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A healthy diet is a powerful tool for the prevention and management of cardiovascular disease (CVD).1 Despite low-fat recommendations, the prevalence of obesity continues to increase steadily,2 because fat intake has not decreased in the United States.3 Accompanying this rise is the proliferation of new diet plans claiming to promote weight loss, improve health and reduce disease risks. While these rapid advances in nutrition science have created confusion among both the general population and healthcare providers, they also provide powerful opportunities to reduce the adverse health and economic impacts of poor diets. Key lessons include shifting the focus to overall diet patterns, rather than isolated nutrients, and recognizing the complex influences of different foods on long-term weight regulation, rather than simply counting calories. This article will review several popular dietary approaches for cardiovascular health and evaluate the available scientific evidence behind these diets.
ISOLATED NUTRIENT TARGETS
Low-Fat Diets
Dietary guidelines traditionally recommended restricting total fat, with a goal of lowering saturated fat and dietary cholesterol and their associated cardiovascular risk. Based on the caloric density of fat and some limited interventional studies, it was theorized that low-fat diets might help prevent obesity.2 Although the initial intention of the low-fat campaign was to reduce saturated-fat intake, it resulted in the unintentional incrimination of all fats. It also spurred a compensatory increase in consumption of refined, processed carbohydrates and added sugars, an unintended consequence that likely worsened the twin epidemics of obesity and diabetes.2,4 Metabolic studies have shown that low-fat, high-carbohydrate diets not only induce high glycemic and insulinemic responses but also increase plasma triglycerides and decrease high- density lipoprotein cholesterol (HDL-C). Observational cohorts and randomized trials have confirmed that little clinical benefit is derived from diets focused on isolated nutrient targets, such as low-fat or low-saturated fat diets, which produce no significant benefits for CVD or its risk factors.2,5-7
Low-Carbohydrate Diets
Controversy surrounding the benefits of low-fat diets contributed to a resurgence of interest in low-carbohydrate and paleo diets. A low-carbohydrate diet first was characterized in the 1860s by William Banting,8 but gained popularity in recent years with the introduction of the Atkins diet.9 The Atkins diet recommends two weeks of extreme carbohydrate restriction, followed by gradually increasing carbohydrates to 35 g/day. It involves obtaining 68% of total calories from fat, 27% from protein and 5% from carbohydrates. Low-carbohydrate diets recommend limiting complex and simple sugars, causing the body to oxidize fat to meet energy requirements. Consumers of these diets are in a perpetual state of ketosis, leading to the disproportionate use of fat stores for energy.
Ketones are renally cleared with fluids, and this diuretic effect may result in rapid initial weight loss. By removing a wide range of carbohydrate-rich foods, caloric intake also is reduced,10 potentially leading to sustained weight loss.11 Trials evaluating the effect of low-carbohydrate versus low-fat diets are uniform in their findings, with better weight loss (about 4 kg to 6 kg) at six months,12-16 but no significant difference at 12 months.12,13 Similarly, studies reported an initial reduction in low-density lipoprotein cholesterol (LDL-C) and triglycerides and an increase in HDL- cholesterol.17 However, these favorable trends of LDL-cholesterol reduction were not consistently demonstrated after the six-month follow-up.12,14 Importantly, low-carbohydrate diets focused on animal protein and fat increase mortality by 22% in the general population,18 and by 53% in patients with prior myocardial infarction;19 Postulated mechanisms include increased inflammation, CVD risk and cancer risk.
The Paleolithic, or Paleo, diet emerged in an effort to return to our nutritional roots. It is centered on the idea that eating like our early ancestors is aligned with our genetics and, therefore, is optimal for health. Paleo diets are limited to protein sources available in “prehistoric times,” such as poultry, fish and red meat, along with vegetables (including tubers) and fruits. Eliminated are dairy, grains, legumes, sugar and salt. Several studies have suggested that a high- protein diet can accelerate weight loss and improve cholesterol, triglycerides, blood pressure and other measures of metabolic syndrome.20,21 However, positive findings should be tempered by the lack of power of these studies, which were limited by small numbers, heterogeneity of study designs and populations, and short duration. Further, this dietary pattern emulates a sustenance pattern, not one designed for longevity, as Paleolithic man is estimated to have lived less than 50 years).22
A main benefit of both low-carbohydrate and Paleo diets is reduced refined grains, starches, and added sugars, which represent the majority of total carbohydrates and ultraprocessed foods in modern diets. However, focusing on carbohydrate restriction could paradoxically reduce the intake of healthful carbohydrate-containing foods such as fruits, legumes and minimally processed whole grains. Furthermore, a higher intake of red meats is associated with multiple adverse effects, notably worsened CVD outcomes and increased colon cancer risk.23-25
DIETARY PATTERNS
Modern evidence demonstrates that nutrient-focused metrics are limited in explaining the effects of diet on chronic diseases. Instead, cardiometabolic diseases largely are influenced by overall diet patterns and their constituent nutrients rather than single nutrients.26,27 Accordingly, the 2015-2020 Dietary Guidelines for Americans shifted recommendations away from a traditional focus on individual foods and nutrients and instead emphasized following “a healthy eating pattern across the lifespan,” asserting that “people do not eat food groups and nutrients in isolation but rather in combination, and the totality of the diet forms an overall eating pattern. The components of the eating pattern can have interactive and potentially cumulative effects on health. These patterns can be tailored to an individual’s personal preferences, enabling Americans to choose the diet that is right for them.”27
A healthy dietary pattern, per the 2015-2020 Dietary Guidelines for Americans, is high in vegetables, fruit, whole grains, legumes, nuts and seafood; can include moderate low- and non-fat dairy products; and is low in red and processed meats, sugar-sweetened foods and beverages, and refined grains.2,28 These diets are higher in fiber, vitamins, antioxidants, minerals, phenolics and unsaturated fats, and lower in glycemic index, glycemic load, salt and trans-fat. The most recent guidelines advocate three dietary patterns to prevent chronic disease: the Healthy U.S.-Style Eating Pattern, the Healthy Mediterranean- Style Eating Pattern and, for the first time, a Healthy Vegetarian-Style Eating Pattern.27 Each can be adapted based on cultural and personal preferences and all share many common food-based features.
