Chronic Disease Prevention
In this section, we briefly review dietary and lifestyle changes that reduce the incidence of chronic disease. The potential magnitude of benefit is also discussed.
Recommended Lifestyle Changes
Specific changes in diet and lifestyle and likely benefits are summarized in table 44.1. These relationships and supporting evidence are summarized here.
Avoid Tobacco Use
Avoidance of smoking by preventing initiation or by cessation for those who already smoke is the single most important way to prevent CVD and cancer (chapter 46). Avoiding the use of smokeless tobacco will also prevent a good deal of oral cancer.
Maintain a Healthy Weight
Obesity is increasing rapidly worldwide (chapter 45). Even though obesity—a body mass index (BMI) of 30 or greater—has received more attention than overweight, overweight (BMI of 25 to 30) is typically even more prevalent and also confers elevated risks of many diseases. For example, overweight people experience a two- to threefold elevation in the risks of CAD and hypertension and a more than tenfold increase in the risk of type 2 diabetes compared with lean individuals (BMI less than 23) (Willett, Dietz, and Colditz 1999). Both overweight and obese people also experience elevated mortality from cancers of the colon, breast (postmenopausal), kidney, endometrium, and other sites (Calle and others 2003).
Many people with a BMI of less than 25 have gained substantial weight since they were young adults and are also at increased risk of these diseases, even though they are not technically overweight (Willett, Dietz, and Colditz 1999). For example, in rural China, where the average BMI was less than 21 for both men and women, F. B. Hu and others (2000) found that the prevalence of hypertension was nearly five times greater for those with a BMI of approximately 25 than for the leanest people. Because many Asians are experiencing adverse consequences of excess body fat with a BMI of less than 25, the definition of overweight for Asia has recently been expanded to include a BMI of 23 to 25 (WHO 2000). For most people, unless obviously malnourished as an adolescent or young adult, bodyweight should ideally not increase by more than 2 or 3 kilograms after age 20 to maintain optimal health (Willett, Dietz, and Colditz 1999). Thus, a desirable weight for most people should be within the BMI range of 18.5 to 25.0, and preferably less than 23.
Additional valuable information can be obtained by measuring waist circumference, which reflects abdominal fat accumulation. In many studies, waist circumference is a strong predictor of CAD, stroke, and type 2 diabetes, even after controlling for BMI (Willett, Dietz, and Colditz 1999). A waist circumference of approximately 100 centimeters for men and 88 centimeters for women has been used as the criterion for the upper limit of the healthy range in the United States, but for many people this extent of abdominal fat would be far above optimal. Because abdominal circumference is easily assessed, even where scales may not be available, further work to develop locally appropriate criteria could be worthwhile. In the meantime, increases of more than 5 centimeters can be used as a basis for recommending changes in activity patterns and diet.
Views about the causes of obesity and ways to prevent or reduce it have been controversial. Diets low in fat and high in carbohydrates were believed to limit caloric intake spontaneously and thus to control adiposity, but such diets have not reduced bodyweight in trials that have lasted for a year or more (Willett and Leibel 2002). Some researchers have suggested that diets with a high energy density, referring to the amount of energy per volume, offer an alternative explanation for the observed increases in obesity (Swinburn and others 2004), but long-term studies have not examined this theory. Sugar-sweetened beverages contribute significantly to the overconsumption of calories, in part because calories in fluid form appear to be poorly regulated by the body (E. A. Bell, Roe, and Rolls 2003). In children, an increase in soda consumption of one serving per day was associated with an odds ratio of 1.6 for incidence of obesity (Ludwig, Peterson, and Gortmaker 2001), and in a randomized trial, replacement of a standard soda with a zero-calorie diet soda was associated with significant weight loss (Raben and others 2002). Reductions in dietary fiber and increases in the dietary glycemic load (large amounts of rapidly absorbed carbohydrates from refined starches and sugar) may also contribute to obesity (Ebbeling and others 2003; Swinburn and others 2004).
Aspects of the food supply unrelated to its macronutrient composition are also likely to be contributing to the global rise in obesity. Inexpensive food energy from refined grains, sugar, and vegetable oils has become extremely plentiful in most countries. Food manufacturers and suppliers use carefully researched methods to make products based on these cheap ingredients maximally convenient and attractive.
