Diet and Chronic Disease

Diet and Chronic Disease

Poor diet is a major contributor to the leading causes of chronic disease and death in the United States, including coronary heart disease, diabetes, hyperlipidemia, and stroke.^[1] In addition, recent data from the National Health and Nutrition Examination Survey show that more than 30% of adults in the United States are obese and that the prevalence of overweight and obesity combined is approximately 70%.^[2] Lifestyle factors, especially healthy dietary practices that influence these trends, have a low level of adherence. Although it is true that the obesity and chronic disease epidemics have complicated origins, the contribution of our current society’s easy access to highly processed foods and low consumption of fresh and whole foods cannot be overemphasized. At the turn of the millennium, the Centers for Disease Control and Prevention’s Behavioral Risk Factor Surveillance Survey showed that less than 25% of Americans consumed the recommended amount of fruits and vegetables and equally few were as physically active as was recommended. More recent studies have not shown much improvement in the low prevalence of healthy lifestyle practices.^[3,4]

In spite of this low level of adherence, a survey of adults in the United States showed that only one third reported receiving dietary advice from a physician on increasing consumption of fruits and vegetables.^[5] Moreover, studies have shown that clinicians’ knowledge and counseling about healthy diets are lacking.^[6,7] In the recent past, numerous diets have been developed to combat weight gain and other disease risk factors, with varying degrees of evidence to show benefit. This article highlights 2 diets, each of which has a robust body of supporting literature: the Dietary Approaches to Stop Hypertension (DASH) diet and the Mediterranean diet.^[8] This article will help clinicians improve their knowledge of healthy diets by reviewing key diets and the dietary components that have been shown to prevent chronic disease.

The DASH Diet

The Dietary Guidelines for Americans were updated in 2010 and the DASH diet was highlighted as a guideline for healthy eating.^[9] DASH has as its core the increased consumption of fresh fruits and vegetables and low-fat dairy products. Fruits and vegetables are important because of the low-caloric and high-nutrient density of such foods and the satiating effects of fiber. Low-fat dairy consumption provides calcium that is needed for bone health and other health outcomes. Other components of DASH include nuts, legumes (dried beans and peas), whole grains, and limiting salt and added sugar intake.

Nuts are rich in beneficial mono- and polyunsaturated fats and are high in fiber and protein. Legumes are a rich source of protein as well as fiber, and lean meats are important sources of protein. Grains provide an important source of energy and should be consumed mostly as whole grains, which will increase fiber consumption. The DASH diet also limits salt to 2300 milligrams per day and added sugar intake to 5 tablespoons per week for the average 2000-calories/day diet. Table 1 shows the recommended intakes of each of these food groups in servings per day for those who consume 2000 calories per day.

Table 1. DASH Diet Food Group Servings for a 2000-Calories-Per-Day Diet

Adapted from the 2010 Dietary Guidelines for Americans.^[9]

DASH was originally developed and studied as a diet to reduce high blood pressure in a randomized controlled trial published in The New England Journal of Medicine ^[10] in 1997. In this study, a diet rich in fruits, vegetables, and low-fat dairy products and low in saturated fat was compared with a control diet and shown to result in substantial reductions in blood pressure. All of the diets had the same 3 grams per day of sodium. After 8 weeks, the systolic blood pressure of the participants assigned to the DASH diet was, on average, 5.5 mm Hg lower and diastolic blood pressure was 3 mm Hg lower compared with participants on the control diet.

Subsequent prospective observational studies have shown the DASH diet to have a number of other beneficial health effects. An analysis of the Nurses’ Health Study showed that women who were in the highest quintile of consumption of a DASH-style diet and also had low consumption of processed meats and sugar-sweetened beverages had lower risk for coronary heart disease over almost 25 years of follow-up.^[11] Other studies have shown beneficial effects in colorectal adenoma, metabolic syndrome, congestive heart failure, and obesity.^[12-14]

One easy way to counsel patients on the basic message of the DASH diet is to describe and encourage the use of the healthy plate method. In 2011, the US Department of Agriculture introduced this diet counseling tool that replaced the food pyramid to make healthy food messages easier to deliver (Figure). The tool shows an easy-to-understand message on consuming the 5 food groups in a healthy way by portion size in relation to a typical dinner plate. Fruit and nonstarchy vegetables should take up one half of the plate; whole grain carbohydrates and lean protein one quarter each. Drinks should be primarily water or low-fat dairy products.^[15]

Figure. The Healthy Plate.

