The 2019 ACC/AHA Guideline on the Primary Prevention of Cardiovascular Disease and the 2015-2020 Dietary Guidelines for Americans provided by the US Department of Health & Human Resources (HHS) and US Department of Agriculture (USDA) have provided evidence-based resources to inform management of patients at risk for atherosclerotic cardiovascular disease (ASCVD).^1,2 Both emphasize the importance of diet and exercise interventions to reduce ASCVD risk. Here we review the recommendations contained in these guidelines as well as the most recent literature on heart-healthy diets.

Carbohydrates
Carbohydrates can be classified as simple or complex and differ in how quickly they are digested and absorbed. Complex carbohydrates include whole grain breads, whole grain pastas, oatmeal and high fiber cereal. They have been associated with a reduced ASCVD risk, cancer risk, and all-cause mortality. Refined grains such as white breads, pastas, chips, and crackers have not demonstrated the same benefits and should be limited.³

Added sugars or syrups are a common dietary source of simple carbohydrates. Increased added sugar consumption has been associated with CVD mortality in a dose-dependent relationship.⁴ This does not include natural sugars found in fruits and milks. However, adults should be reminded that many fruit juices and smoothies contain added sugars.

Furthermore, many food items not traditionally considered sweet, such as tomato sauce, salad dressings, and even yogurt can contain significant amounts of added sugar. Thus, adults should be encouraged to read food labels. Many food labels are now specifying 'added sugar', which can aid the consumer in reducing added sugars. Several guidelines recommend consuming <25 grams of sugar a day.   

Fruits and Vegetables
Per the ACC/AHA and USDA, fruits and vegetables are the mainstays of a healthy diet. All vegetables and fruits have health benefits; however, leafy green vegetables and berry consumption (3 servings per week) have additional benefits.⁵

Starchy vegetables (white potatoes, corn, green peas) contain higher amounts of complex carbohydrates but are calorie-dense and should be consumed in moderation. Individuals should choose whole fruit over dried fruit since whole fruit is more filling, has lower sugar content (many dried fruits have added sugars) and is less calorically dense.⁵

Fats
Dietary fats can be categorized into three main categories: unsaturated (mono- and polyunsaturated), saturated, and trans-fats.⁶ Trans-fat can be found in a variety of dairy and meat products as well as many processed foods to provide longer shelf life (frozen pizza, fast food, crackers, cookies, cakes, canned frostings, potato chips). Trans-fats are strongly associated with adverse cardiovascular outcomes and should be minimized.⁵

Saturated fats are mostly found in dairy products and meats. Reduction of saturated fat has been associated with lower LDL-C levels⁷ and limited saturated fat intake should be encouraged. While the literature currently shows unclear associations between dairy products and cardiovascular health, lower saturated fat versions should be encouraged over full-fat and trans-fat containing products.⁵

Monounsaturated and polyunsaturated fats can be found in olives, nuts, seeds, oil, and fish. Interestingly, consumption of unsaturated fats has been inversely correlated with ASCVD risk, with replacement of saturated fats with calorically-equivalent forms of mono or polyunsaturated fats associated with a reduction in total mortality.⁸ Patients should be encouraged to consume unsaturated fats over saturated fats, albeit sparingly as unsaturated fat sources are still fairly calorically dense.

Protein
In terms of animal protein sources, current guidelines recommend moderate consumption of lean poultry and seafood over red meat, which contains higher levels of saturated and trans fats.^1,2 Processed meats such as deli/cold cuts, sausage, bacon, and dried meat products have been associated with the most severe adverse health outcomes and should be avoided.⁹ Fish is rich in omega-3 fatty acids (a type of polyunsaturated fat) and has been touted as a healthy source of animal protein.¹⁰

Overall, animal protein intake has been associated with higher cardiovascular mortality, while plant protein has been inversely correlated with cardiovascular mortality and all-cause mortality.¹¹ Thus, an emphasis should be placed on plant-based protein over animal proteins, and lean protein sources over those high in saturated and trans fats. Patients should be warned, however, that not all plant-based protein sources are created equal.

Of note, there has been a recent increase in the popularity of plant-based burgers and sausages from brands such as Impossible™ Foods and Beyond Meat - Go Beyond® that have similar levels of saturated fat to their animal-based counterparts. Patients should minimize consumption of these products due to their high fat content.