Healthy US Style Eating Pattern
This pattern is based on the types and proportions of foods that Americans typically consume, but in nutrient-dense forms and recommended amounts. Specific recommendations have been made for saturated fat, sodium and added sugars: < 10% of calories from saturated fat; < 2,300 mg of sodium/day; and < 10% of calories from added sugars.27 The typical eating patterns currently consumed by many in the United States do not align with the dietary guidelines.
Compared to the described Healthy U.S.-Style Eating Pattern, about 75% of Americans are consuming a dietary pattern that is low in vegetables, fruits, dairy and oils. The guidelines advise shifting away from solid fats (i.e. butter, stick margarine, shortening, lard, coconut oil) to foods naturally containing oils (i.e. nuts, seafood). A shift toward vegetables, fruits and fat-free or low-fat dairy products also is recommended. More than half of the U.S. population exceeds recommendations for total grains and protein foods without meeting the recommendations for subgroups within these food groups. Increasing the variety of nutrient-dense protein food choices and ensuring that half of all consumed grains are whole grains is suggested. In addition, the majority of Americans exceed recommendations for saturated fat, sodium and added sugars.27 Average sodium intake is 3,440 mg a day for the U.S. population, and added sugars contribute an average of nearly 270 calories, or more than 13 percent of calories a day, which is approximately 70% above the proposed limit.27
Mediterranean-Style Dietary Patterns
The traditional Mediterranean and Dietary Approaches to Stop Hypertension (DASH) diets are the most well-studied dietary patterns. The traditional Mediterranean diet is characterized by a high intake of olive oil, fruits, vegetables (particularly green and root types), whole grains and nuts; a moderate intake of fish and poultry; and a very low intake of dairy products, red meat, processed meats and sweets.29,30 Total fat may be high (approximately 40% of total energy intake), and the monounsaturated-to-saturated fat ratio should be around 2. Starting with the Seven Countries Study of Cardiovascular Diseases (SCS),31 strong evidence from prospective cohort studies and randomized trials shows reduced CVD with the consumption of a higher-fat Mediterranean diet.32-34 The Lyon Diet Heart Study found regular consumption of alpha-linolenic acid (canola oil) significantly reduced cardiac deaths and nonfatal coronary heart disease (CHD),35 whereas the Prevención con Dieta Mediterránea (PREDIMED) randomized trial found that a Mediterranean diet (50 g/day extra virgin olive oil) reduced CVD events by 30% over a 5-year period(34). There is, however, the limitation that residual risk of CVD events is not eliminated by this dietary pattern.
The DASH diet emphasizes high intake of fruits, vegetables, low-fat dairy products, whole grains, nuts, fish and poultry, and discourages sugar-sweetened foods and beverages, red meat, and added fats. The result is a diet high in potassium, magnesium, calcium and fiber and lower in sodium. Compared with a typical Western diet, the DASH eating plan reduces blood pressure, total cholesterol, LDL-C and HDL-C without affecting triglycerides.36 Further cardiometabolic benefits are seen when part of the carbohydrate in DASH is replaced with either monounsaturated fat or protein, more closely resembling a Mediterranean pattern.37 A meta-analysis of three prospective cohorts (3,415 cases) found high adherence to DASH reduced diabetes risk by 27%.32
Both Mediterranean and DASH eating patterns improve a range of risk factors, reduce long-term weight gain and are consistently associated with a lower risk of clinical events.2,26,27,38 Pathways of benefit are broad, including effects on blood pressure, glucose-insulin homeostasis, lipid profiles and inflammation.2,27 Potential side effects of these diet patterns include a fishy aftertaste, gastrointestinal discomfort, and mercury exposure.39
Adapted from the Healthy U.S.-Style Pattern, the Guidelines’ Healthy Mediterranean-Style Pattern contains more fruits and seafood and less dairy to reflect eating patterns that have been associated with positive health outcomes in studies of Mediterranean-style diets.27 The amounts of oils, meats and poultry were not adjusted. The pattern is similar to the U.S. pattern in nutrient content, with the exception of lower calcium and vitamin D.