The Mediterranean Diet

The Mediterranean diet is a food plan that came from the olive-growing countries bordering the Mediterranean Sea (Greece, Spain, and Italy) in the 1960s. A landmark ecological study that began in the 1950s was important to the origins of this eating pattern’s health benefits and showed that mortality rates in selected countries near the Mediterranean Sea were substantially lower compared with those in westernized countries, such as the United States and Great Britain.^[16] In 1993, the dietary pattern was defined at the International Conference on the Diets of the Mediterranean as being characterized by high consumption of olive oil, legumes, unrefined grains, fruits, and vegetables; moderate consumption of dairy products; moderate-to-high consumption of fish; low consumption of meat; and moderate consumption of alcohol. In early studies, the cardiovascular benefits of this eating plan were thought to be primarily from its lower saturated fat content.^[17] It was later understood that the health benefits with respect to fat intake were not only the result of the low saturated fat content but also from the high content of beneficial fats. Monounsaturated fats, found in high levels in olive oils, and polyunsaturated fats, found in both fatty fish and nuts, were key reasons for the beneficial health effects of the diet (Table 2).

Table 2. Mediterranean Diet

Protection from a number of disease outcomes has been correlated with the Mediterranean eating pattern. Prospective observational studies in both European and US populations have shown that higher adherence to a Mediterranean food plan is associated with lower all-cause mortality, coronary heart disease, and cancer mortality. Those who more closely adhered to a Mediterranean dietary pattern had a higher monounsaturated-to-saturated fat ratio; higher consumption of vegetables, fruits, whole grains, and fish; and lower intake of red and processed meats. Moreover, specific intake levels of alcohol currently considered low to moderate were part of the diet. For men, this was 1-4 drinks per day and for women, it was one third to 2 drinks per day. Of interest, it was the overall dietary pattern that was linked with lower mortality rather than the specific food groups that make up the eating plan. It is possible that the synergy between different healthy food groups partly accounts for the benefits.^[18,19]

A Primer on Fats

An understanding of the different types of dietary fat is critical to giving well-informed dietary counsel. Advice on the intake of dietary fat has a checkered history. As recently as the 1990s, diets low in total fat were very popular as a means to better health and resulted in the development of many commercial nonfat and low-fat products that were high in processed carbohydrates — a combination that did not prove to be healthy.

We know now that a more nuanced approach to fat intake is necessary to take advantage of the health benefits from certain types of fats — monounsaturated and polyunsaturated — while avoiding the more harmful fats, saturated and trans-unsaturated fat. Saturated fat is the principal dietary contributor to low-density lipoprotein (LDL) cholesterol and is abundant in high-fat animal products including nonlean cuts of beef and full-fat dairy products. Minimizing intake of saturated fat is key to a healthy diet, and intervention studies have shown that doing so can dramatically reduce markers of disease such as LDL cholesterol or coronary stenosis.^[20]

Replacing saturated fat with monounsaturated and polyunsaturated fats rather than eliminating fat intake altogether is preferred and results in reductions in cardiovascular disease risk as well as other disease outcomes such as diabetes.^[21] Foods, such as olive and canola oils, avocados, and certain nuts, such as almonds and cashews, are high in monounsaturated fats. Monounsaturated fat has been shown to lower LDL cholesterol, insulin resistance, and thrombotic potential. Foods that are high in omega-3 polyunsaturated fat include fatty fish (such as salmon) and certain nuts (such as walnuts). Omega-6 polyunsaturated fat is found in safflower, sunflower, and corn oil and may lower healthy high-density lipoprotein (HDL) cholesterol, although its overall effect is probably beneficial. Health benefits include lowering LDL cholesterol, triglycerides, and platelet aggregation as well as reducing coronary heart disease-related death. A recent intervention study on newly diagnosed patients with diabetes showed that patients who followed a Mediterranean-style diet (which included increased intake of poly- and monounsaturated fats in place of carbohydrates) had a markedly lower need for diabetic medications than those who consumed an American Heart Association low-fat diet. Those who were on the Mediterranean diet consumed at least 30% of calories as fat, primarily from olive oil, which is high in monounsaturated fat.^[22]