Diet Patterns
These basic principles are embodied in three diets associated with a decreased ASCVD risk, CHD, and mortality: The Mediterranean Diet, Dietary Approaches to Stop Hypertension (DASH) diet, and Healthy Vegetarian Diet (Table 1). We discuss these diets in more detail below.

Table 1: Current Nationally Endorsed Heart Healthy Dietary Patterns

Dietary Pattern	Includes	Restricts	Health Benefits	Key Differences
Dietary Approaches to Stop Hypertension (the DASH diet)17	Vegetables, fruits, low-fat dairy products, whole grains, lean meats, fish, poultry, fish, beans, and nuts	Sodium intake ≤2,300mg or ≤1,500mg per day.	Lower blood pressure Lower LDL cholesterol level Reduced cardiovascular disease risk	More emphasis on restricting sodium intake than other diets. Less emphasis on sea food.
The Mediterranean Diet12-15	Vegetables, fruits, nuts, legumes, whole grains, and extra-virgin olive oil, lean meats, fish, and poultry.	Limited red meat, processed meats, low-fat diary, and sweets intake.	Primary and secondary prevention of cardiovascular disease Reduced risk of cardiovascular disease mortality Reduced risk of myocardial infarction and stroke Reduced risk of all-cause mortality	More emphasis on nuts, fish, and olive oil. Less emphasis on dairy.
Healthy Vegetarian Eating Pattern3,20	Vegetables, fruits, whole grains, legumes, soy products, nuts, low-fat dairy products, and seeds.	All meats, poultries, and sea food.	Lower blood pressure Lower LDL cholesterol level Reduced cardiovascular disease risk	More emphasis on soy products, legumes, and dairy products. Lean protein is plant-based.

Mediterranean Diet
The Mediterranean diet focuses on 1) high leafy green vegetable intake, fruits, whole grains, nuts, legumes, and extra virgin olive oil; 2) moderate intake of fish, lean meats, low fat dairy, poultry; 3) low intake of red meat and sweets; 4) wine in moderation. This diet has been found beneficial for primary cardiovascular disease (CVD) prevention and is associated with reduced risk of ASCVD mortality and all-cause mortality.^12-14

There was recent concern about the methodology of the PREDIMED study, the first randomized controlled trial (RCT) showing the benefits of the Mediterranean diet on cardiovascular health, but a retracted and revised analysis showed similar results to the original study.⁹ This diet has also been shown to be beneficial for secondary prevention of cardiovascular disease and is associated with reduced risk of major adverse cardiovascular events such as recurrent myocardial infarction (MI) in patients with high-risk stable coronary artery disease.¹⁵

DASH Diet
The DASH diet is similar to the Mediterranean diet with the following notable differences: 1) greater allowance for dairy; 2) limit on sodium intake; 3) less emphasis on seafood. This diet was first proposed to combat hypertension and has been shown to reduce blood pressure in both hypertensive and normotensive individuals.¹⁶

The OmniHeart RCT in 2013 demonstrated a reduction of systolic blood pressure (SBP) by 5-6 mmHg, diastolic blood pressure (DBP) by 3 mmHg, and LDL-C levels by 11 mg/dL in patients on DASH diet compared to a typical American diet.¹⁷ The DASH diet has also been associated with decreased incidence of CVD and diabetes.¹⁸ The DASH diet is especially good for patients with hypertension and cardiovascular disease or risk factors.

Healthy Vegetarian Diet
This diet is similar to the DASH and Mediterranean diets but substitutes soy products, legumes, and nuts for meat, seafood, and poultry. Overall, the vegetarian diet has been associated with improvement in systolic blood pressure, LDL-C levels, and reduced CVD risk.^19-21 A common pitfall of this diet is the substitution of lean meats for refined grains, sweets, and sugar-sweetened juices and beverages.

One meta-analysis of 210,928 participants eating vegetarian diets showed that adherence to a more healthful vegetarian diet was inversely correlated with risk of developing coronary heart disease (CHD), whereas adherence to an unhealthy vegetarian diet (refined carbohydrates, added sugars, etc.) was correlated with increased risk of developing CHD.²² Thus, healthy food choices must still be emphasized in patients with a vegetarian diet.

