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Recognizing that almost no family is spared and that even the very young are afflicted, the US government has finally declared war on obesity. Suddenly, obesity is everywhere: on the news; in magazines, popular books, and scientific journals; on political agendas; and as a high priority at federal funding agencies. Everyone seems to have the answer to lifelong weight control, but the answers are all different and no validated, science-based, long-term solutions are in sight.
Accordingly, the American Society for Nutritional Sciences/American Society for Clinical Nutrition Public Information Committee (PIC) selected the timely topic "The Weight Debate: Balancing Food Composition and Physical Activity" for its annual symposium. The session was held on April 19 at Experimental Biology 2004 in Washington, DC.
What can explain the apparently sudden appearance of obesity as a major national health problem? Industrialization, beginning in the 19th century, brought with it many of the wonders we take for granted today: improved agricultural methods leading to high-quality foods at relatively low cost, reductions in occupational physical activity, medical treatments for killer infectious diseases and other previously fatal health conditions, and public health measures for countering epidemics such as smallpox, cholera, and tuberculosis. The gains of modern civilization removed many of the natural processes that kept body weights at what are now recognized as healthy levels. Moreover, the last major war with famines in some parts of Europe and Asia was more than half a century ago. Aggressive food marketing, even in schools, and a surfeit of public transportation and labor-saving devices conspire to drive weights upward unchecked by physiologic mechanisms of equivalent potency to those that slow weight loss with food deprivation (1).
Although popular views hold that our excess weight has accumulated because of massive food portions or couch potato behaviors, the average American adult gains <1 kg/y (2). Only small, consistent increments in energy balance, <100 kcal/d, are needed to incite weight gain rates of this magnitude. The search for means of preventing overweight and obesity must by necessity consider approaches by which subjects in the general population can maintain energy equilibrium at healthy weights. For the 2004 symposium, the PIC selected topics that bring new or updated insights to both sides of the energy balance equation: energy intake and output.
In his presentation, Zemel (3) challenges the traditional view that considers only metabolic fuels as components of energy regulation and balance. Our usual teaching is that fuels of known energy value are ingested, absorbed, metabolized, and then either immediately oxidized or stored in fat or lean tissues for later use (Figure 1). Zemel reviews a growing body of literature supporting the view that factors other than metabolic fuels play an important role in the metabolism and storage of fat. Specifically, Zemel suggests that dietary calcium plays an important role in the regulation of energy metabolism, with high-calcium diets attenuating adipocyte lipid accumulation and weight gain during periods of overconsumption and increasing lipolysis and maintaining thermogenesis during caloric restriction, thus accelerating weight loss. Additional components of Zemel's calcium-weight control theory are presented in the review by Parikh and Yanovski (4).
FIGURE 1.. The traditional view of energy exchange in humans consisting of 3 components: intake, output, and balance (ie, intake – output) or fuel deposition in the form of fat and lean tissue.
Although the proposed magnitude of calcium-induced effects on body weight are small (4), so too is the average weight gain of Americans during the adult years (2). Combinations of what may appear to be small modifications in diet or physical activity are likely in the long run to have a large public health impact.
Whereas preventing weight gain is a key long-term consideration in curbing the obesity epidemic, weight loss for those who are now overweight or obese remains a high priority. The traditional energy balance model as outlined in Figure 1, according to Buchholz and Schoeller (5), states that a calorie is a calorie regardless of the macronutrient composition of the diet. Accordingly, these authors examine possible mechanisms for the increased rates of weight loss observed with high-protein, low-carbohydrate diets. Buchholz and Schoeller evaluate studies that examined weight loss and energy expenditure in adults ingesting diets either high in protein and low in carbohydrate or low in fat. The authors observe that high-protein, low-carbohydrate diets produced 2.5-kg greater weight losses than did low-fat diets after 3 mo of treatment. The observed differences in weight loss could not be explained by macronutrient-specific differences in dietary energy availability or energy expenditure. To reconcile these observations, Buchholz and Schoeller propose that further research is needed to examine between-diet differences in weight-loss composition and satiety-related compliance. In other words, can the greater weight loss with high-protein, low-carbohydrate diets be explained by differences in the composition of the weight loss or in the actual amount of calories ingested during the treatment period?
On the output side of the energy balance equation (Figure 1), we can consider 2 important components, one nonvolitional and the other volitional. The nonvolitional component primarily includes 2 portions: energy expended at rest and the thermic effect of food. The volitional portion includes energy expended mainly with occupational and recreational physical activities. Physical inactivity may also lead to loss of metabolically active tissues through disuse and thus secondarily lower nonvolitional energy expenditure. Modernization has led to large reductions in physical activity (6) that are obviously not completely compensated for by corresponding reductions in food intake. The question of how much physical activity to recommend for weight maintenance within the healthy range, or just to maintain optimum health, is thus of critical importance.
Although most investigators and federal agencies agree that Americans need to be more physically active, the required dose and type of recommended activity remain controversial. Brooks et al (7), in their extensive analysis, provide the rationale for the recent Institute of Medicine physical activity recommendations based on doubly labeled water and indirect calorimetry measurements of total energy expenditure and basal energy expenditure, respectively (8). Blair et al (9) examine the evolution of physical activity recommendations, discuss why there are differences in these recommendations, and then summarize the recommendations in their effort to reconcile existing differences. Critical issues in both reviews surround the associations between physical activity levels, body weight, adequacy of nutrient intakes, and health outcomes. Physical activity recommendations, in addition to these important issues, must be considered in the context of time available; economic, social, and cultural issues; availability of resources and facilities; and many other factors conspiring to maintain our sedentary behaviors.
The PIC symposia provide an open forum for the timely exchange of new, and in some cases controversial, emerging concepts in clinical nutrition. The session this year was created with this spirit in mind.
ACKNOWLEDGMENTS
The organizers extend their thanks to The National Dairy Council and Wyeth Nutrition for their generous support of the symposium.
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