Maintain Daily Physical Activity and Limit Television Watching
Contemporary life in developed nations has markedly reduced people's opportunities to expend energy, whether in moving from place to place, in the work environment, or at home (Koplan and Dietz 1999). Dramatic reductions in physical activity are also occurring in developing countries because of urbanization, increased availability of motorized transportation to replace walking and bicycle riding, and mechanization of labor. However, regular physical activity is a key element in weight control and prevention of obesity (IARC 2002; Swinburn and others 2004). For example, among middle-aged West African women, more walking was associated with a three-unit lower BMI (Sobngwi, Gautier, and Mbanya 2003), and in China, car owners are 80 percent more likely to be obese (Hu 2002).
In addition to its key role in maintaining a healthy weight, regular physical activity reduces the risk of CAD, stroke, type 2 diabetes, colon and breast cancer, osteoporotic fractures, osteoarthritis, depression, and erectile dysfunction (table 44.1). Important health benefits have even been associated with walking for half an hour per day, but greater reductions in risk are seen with longer durations of physical activity and more intense activity.
The number of hours of television watched per day is associated with increased obesity rates among both children and adults (Hernandez and others 1999; Ruangdaraganon and others 2002) and with a higher risk of type 2 diabetes and gallstones (F. B. Hu, Leitzmann, and others 2001; Leitzmann and others 1999). This association is likely attributable both to reduced physical activity and to increased consumption of foods and beverages high in calories, which are typically those promoted on television. Decreases in television watching reduce weight (Robinson 1999), and the American Academy of Pediatrics recommends a maximum of two hours of television watching per day.
Eat a Healthy Diet
Medical experts have long recognized the effects of diet on the risk of CVD, but the relationship between diet and many other conditions, including specific cancers, diabetes, cataracts, macular degeneration, cholelithiasis, renal stones, dental disease, and birth defects, have been documented more recently. The following list discusses six aspects of diet for which strong evidence indicates important health implications (table 44.1). These goals are consistent with a detailed 2003 World Health Organization (WHO) report (WHO and FAO 2003).
Replace saturated and trans fats with unsaturated fats, including sources of omega-3 fatty acids. Replacing saturated fats with unsaturated fats will reduce the risk of CAD (F. B. Hu and Willett 2002; Institute of Medicine 2002; WHO and FAO 2003) by reducing serum low-density lipoprotein (LDL) cholesterol. Also, polyunsaturated fats (including the long-chain omega-3 fish oils and probably alpha-linolenic acid, the primary plant omega-3 fatty acid) can prevent ventricular arrhythmias and thereby reduce fatal CAD. In a case-control study in Costa Rica, where fish intake was extremely low, the risk of myocardial infarction was 80 percent lower in those with the highest alpha-linolenic acid intake (Baylin and others 2003). Intakes of omega-3 fatty acids are suboptimal in many populations, particularly if fish intake is low and the primary oils consumed are low in omega-3 fatty acids (for example, partially hydrogenated soybean, corn, sunflower, or palm oil). These findings have major implications, because changes in the type of oil used for food preparation are often quite feasible and not expensive.
Trans fatty acids produced by the partial hydrogenation of vegetable oils have uniquely adverse effects on blood lipids (F. B. Hu and Willett 2002; Institute of Medicine 2002) and increase risks of CAD (F. B. Hu and Willett 2002); on a gram-for-gram basis, both the effects on blood lipids and the relationship with CAD risk are considerably more adverse than for saturated fat. In many developing countries, trans fat consumption is high because partially hydrogenated soybean oil is among the cheapest fats available. In South Asia, vegetable ghee, which has largely replaced traditional ghee, contains approximately 50 percent trans fatty acids (Ascherio and others 1996). Independent of other risk factors, higher intakes of trans fat and lower intakes of polyunsaturated fat increase risk of type 2 diabetes (F. B. Hu, van Dam, and Liu 2001).