Emerging Diet Trends
Here we will review three emerging diet trends that have gained popularity in recent years.

Ketogenic diet
The ketogenic diet emphasizes high fat and protein intake, while restricting carbohydrate consumption to as low as 20-50 g daily.²³ The diet is intended to induce a state of ketosis in the body, promoting fat loss. Furthermore, the high fat and protein content of the diet may promote satiety and decrease hunger.

Several meta-analyses have produced contrasting results of cardiovascular health benefits, weight loss, and long-term mortality.^24-26 The data is complicated by different total protein and fat compositions between studies, as well as the questionable degree of ketosis in some studies.²⁷ The impact of the ketogenic diet on cardiovascular health is still unclear.

Paleolithic Diet
Also known as the Paleo diet, this diet is based on eating foods that existed prior to human agricultural practices such as lean meat, fish, vegetables, fruits, roots, eggs, and nuts. Grains, legumes, dairy, salt, refined sugars and processed oils are avoided. Initial studies have shown improvements in waist circumference, triglyceride concentrations, HDL concentrations, and blood pressure, but larger, long-term studies are needed.²⁸

Intermittent Fasting
Intermittent fasting is the practice of alternating periods of normal food intake with periods of little to no caloric intake. For example, >60% caloric restriction on 2-3 days per week or on alternating days. A common weekly regimen is 5 days of normal eating with 2 days of restricted eating (400 calories per day).

Initial results have been promising, with several studies showing reduced LDL-C levels, decreased systemic inflammatory biomarkers, decreased systolic and diastolic blood pressure, and improved glycemic profile.²⁵ However, this diet may be challenging or even dangerous for those with hormonal imbalances, pregnancy, diabetes, or eating disorders.

Time Restricted Feeding
Time restricted feeding (TRF or time restricted eating, TRE) is another strategy that limits the daily period of food intake to 8-10 hours or less on most days of the week. A common daily regimen is 8 hours of healthy eating followed by 16 hours of fasting.

Animal studies of TRF have demonstrated reductions in body weight, cholesterol, triglycerides, glucose, insulin, IL-6, and TNF α, suggesting that TRF may have beneficial cardiometabolic effects.^29-31 TRF has been shown to be well tolerated in non-obese middle-aged adults. It seems like beneficial cardiometabolic effects are independent of weight change.³²

The evidence for these emerging diets is still limited and thus no major societal guidelines have endorsed these diets yet. Rigorous studies with larger sample sizes and longer follow-up periods are needed before more definite conclusions can be drawn.

Conclusions
The above principles and evidence-based diets provide a framework with which clinicians can counsel patients on their eating habits to reduce ASCVD risk. In addition to diet content, patients should also be educated on appropriate portion sizes and gradual caloric restriction if appropriate. Furthermore, patient lifestyles, goals, and barriers to care should be considered and addressed when formulating a diet plan.

Diets are only effective when they can be implemented and sustained by patients. Not all patients can afford to buy extra virgin olive oil and fish such as salmon regularly, but simple steps such as eliminating sugar-sweetened beverages and reducing fast food intake can be very effective. Patients should also not feel pressured to change their entire diet immediately and can adopt a stepwise approach.

Clinicians can discuss one or two concepts or changes per visit to avoid overwhelming patients with too much information at once. The choice of diet should also take into consideration patient comorbidities, socioeconomic status, and cultural preferences. Those with hypertension may benefit more from the low sodium DASH diet, while those who prefer vegetarianism should be counseled on healthy vegetarian options. In summary, clinician expertise should complement patient goals and resources to create a dietary plan for optimal health. Adults should be encouraged to read food labels and control portion sizes.

References

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Clinical Topics: Diabetes and Cardiometabolic Disease, Dyslipidemia, Prevention, Lipid Metabolism, Nonstatins, Diet

Keywords: Dyslipidemias, Cholesterol, LDL, Vegetables, Dietary Fats, Diet, Diet, Mediterranean, Lycopersicon esculentum, Blood Pressure, Caloric Restriction, Portion Size, Fabaceae, Pregnancy, Fast Foods, Poultry, Nuts, Fruit, Zea mays, Interleukin-6

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