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The Public Health Effects of Food Deserts: Workshop Summary 4 Diet and Health Evidence to Support Improved Food Access The interventions to improve food deserts center on increasing the intake of healthy foods. Those healthy foods include whole grains, fruits and vegetables, fat-free rather than whole milk, and drinking fewer calorically sweetened beverages. The excess availability of energy-dense snacks and fast foods in food deserts is a concern because both have been linked to obesity, and current interventions have attempted to increase the availability of healthy foods to mitigate those effects in food deserts; thus, presentations in this session addressed the possible public health outcomes of increasing healthy food intake. The speakers in this session focused on evidence-based health consequences of these changes in terms of obesity, cancer, and cardiovascular diseases. EFFECTS OF SELECTED DIETARY FACTORS ON OBESITY Richard Mattes, of Purdue University, stated that an increase in the consumption of healthy foods will not necessarily reduce body weight. In fact, only the case of reducing caloric beverage intake showed consensus on the link between change in diet and weight loss. Culture and learned associations often govern what people prefer to eat. He counseled caution in making the best choices about the interventions to pursue if the goal is reducing obesity. In short, there are no easy fixes.
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The Public Health Effects of Food Deserts: Workshop Summary Healthy Food and Changes in Weight Fruits and Vegetables Mattes suggested that the message in the media to “load up” on fruits and vegetables as a way to lower weight is misleading without considering overall energy intake. The Nurses Health Study, for example, which tracked almost 75,000 people over a 12-year period, showed that greater fruit and vegetable intake led to lower weight gain in women but not reduced weight for participants or for their children (He et al., 2004). Greater fruit and vegetable consumption alone will not reduce weight without the qualification to moderate energy intake. Whole Grains The next food category that Mattes discussed was whole versus refined grains. The line of reasoning behind encouraging consumption of whole grains is that they are higher in fiber and increase satiety, and therefore, people will eat less. Data from the Nurses Health Study indicate that greater intake of whole grain products was associated with reduced weight gain but provided little or no benefit for weight loss compared to consumption of refined grain products over the course of the 12-year study period (Liu et al., 2003). Other recent studies, both short and longer term, have shown similar results. Milk Drinking reduced-fat versus whole milk does not benefit weight management. Higher-income people purchase more low-fat milk and lower-income people purchase more whole milk, even when prices are the same, according to the Continuing Survey of Food Intake by Individuals (CSFII). The prevailing belief is that weight improves by switching to lower-fat dairy products. However, the Growing Up Today study (Berkey et al., 2005) and the National Health and Nutrition Examination Survey (Beydoun et al., 2008) actually show an increase in body mass index among children who drink fat-free and low-fat milk. This may reflect reverse causality in that heavier individuals choose lower-fat products to manage their weight, but it cannot be concluded that simply including lower-fat dairy products in the diet or substituting them for higher-fat products will promote weight loss.
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The Public Health Effects of Food Deserts: Workshop Summary Sweetened Beverages Whereas eating or drinking these healthier foods does not reduce weight, evidence is stronger that drinking caloric beverages has a detrimental effect. Consumption of sweetened beverages is now about 40 gallons per capita and has clearly gone up in concert with the rise in BMI and obesity in the population (see Figure 4-1). On average, Americans now get about 21 percent of their total energy intake from beverages, almost double the amount in 1965 (Duffey and Popkin, 2007). Beverages of all types seem to increase energy intake. In a study in which participants consumed various foods in liquefied and whole form, total energy intake was higher over the course of a day with the beverage form. The consumption of energy-yielding beverages seems to lead to a lack of dietary compensation, positive energy balance, and weight gain, although he acknowledged some controversy about whether there are sufficient data to move forward in terms of policy. Data specific to soft drink consumption from the Nurses Health Study showed that the weight FIGURE 4-1 Soft drink consumption, 1970-2000. SOURCE: USDA ERS, 2000. Reprinted with permission from Susan E. Swithers, Purdue University.
