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Effects of Fruits and Vegetables on Dietary Patterns and Energy Intakes
Adam Drewnowski, PhD
University of Washington
Obesity is a major health concern, linked to higher rates of digestive and pulmonary diseases, diabetes and cardiovascular disease, as well mortality from these conditions. In fact, there is a direct correlation between Body Mass Index (BMI) and disease and mortality: disease rates begin to climb slowly when BMI exceeds 25 and/or when body weight is 100-115% of ideal weight, then rises dramatically with BMI of 30 or greater and/or body weight more than 120% of ideal weight.
The increasingly energy-rich but nutrient-poor diet of Western societies is said to be responsible for the obesity epidemic. Researchers at the U.S. Department of Agriculture compared Americans’ actual intakes from the various food groups to intakes recommended in the Food Guide Pyramid and found the most glaring discrepancy to be at the top of the pyramid: Americans consume an average 34 teaspoons of added sugars daily versus the six to18 recommended (for diets of1,600 to 2,800 calories) and 64 grams of added fat. The Dietary Guidelines suggest that Americans limit fat intake to 30% of daily calories, which translates to a range of 53-73 grams of total fat from all sources, both added and naturally occurring. An intake of 64 grams of added fat daily is already at 97% of this upper limit. Between 1970 and 2000, the average American increased consumption of flour and cereal, added sugars and added fats by about 100 pounds per year. Of this, approximately 30 pounds per person per year is added sugar in the form of corn sweeteners, which have displaced cane and beet sugars as the dominant sweetener added to processed foods.
In the protein category, Americans are eating more poultry and fish, less red meat and fewer eggs, but more protein foods overall. Fruit consumption has increased slightly but is still only half the recommended intake and is lacking in variety. Out of 60 fruits monitored, six accounted for 51% of all fruit servings: orange juice, bananas, apple juice, grapes and watermelon. Vegetable consumption has increased slightly but also lacks variety: iceberg lettuce, frozen potatoes and potato chips accounted for 33% of all vegetable servings in 2000. Consumption of dark green leafy and deep yellow vegetables combined was only 0.4 servings per capita per day, well below the 1.3 servings suggested for a 2,200-calorie diet.
The quality of diets, as measured by the Healthy Eating Index (HEI), correlates directly with both income and education, and the rate of obesity correlates with diet quality. Another analysis by USDA researchers showed that high-energy, low-nutrient-dense foods consistently cost less than more prudent choices, which provides at least a partial explanation for the correlation between lower income and obesity.
The 2005 Dietary Guidelines for Americans recommend that consumers give priority to nutrient-dense foods, that is, foods with a high nutrient-to-energy ratio, with the remaining discretionary calories being consumed as energy needs allow. The most desirable nutrient dense foods are those that are rich sources of vitamins, minerals, fiber and phytochemicals per calorie. Because of their low energy density and high nutrient values, vegetables and fruits fulfill these criteria. The World Health Organization (WHO) and others have suggested that one way to reduce the energy density of the diet is by increasing consumption of fruits and vegetables. If nutrient density could be calculated and scored in a convenient way for use on food labels and in health claims, food selection could be based on a nutrients-to-calorie ratio as an alternative to a specific minimum amount of a nutrient per serving size. Such a calculation, however, must also relate to food costs.
As yet there are no universally agreed-upon measures of nutrient density. There are no criteria for specifying which nutrients or in what amounts define a nutrient-dense food or beverage. Attempts to evaluate nutrient density have been based on calorie-to-nutrient scores, nutrient-to-nutrient ratios and nutrient-per-calorie indices. None of these approaches has been entirely successful.
One new approach, the Naturally Nutrient Rich or NNR score, is based on mean percent daily values for 16 nutrients including fiber. It can be used to rank foods along a dimension of nutrient density and to calculate nutrient adequacy and nutrient density scores, as well as a nutrient-to-price ratio. This approach has major implications for regulatory agencies and food labeling, for implementation of nutritional policies in schools, and for consumer education. The NNR approach would allow consumers to select nutrient-dense foods first and monitor the nutrients-to-calorie ratio. Still, a number of issues remain to be resolved:
- Should the score favor hard-to-get nutrients?
- Should it be based on a 2000-calorie diet, or on 100-gram edible portions, or on standard servings sizes?
- How will it account for bioavailability?
- Should there be separate scores for natural versus fortified foods?
- Should there be negative points for total fat, saturated fat or dietary cholesterol content?
- Should there be negative points for added sugar?
- How should it score nutrients for which there is no DV, such as phytonutrients, carotenoids or other antioxidants?
Work continues on this approach as a potentially valuable tool for dietary guidance and nutrition education. The end goal is to help people select naturally nutrient-rich foods so as to make every calorie count. The NNR can be linked to other measures of diet quality to help people achieve and maintain healthy weight. A nutrient-dense diet that maintains healthy weight may be one and the same with a “digestive health diet.”
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