Energy balance in human body undergoes constant change, leading to a change in the weight and body composition throughout life. Energy balance at a particular time point is influenced by the psychological, physiological, sociological, and environmental factors of that moment. In addition, the regulation of homeostasis continuously monitors and maintains the energy balance; however, it complicates the identification of factors influencing the energy balance. For understanding these factors, creating a model with comprehensive factors and testing it among a substantial number of individuals for dynamic changes in the energy balance may be warranted. However, till date, to the best of our knowledge, no studies report on comprehensive modeling, including homeostasis and the other factors. Thus, at this moment, summarizing previous studies for further research is required. Accordingly, this review summarizes 1) the basic factor of energy expenditure and intake; 2) interactive relationship between energy expenditure and intake; and 3) energy expenditure and intake during dynamic changes in the body weight caused by events such as overfeeding, underfeeding, growth and aging, and pregnancy.

Obesity and diabetes have become a major epidemic problem. Preventing obesity has proven a challenge because this would require increased physical activity and/or reduced energy intake; both of which are difficult to enforce. Brown adipose tissue (BAT) is a major metabolic organ that contributes to the regulation of energy metabolism in small rodents. In humans, since its rediscovery in 2009, BAT has emerged as a potential target to fight obesity and related metabolic diseases such as type 2 diabetes. The BAT of humans is activated by acute cold exposure and mediates cold-induced thermogenesis. The metabolic activity of BAT demonstrates huge individual variation and is negatively associated with body fat accumulation and glucose intolerance. The decreased BAT can be reactivated and recruited by repeated cold exposure, which has the effects of increasing thermogenesis and decreasing body fat. Such effects of cold exposure can be mimicked by oral ingestion of some food ingredients with agonistic activity at temperature-sensitive transient receptor potential channels. To develop strategies to boost BAT thermogenesis for obesity prevention, more insight is needed into the physiological significance of human BAT and mechanisms by which BAT function is regulated by aging, as well as endogenous and environmental factors. This review will summarize the current knowledge on the activation of human BAT thermogenesis and its roles in obesity-related disorders.

This review aimed to verify the effect of exercise and meal timing on energy metabolism. Many people are exercising and playing sports in their own spare time. Although guidelines for daily exercise for healthy life suggest indications of intensity and frequency, there is no instruction about when exercise should be performed. However, there are some diurnal variations in energy metabolism responses to exercise and food intake. In addition, exercise performed before meals and vice versa are different stimuli to whole body energy metabolism, respectively. Further research is required to optimize that translating the results in laboratory to real life, because with growing diversity in lifestyle.

In this review, the energy metabolism and body composition in athletes are discussed on the basis of an energy balance study using a doubly labeled water method. In athletes, the energy expenditure induced by sports activities widely differs according to the type of sports event. In addition, the physical activity of athletes can reach more than twice the basal metabolic rate throughout the sports season. It may be also more than three times depending on the duration of the tournament and/or the training camp. Thus, the event type, and training time and intensity must be considered when estimating the energy requirement of athletes.

The scarce evidence on dietary reference intakes (DRIs) for people with physical impairments has made estimation of total energy expenditure (TEE) and nutritional requirements difficult. The first reason for this difficulty is that estimating basal metabolic rate (BMR) or resting energy expenditure (REE) is challenging. The second reason is the difficulty of estimating body composition, which is useful for calculating BMR, and the third reason is the difficulty of estimating physical activity level. The reason for all these difficulties is that DRIs applies to all types of physical disabilities. Thus, it turns out to be unenviable for planning nutritional programs and evaluating physical impairments. However, notable references are available on energy requirements not only for the corresponding health maintenance and promotional information for preventing physical impairments caused by lifestyle diseases in middle and old age but also for improving performance of Paralympic athletes. This review article focuses on the estimation of the energy requirements for people with physical impairments and discusses the possibilities and limitations of methods.

The National Institute of Health and Nutrition, as a part of the National Institutes of Biomedical Innovation, Health and Nutrition at present, has put emphasis on energy metabolism research since its establishment in 1920. After 2000, the National Institute of Health and Nutrition introduced doubly labeled water method and human calorimeters and has also contributed to the establishment of energy requirements in the Dietary Reference Intakes for Japanese. Inter-individual differences in physical activity level are large and difficult to accurately evaluate. Therefore, various studies have been conducted using not only subjective methods such as questionnaire but also objective methods such as accelerometry. Moreover, determinants of total energy expenditure and physical activity have been investigated, including comparison of physical activity level between normal-weight and obese adults. New evidences on contribution of brown adipose tissue activity and diseases on energy metabolism have been obtained. In the near future, research to elucidate the mechanism of energy balance, more accurate and variety of assessment of physical activity, and examination from the viewpoints of chrono-nutrition and chrono-exercise are expected.