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The Public Health Effects of Food Deserts: Workshop Summary of individuals who drank nutritively sweetened beverages increased, while those who switched to nonnutritive “diet” drinks had a weight plateau (Schulze et al., 2004). EFFECTS OF SELECTED DIETARY FACTORS ON CARDIOVASCULAR DISEASE AND CANCER In contrast to obesity, research into the effects of diet on cardiovascular disease and cancer has shown a more positive link, said Frank Hu of Harvard University. Before discussing different categories of foods, he reviewed the hierarchy of evidence that nutrition researchers use in making clinical recommendations. The strength and consistency of evidence across different studies, biological plausibility, and responsive relationships are all needed to assess causal relationships between a food and a health outcome. Fats and Carbohydrates “Good” and “bad” fats have received much attention, with Americans encouraged to eat, for example, more healthy fats from plant-based oils and nuts rather than deep-fried food and stick margarine. The type of fat—the quality of the fats consumed, rather than total fat—has been shown to have a relationship to coronary heart disease (Hu et al., 1997) (see Figure 4-2). Fats have not generally increased breast cancer risk. There is fairly consistent evidence that higher consumption of red and processed meats is associated with increased risk of colorectal cancer, although it may not be due to the saturated fat in those products. The study of carbohydrates has shifted from classification by their chemical structure to a focus on glycemic index and glycemic load. In this paradigm, the greater the amount of refined carbohydrates and sugar, the higher the glycemic load. He reported on research that found a strong positive association between a high glycemic load diet and the risk of coronary heart disease (CHD), especially among overweight and obese individuals who are more insulin resistant (Liu, 2000). Plant-Based Foods A systematic review of nuts, fruits, vegetables, and whole grains consistently showed that higher consumption of these foods is significantly associated with decreased risk of both coronary heart disease and stroke (Hu and Willett, 2002). They have not been associated with overall reduced cancer mortality, but have shown benefits for some individual types of cancers, including mouth, lung, and stomach cancers. The World
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The Public Health Effects of Food Deserts: Workshop Summary FIGURE 4-2 Effect of types of fat on coronary heart disease. SOURCE: Hu et al., 1997. Cancer Research Fund concluded that some non-starchy fruits and vegetables may protect against specific cancers. Dairy Products Hu characterized dairy as a “much more complicated story,” with potential benefits and potential problems. Although they are a good source of many important nutrients, dairy products have also been associated with higher body weights among children and may increase risks of some hormone-related cancers. He distinguished between fat-free and whole milk. Replacing high-fat dairy with low-fat was associated with lower risk of CHD and Type 2 diabetes. Soft Drinks Hu concurred with Mattes about the problems of soft drinks. In addition to weight gain, the Nurses Health Study and other research has shown an association between soft drink consumption and the risk of diabetes and CHD. Limited evidence from that study and several others showed an association with pancreatic cancer, although those findings were not unanimous.
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The Public Health Effects of Food Deserts: Workshop Summary The Bottom Line Hu said the evidence is “pretty solid” that plant-based foods—including whole grains, fruits and vegetables, nuts, legumes, and healthy vegetable oils—are beneficial for cardiovascular disease (CVD) prevention. These foods are basically an indication of a high-quality diet. Diets high in saturated fat, trans fat, or refined sugars, including some starchy food, are detrimental for both diabetes and CVD. Sugar-sweetened beverages increase the risk of obesity, diabetes, and perhaps CVD. The findings are less specific about the link between diet and cancer. The recommendation of the World Cancer Research Fund focuses on body weight and physical activity because these are more important than individual foods and nutrients in terms of cancer prevention. DISCUSSION: HEALTH CONSEQUENCES Wendy Johnson-Askew, of the National Institute of Diabetes and Digestive and Kidney Diseases at the National Institutes of Health (NIH), moderated the discussion on health consequences of healthy foods and, particularly among the less healthy options, sweetened beverages. Understanding the science can help guide the policy-making process, by either encouraging or discouraging the intake of specific foods. Small Changes Johnson-Askew launched the session by suggesting to Mattes that even the slightly reduced intake caused by eating more fiber, as reported in the Nurses Health Study, may have some significance in the long term. Mattes replied that small imbalances do add up over time, but not to as great an extent as reported in the popular media. Because the initial weight gain also increases the energy needed to sustain that weight, the original small positive energy balance does not continue to increase weight gain. Along these lines, a participant noted that since the interest in weight stems from an interest in people’s health, maybe separating the two does not move the conversation further. Any action, said Mattes, has positives and negatives. So a particular dietary intervention may cause weight gain at the same time that it reduces the risk for certain chronic diseases. The role of scientists is to provide policy makers with this information so that they can make evidence-based decisions.