Plant-Based Diets
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Plant-based dietary patterns are becoming increasingly popular because of a variety of reported benefits to overall health and cardiovascular risk and disease, in particular.28,40 The concept of plant-based nutrition varies widely in definition, ranging from exclusion of all animal products to emphasizing increased fruits and vegetables, while including fish and poultry. Plant-based diets also are characterized by actual content, (e.g., semi-vegetarian [typical American diet with smaller portions or lower frequency of animal products], pescovegetarian [seafood with or without eggs and dairy], lacto- ovovegetarian [eggs and dairy], and vegan [no animal products] diets).
Randomized controlled trials and epidemiological studies indicate that plant- based diets are associated with significant improvement in CVD risk factors and events. The large, prospective Adventist Health Study 2 found that vegetarian and vegan dietary patterns were associated with less hypertension, obesity, type 2 diabetes and metabolic syndrome compared with omnivore patterns.41 A 12- year follow-up of the European Prospective Investigation into Cancer and Nutrition- Oxford (EPIC-Oxford) trial demonstrated that vegetarian diets were associated with lower ischemic heart disease risk compared with a nonvegetarian diet, a difference likely mediated by more favorable systolic blood pressures and lipid profiles.42 A recent large prospective cohort study of U.S. nurses and other healthcare professionals found that higher intake of animal protein (including processed red meat, unprocessed red meat, dair y, poultr y and eggs) was positively associated with mortality, whereas the inverse was true for high intake of plant protein.43 Another recent meta-analysis44 reported similar results, with vegetarians experiencing a 29% lower risk of CVD mortality relative to nonvegetarians. These findings emphasize the importance of protein source and support recommendations to increase plant protein intake. Lastly, whole-food plant- based nutrition has been shown to prevent progression of coronary atherosclerosis and reverse angiographic disease.45-47
The USDA Guidelines’ Healthy Vegetarian Pattern is adapted from the Healthy U.S.-Style Pattern, modifying amounts recommended from some food groups to more closely reflect eating patterns reported by self-identified vegetarians in the National Health and Nutrition Examination Survey (NHANES).27
Based on this, amounts of soy products (particularly tofu and other processed soy products), legumes, nuts and seeds, and whole grains were increased, and meat, poultry and seafood were eliminated. Dairy and eggs were included because they were consumed by the majority of these vegetarians. This pattern can be vegan if all dairy choices are comprised of fortified soy beverages (soymilk) or other plant-based dairy substitutes. The pattern meets similar nutrient standards as the U.S pattern but is somewhat higher in calcium and fiber and lower in vitamin D because of differences in the foods included.27
SHIFTING FORWARD
The global challenges of diet-related obesity and CVD present large health and economic burdens. Dietary patterns offer a promising approach for informing individual dietary guidance as well as national dietary guidance for chronic disease prevention. Although data support a remarkable consistency in the characteristics of a healthy dietary pattern across a wide range of disease outcomes, the evidence base for CVD risk reduction is particularly robust.
Across different study types, dietary patterns protective for CVD prominently feature fruits, vegetables, whole grains, nuts, legumes and, optionally, low-fat dairy and fish, and reduced consumption of red and processed meats. Many of these cardioprotective regimens are rich in fiber and potassium, and moderate consumption of alcohol and coffee can be included in a healthy dietary pattern. These diets typically are limited in their amounts of saturated fats, added sugars and sodium. Notably, there are no comparative randomized trials of exclusively whole-foods, plant-based diets versus partially plant-based diets, such as the Mediterranean or DASH diets. The results of such a study could be particularly impactful in terms of medical practice. Further, the common threads in nearly all of the healthful dietary patterns described include whole plant-based foods, and some suggest eating minimally processed and low- or no-fat dairy and shying away from red meats altogether. Finally, looking at life and health together, the negative environmental impact of many Western diets that are higher in meats, dairy and processed foods and oil also are quite high.48,49 Several large international scientific bodies now are suggesting a more plant-based dietary approach for environmental reasons, independent of their health outcomes.23,24
A healthy dietary pattern can be achieved in many ways and should be adapted to an individual’s food and cultural preferences. Large bodies of evidence imply that the dietary-pattern message is more complicated than simply prescribing a “low-fat” or “low-carbohydrate” diet.2 Improving the overall diet quality of individual patients will confer substantial, long-term reductions in chronic disease and improvements in quality of life.
Disclosure statement: Dr. Freeman has received honoraria from Boehringer-Ingleheim. Dr. Patel and Dr. Williams have no disclosures to report.
References are available here.
Article By:
Department of Medicine
Division of Cardiology
Rush University Medical Center
Chicago, IL
Department of Medicine
Division of Cardiology
National Jewish Health
Denver, CO
Department of Medicine
Division of Cardiology
Rush University Medical Center
Chicago, IL