Ensure generous consumption of fruits and vegetables and adequate folic acid intake. Strong evidence indicates that high intakes of fruits and vegetables will reduce the risk of CAD and stroke (Conlin 1999). Some of this benefit is mediated by higher intakes of potassium, but folic acid probably also plays a role (F. B. Hu and Willett 2002). Supplementation with folic acid reduces the risk of neural tube defect pregnancies. Substantial evidence also suggests that low folic acid intake is associated with greater risk of colon—and possibly breast—cancer and that use of multiple vitamins containing folic acid reduces the risk of these cancers (Giovannucci 2002). Findings relating folic acid intake to CVD and some cancers have major implications for many parts of the developing world. In many areas, consumption of fruits and vegetables is low. For example, in northern China, approximately half the adult population is deficient in folic acid (Hao and others 2003).
Consume cereal products in their whole-grain, high-fiber form. Consuming grains in a whole-grain, high-fiber form has double benefits. First, consumption of fiber from cereal products has consistently been associated with lower risks of CAD and type 2 diabetes (F. B. Hu, van Dam, and Liu 2001; F. B. Hu and Willett 2002), which may be because of both the fiber itself and the vitamins and minerals naturally present in whole grains. High consumption of refined starches exacerbates the metabolic syndrome and is associated with higher risks of CAD (F. B. Hu and Willett 2002) and type 2 diabetes (F. B. Hu, van Dam, and Liu 2001). Second, higher consumption of dietary fiber also appears to facilitate weight control (Swinburn and others 2004) and helps prevent constipation.
Limit consumption of sugar and sugar-based beverages. Sugar (free sugars refined from sugarcane or sugar beets and high-fructose corn sweeteners) has no nutritional value except for calories and, thus, has negative health implications for those at risk of overweight. Furthermore, sugar contributes to the dietary glycemic load, which exacerbates the metabolic syndrome and is related to the risk of diabetes and CAD (F. B. Hu, van Dam, and Liu 2001; F. B. Hu and Willett 2002; Schulze and others 2004). WHO has suggested an upper limit of 10 percent of energy from sugar, but lower intakes are usually desirable because of the adverse metabolic effects and empty calories.
Limit excessive caloric intake from any source. Given the importance of obesity and overweight in the causation of many chronic diseases, avoiding excessive consumption of energy from any source is fundamentally important. Because calories consumed as beverages are less well-regulated than calories from solid food, limiting the consumption of sugar-sweetened beverages is particularly important.
Limit sodium intake. The principle justification for limiting sodium is its effect on blood pressure, a major risk factor for stroke and coronary disease (chapter 33). WHO has suggested an upper limit of 1.7 grams of sodium per day (5 grams of salt per day) (WHO and FAO 2003).
Potential of Dietary and Lifestyle Factors to Prevent Chronic Diseases
Several lines of evidence indicate that realistic modifications of diet and lifestyle can prevent most CAD, stroke, diabetes, colon cancer, and smoking-related cancers. Less progress has been made in identifying practically modifiable causes of breast and prostate cancers.
One line of evidence is based on declines in CAD in countries that have implemented preventive programs. Rates of CAD mortality have been cut in half in several high-income countries, including Australia, the United Kingdom, and the United States. The most dramatic example is that of Finland (box 44.2).
Other evidence derives from randomized intervention studies. These often have serious limitations for estimating the potential magnitude of benefits, because typically only one or a few factors are modified, durations are usually only a few years, and noncompliance with lifestyle change is often substantial. Nevertheless, some examples are illustrative of the potential benefit. In two randomized studies among adults at high risk of type 2 diabetes, those assigned to a program emphasizing dietary changes, weight loss, and physical activity experienced only half the risk of incident diabetes (Knowler and others 2002; Tuomilehto and others 2001). The Lyon Heart Study, conducted among those with existing heart disease, found a Mediterranean-type diet high in omega-3 fatty acids reduced recurrent infarction by 70 percent compared with an American Heart Association diet (de Lorgeril and others 1994).
A third approach is to estimate the percentage of disease that is potentially preventable by reducing multiple behavioral risk factors using prospective cohort studies. Among U.S. adults, more than 90 percent of type 2 diabetes, 80 percent of CAD, 70 percent of stroke, and 70 percent of colon cancer are potentially preventable by a combination of nonsmoking, avoidance of overweight, moderate physical activity, healthy diet, and moderate alcohol consumption (Willett 2002).
Collectively, these findings indicate that the low rates of these diseases suggested by international comparisons and time trends are attainable by realistic, moderate changes that are compatible with 21st-century lifestyles.