Nutrition and Calorie Consumption
NUTRITION AND CALORIE CONSUMPTION
Net nutrition (diet in relation to claims made on food intake by basal metabolism, physical activity, and disease) is an important influence on overall health. Poor nourishment impairs child growth and development, which in turn increases the risk of mortality, raises morbidity, and reduces physical capacity. These consequences are not limited to the growing years, and adversely affect adults who had poor nutritional experiences as children. Numerous debates and controversies surround the study of nutrition. These controversies historically have been fed by a lack of scientific knowledge, by varying goals and methods for studying nutritional needs, and by the confusion arising from simplifying complex material for a wide audience.
A brief history of the evolution of approaches to calibrating requirements for human nutrition can illuminate some of the central issues around dietary standards. The nutritional values of certain foods–such as limes in combating scurvy–have been known for over two centuries, but it was food shortages associated with World War I that established the need for dietary standards in planning food shipments. In the 1930s the League of Nations issued a series of reports on nutrient needs according to age, sex, and activity patterns; these were later drawn on in responding to the food crises created by World War II. Beginning in the late 1940s the United Nations, the League's successor, coordinated a program of nutritional requirement reports produced by individual countries. These reports were often local adaptations of Food and Agriculture Organization/World Health Organization recommendations, but they varied widely on some components.
Research on dietary standards began with tabulations of average intakes of nutrients thought essential to life among "healthy" people. Over time this list expanded from items such as energy and carbon to include protein, iron, zinc, and a host of other ingredients. Scientific studies that varied or observed these intakes across people, looking for readily observable effects on health (such as growth failure or specific signs of disease), found both systematic patterns and also considerable differences in individual needs. Biochemical markers later provided a means to identify subclinical deficiencies.
An important question to ask of dietary recommendations is how they are to be used. The standards may differ widely depending on whether they are used to implement a program of organizing food shipments to address a crisis; to protect against obvious deficiency diseases in the vast majority of a population that is relatively sedentary; or to insure against any subclinical deficiencies for all people in a physically active group.
Another area of concern in the field of nutrition is assessment, or measuring dietary intake in relation to dietary standards. Several approaches historically have been used, but each has limitations. Under the "disappearance" method, human consumption is a residual calculated as the supply of food (production, plus beginning stocks, plus imports) minus utilization (the sum of exports, ending stocks, nonfood uses, feed, spoilage, and seed). This method ignores the unequal distribution of food across regions, families, and individuals, and so malnutrition may exist even though per capita amounts are adequate. In addition, methods of preservation and cooking affect food's nutritional value. These changes are not acknowledged by the disappearance method of nutritional assessment.
Surveys are another method of assessment. While much has been learned through dietary surveys, one may question their accuracy as they are affected by the limited window of time during which food intakes are observed. Because diets vary by season, it is desirable to gather information throughout the year, but this takes time and imposes high costs. Moreover, even well constructed studies across seasons cannot detect annual fluctuations. Surveys conducted through recall methods may undercount or misreport consumption. This can be remedied in principle by placing an observer in the household; however, the family may then try to impress the observer by preparing unusually good meals (a situation that is analogous to the Heisenberg principle).
Anthropometric approaches to measuring nutritional status have the virtue of accounting for biological individuality while simultaneously measuring net nutrition, or dietary intake minus claims made by work and by disease. Disease is too seldom recognized as a factor in nutritional status. A person's need for iron in the diet, for example, is very much a function of exposure to hookworm and other parasites. Similarly, gastrointestinal diseases may divert dietary intake, resulting in malnutrition even though disappearance methods or dietary surveys would indicate that food supplies or intake were adequate. Anthropometric measures consider biological performance or failure to thrive as motivating principles.
Numerous studies of height and weight of healthy groups around the world suggest that a wide variety of human populations have a similar potential for growth. Therefore, if a particular group falls substantially below standard, one may infer that components of net nutrition are inadequate. The two most widely used anthropometric measures are height and weight-for-height (sometimes expressed as the body mass index, which is weight in kilograms divided by the square of height in meters). Height is an indicator of a person's net nutrition from conception through the growing years, which if poor, results in "stunting." Weight-for-height is a measure of recent nutritional status, which if poor, results in "wasting." If it can be established that a particular group's potential for growth differs from commonly used norms, it may be appropriate to utilize other standards.
A significant advantage of anthropometric measures is their low cost of collection. They are also broad-spectrum measures, incorporating a wide variety of factors that affect height and weight. This has the advantage of reflecting all variables that affect growth, but complicates the analysis of results. Simply identifying groups that fail to grow adequately does not provide information on causes or remedies. Other detective work (and expense) may be required.
Historically a major challenge facing humans has been acquiring enough food. In the early twenty-first century, however, the world faces two food problems: undernutrition and overnutrition. The first is primarily, but not exclusively, an issue for poor countries, many of which have low agricultural productivity yet limited capacities to pay for food imports. Nutritional problems can be made worse by policies that artificially reduce prices received by farmers, thereby discouraging production and agricultural innovation. Wars and rivalries within and across countries can be significant in interrupting the delivery of food that would alleviate malnutrition. The largest regions of contemporary nutritional distress are found in Africa and Asia. Pockets of malnutrition are widely distributed and can be found throughout the world, even among some subpopulations in industrialized countries.
Obesity is a growing problem in developed countries, and is especially noteworthy in the United States. In more recent years, some children in some developing countries also have acquired this "disease." Two causes are widely cited: the declining cost of food and sedentary lifestyles. Rapid increases in agricultural productivity have occurred since the 1950s, and the price of food has fallen relative to income and many other commercial goods, which tempts people to consume more. Evidence for sedentary lifestyles is less well established, in part because energy expenditure is difficult and expensive to measure. But the growing use of automobiles to replace walking or cycling, falling demand for manual labor, and a host of household conveniences do make a case for reduction in calorie expenditures. Against this one may cite growth of health clubs and the rise of recreational sports played by a segment of the population. If these trends continue, the populations within industrialized countries may become divided increasingly on the basis of weight and physical fitness.
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Richard H. Steckel