Trans Fat Enters, and Exits, the Dietary Scene

Trans fat is the most unhealthy of the dietary fats and its source is primarily manufactured. Trans fat is created by partial hydrogenation of vegetable oils, a process first used in the early 1900s to increase the stability of certain oils. In 1911, Crisco was the first product made with trans fat, and in the 1950s trans fat was used increasingly in manufactured food products owing to its ability to increase the shelf life of store-bought foods. By the 1970s its use in margarine was promoted as a healthier alternative to the saturated fat in butter, and not until the 1980s-1990s were its health hazards fully appreciated. Trans fat has been shown to increase LDL cholesterol, reduce HDL cholesterol, and play a role in inflammation. It is found in fried foods as well as in many store-bought cookies and crackers. Trans fat received recent press attention when it was banned in New York City restaurants and other cities soon followed, resulting in its removal from products of major national food chains such as McDonald’s and Arby’s.^[23] Risk models have shown that its elimination in the food supply and its replacement with healthy fats could substantially reduce the burden of heart disease.^[24]

Although it is difficult to measure our daily intake of types of fat by counting percentages, it is useful to counsel patients to read the Nutrition Facts labels on manufactured foods. Advise patients to avoid all food with the ingredient “partially hydrogenated oil,” even if the Nutrition Facts label reports 0 grams of trans fat. The Nutrition Facts label is permitted to list 0 grams of trans fat as long as the product contains less than 0.5 gram per serving. Trans fat could be inadvertently consumed in significant levels if enough of these less-than-0.5 gram-per-serving foods are consumed, and this consumption may be harmful even in small amounts. Also advise consuming foods very low in saturated fat and with higher amounts of mono- and polyunsaturated fats.

What to Do in the Office

The busy primary care provider may ask whether it is useful to perform in-office nutritional counseling. The US Preventive Services Task Force addressed this question in 2003 and concluded that the evidence was insufficient to show benefit of primary care providers giving dietary counseling in unselected populations. However, the report stated that low-intensity counseling of primary care patients, such as repeated 5-minute counseling sessions coupled with patient education materials, is promising, and that more evidence is needed to assess the long-term efficacy. In addition, evidence shows the benefits of intensive dietary counseling in patients with hyperlipidemia and other risk factors for cardiovascular and diet-related chronic diseases. Intensive dietary counseling has been shown to produce significant changes in daily intake of important components of a healthy diet, including fruits, vegetables, and healthy fats.^[25]

The 24-hour recall can be a useful tool to get an idea of a patient’s nutritional patterns. It can be done during the office visit, often in less than 10 minutes, by reviewing in detail each of the patient’s meals and snacks over the past 24 hours. One strategy is to start by asking the patient to describe his or her most recent meal or snack, and then to go backwards until the previous 24 hours are covered. After each meal or snack is described, it is important to ask if the patient consumed anything else because food items are often forgotten until prompted (eg, “What did you have for dessert?”). Remember to ask specifically what beverages were consumed during and between meals. Without prompting, patients may not report beverage intake, and sugar-sweetened beverages can be a substantial contributor to total calories. Replacing them with water can lead to important reductions in caloric intake.^[26]

In addition to the 24-hour recall, a number of validated tools can be used to assess a patient’s diet. Although the gold standard for nutritional assessment is either the multiday food diary or the food frequency questionnaire, these detailed and lengthy tools are often used in nutritional research and are not practical to use in the typical primary care office setting. As a result, a number of authors have developed tools^[27] designed specifically for dietary assessment in primary care settings. Many of the tools focus on specific aspects of the diet, such as intake of fruits and vegetables, dietary fat, and fiber. Most have a low response burden and can be administered in fewer than 10 minutes, making them practical for a busy office practice. Although these tools may not be as specific as the food frequency questionnaire or food diary, they are often able to differentiate between healthy and unhealthy eating, directing providers towards patients who would benefit from dietary counseling.^[27]

The following resources provide a number of validated options for in-office nutritional assessments as well as more information about the diets discussed earlier.

Online Resources

USDA Dietary Guidelines for Americans, 2010: Appendix 10 — The DASH Eating Plan at Various Calorie Levels

USDA: Choose My Plate.gov: Food Groups