascular and mortality risks, whereas trans fats and, to a lesser degree, saturated fats are associated with negative impacts on health, including increased mortality risk [36,37]. Two families of polyunsaturated fatty acids, omega-3 and omega-6, are described as essential fatty acids, because they are required for normal growth and reproduction but are not produced by the body and, therefore, must be obtained from dietary sources [10]. Omega-3 fatty acids, in particular, eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have been widely studied for their potential health benefits, with evidence suggesting positive effects including cardioprotection, preventing cognitive decline, reducing inflammation, sustaining muscle mass, and improving systemic insulin resistance [38,39,40]. Seafood, especially oily fish, provides EPA and DHA, and supplements are widely available for those not meeting recommended intakes with diet alone [41,42]. Nuts and some seeds and plant oils provide alpha-linolenic acid, the major plant omega-3 fatty acid [43].
Although required in trace amounts compared with macronutrients, micronutrients are necessary for normal growth, metabolism, physiologic functioning, and cellular integrity [12,13]. The shift from whole foods to processed, refined foods has reduced the micronutrient quality of the modern Western diet [44]. Vitamin and mineral inadequacies have been implicated in cellular aging and late-onset disease, as scarcity drives chronic metabolic disruption. Keeping with these observations, adequate dietary intake of, or supplementation with, micronutrients that have antioxidant properties (e.g., vitamins A, C, and E, copper, zinc, and selenium) has been suggested as a means to reduce the risk for and progression of age-related diseases [45].
Water is the principal component of the body, constituting the majority of lean body mass and total body weight [13]. Water not only provides hydration but also carries micronutrients, including trace elements and electrolytes [46,47]. Drinking water may supply as much as 20% of the daily recommended intake of calcium and magnesium [47]. Our understanding of water requirements and water’s effect on health and disease is limited, although the global increase in intake of high-calorie beverages has refocused attention on the importance of water for maintaining health and preventing disease [46].
Based on our understanding of nutritional requirements and their likely health impacts as described above, healthy dietary patterns can be generally described as those that are rich in health-promoting foods, including plant-based foods, fresh fruits and vegetables, antioxidants, soya, nuts, and sources of omega-3 fatty acids, and low in saturated fats and trans fats, animal-derived proteins, and added/refined sugars [48]. Patterns such as these are naturally occurring in certain regions of the world and rooted in local/regional tradition and food sources, as is the case for the traditional Mediterranean and Asian diets. Healthy dietary patterns have also been developed based on studies of nutrient intake and subsequent health measures or outcomes (e.g., the DASH [8] and MIND [9] diets) that share some common characteristics ( Figure 1 ).