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The Public Health Effects of Food Deserts: Workshop Summary Healthy Eating One workshop participant suggested focusing on the Healthy Eating Index (HEI, developed by the U.S. Department of Agriculture), rather than BMI, as a proximal indicator of access to good food. Hu agreed that the revised HEI is a good measure of overall diet quality, and it is overall quality, rather than just individual nutrients, that contribute to our health. Mattes said from an obesity perspective, the time has come to abandon the idea that there is a single cause of obesity that a particular diet will correct. Just as it was once believed that a single treatment for cancer could be possible, it is now clear that obesity is caused by diet composition for some people, energy expenditure for others, and eating frequency or portion size for someone else. Mattes mentioned that more individualized interventions may be more appropriate for changing health outcomes. Another concern about more generalized recommendations is that some people have counterreactions to various changes: for example, about 15 percent of people with elevated cholesterol or blood pressure show an increase when they eat a fat-restricted or low-sodium diet, so this recommendation actually runs counter to this subgroup’s good health. Hu agreed that there may be no silver bullet for curing obesity, and the data suggest that diet quality is more important than a specific type of low-fat or low-carbohydrate diet. Johnson-Askew noted that the issue of BMI as a marker of health is under debate by her and her colleagues at NIH. Taxes and Subsidies Should soda and other sugar-sweetened beverages be taxed? Mattes said the issue from an obesity perspective needs to be more about caloric beverages in general, not just soft drinks. Although sweetened soft drinks and fruit drinks are the largest source of refined carbohydrates and thus are a good target, the probability of a positive energy balance is likely to be as great from consuming milk, sports drinks, sugary gourmet teas and coffees, or fruit juice. The issue stems from the medium in which the energy is derived. Beverages, for reasons still unknown, seem to escape regulatory mechanisms. Mattes noted that people do not reduce their food intake when they consume beverages with calories. Hu supported a soda tax for sweetened soft drinks, but not for diet soda, fruit juices, or other beverages for two reasons: (1) the evidence is more solid for sugar-sweetened beverages, and (2) these drinks are a clear and easily defined target. Given the situation with food deserts and the prices of many healthy foods, one participant wondered about the fairness of promoting foods that not everyone can afford. Hu acknowledged that the prices of fruits and vegetables have increased, which perhaps should be addressed on a
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The Public Health Effects of Food Deserts: Workshop Summary policy level. To make the consumption of healthier foods more affordable, government subsidies could be provided to specialty crops rather than to commodity crops such as corn. Employers perhaps could reward workers who follow a healthy diet and exercise regularly. Mattes said fairness is one reason he is not in favor of a soda tax, because it differentially isolates low-income populations, even though he recognizes the role of these beverages in the energy-balance problem. Snacks and Extra Calories Mattes said his review of the literature indicates that meal frequency, particularly snacking, may be a substantive contributor to weight gain. Americans are eating perhaps an extra half-meal or so per day, often a high-calorie snack. Hu said reducing soft drinks and unhealthy snacks are two main problems to address in improving health.