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This chapter deals with the importance of considering total energy intakes in nutritional epidemiology. Topics discussed include the physiologic determinants of energy utilization, adjustment for energy intake in epidemiologic analyses, and alternate approaches to adjust for total energy intake.

This chapter reviews information on temporal trends in energy intake and energy expenditure in the USA and discusses the quality and limitations of available data. The focus is on trends in the USA, firstly because surveillance data is available for the USA, and secondly because of the complications of differences in obesity trends across countries. The surveillance issues that have arisen in the USA serve as a valuable example for other countries regarding the pitfalls of tracking risk factors for obesity. Further, because the USA has experienced a steady and steep increase in the prevalence of obesity over the past twenty-five years, it serves as good case study for attempts to understand the relative contributions of diet and physical activity to the development of obesity.

This chapter presents a systematic review of available data on environmental factors that may have contributed to recent trends in population body weight. It focuses especially on the US population, for which data on environmental change and body weight are most complete. The cause of the obesity epidemic that has affected the world for the last thirty years remains unknown. Although changes in body weight and fatness are surely the result of changes in energy intake and energy expenditure that are mediated by changes in food and activity choices, there are no clear data identifying the specific contribution of energy intake versus energy expenditure, or the specific contributions of specific behavioural choices. It is argued that consideration of biological and behavioural aspects of energy intake and use strongly favour change in energy intake as the most likely cause. However, the fact that population data on energy intake and energy expenditure cannot elucidate this issue is a cause for concern.

This chapter provides a conceptual background regarding sources of variation in diet for the novice to this field and also assembles data on dietary variation that may be of use to the serious investigator. The daily variation in nutrient intake among free-living subjects has consistently been found to be large, although the degree of variation differs greatly according to nutrient. The ratio of within-person to between-person variation is usually substantially greater when the nutrient is expressed in relation to energy intake (energy-adjusted). Measurements of dietary intake based on a single or small number of 24-hour recalls per subject can provide an accurate estimate of the mean for a group, but the standard deviation will be greatly overestimated. Furthermore, measurements of association in epidemiologic studies, such as correlation and regression coefficients and relative risks, are substantially weakened, possibly to the point of being undetectable.

This chapter analyses the data on obesity, healthy eating, and health by taking a closer look at the numbers behind the obesity epidemic. It explains the meaning of overweight and obesity by providing illustrations. It points out the distribution of the body mass index (BMI) and its trends. It shows that many non-communicable diseases associated with obesity are concentrated in disadvantaged groups, those of lower socioeconomic group, and lower education, which suggests a behavioral explanation. It attributes the biological cause of weight gain over time is higher energy intake than expenditure. It enumerates the adverse health consequences of obesity like heart disease, stroke, diabetes, and several cancers. It also offers that the study of economics on obesity can help address these issues.

The brain consumes about 20 per cent of the total energy intake in human adults. Primates, and especially humans, have unusually large brains for body size compared with other vertebrates, and fuelling these is a significant drain on both time and energy. Larger-brained primates generally eat fruit-intense diets, but human brains are so large that a reduction in gut size is needed to free up sufficient resources to allow a larger brain to be evolved, placing further pressure on foraging. The early invention of cooking increased nutrient absorption by around 30 per cent over raw food. Increasing digestibility in this way perhaps inevitably leads to risk of obesity when food is super-abundant, as it is in post-industrial societies. However, obesity has clearly been around for a long time, as suggested by the late Palaeolithic Venus figures of Europe, so it is not a novel problem.

This chapter focuses on the energy budget of a single species (a tuco-tuco, one of Pearson's favorite beasts). It characterizes the energy budget for the South American subterranean rodent Ctenomys talarum (Talas tuco-tuco), based on new and published data on energy intake (ingestion and digestion) and expenditure (cellular maintenance, thermoregulation, digging activity, movement, pregnancy, lactation, and growth). The chapter observes that animals fed on an low-quality diet ingested more food, produced more feces, and showed lower incidence of coprophagy, lower digestibility, and reduced transit time relative to those fed a high-quality diet. It notes that this increase in food intake would assure similar total-energy assimilation as a high-quality diet, and demonstrates that tuco-tucos can adjust their ingestive and digestive processes according to the quality of the available food resources. The chapter also discusses possible extrinsic and intrinsic limitations on the C. talarum's energy budget.

This chapter explores the role of diet and nutrition on cognitive aging and the cognition of animals and human beings. The association of dietary restriction with positive effects on cognition and health are explored along with the role of moderate physical stress in improving health. The chapter explores the side effects of excess energy intake on human beings’ cognitive functioning. The increased risk of Alzheimer’s disease is also linked to excess energy consumption. In animals, excess fat intake reduces the neuronal plasticity and learning capacity of the brain. Mechanisms including glucose regulation, the hippocampal circuits’ plasticity and oxidative stress, among others, are being studied to provide answers regarding the positive effect of dietary restriction.