When using indirect caloric measurement through monitoring oxygen intake, carbon dioxide production, and nitrogen excretion in urine to investigate energy metabolism, exhalation samples are taking using a mask and hood or a metabolic chamber. Since the metabolic chamber method (also called the human calorimeter test) analyzes the diffusion of exhalation in a room, although it is the less precise method of the two mentioned above, it can be used for long-duration measurement including meals and sleep. For this reason, human calorimeter method testing up until now has been used for 24-hour total energy expenditure and total fat oxidation measurement. The metabolic chamber set up at the National Institute of Health and Nutrition was the first facility in Japan, and there are currently a total of 11 in operation in this country. The majority of the human calorimeters in Japan use magnetic sector type mass spectrometers for gas analysis, and compared to the research facilities in the United States where this research was launched 30 - 100 years earlier, the precision is one digit higher (there is also an additional digit tacked on to the price for the machines). Since the equipment measures the content of nitrogen and argon in the air prior to calculating the percentage of oxygen and carbon dioxide, gas concentration measurement value drift is avoided when conducting long-duration measurements. In addition, an algorithm has been developed to improve time resolution of these human calorimeter's metabolic rate measurements (J Appl Physiol 106: 640, 2009). These improved Japanese human calorimeters may also be used for large-scale research projects in China, and I expect they will become the international standard in short time.

If energy metabolism is measured with factors such as exercise, meals, and sleep managed, a second measurement conducted under the same conditions will differ by just 50 - 100 kcal per 24 hours. In order to understand the pathological physiology of lifestyle diseases, I feel that measurements of post-exercise, post-meal, and during sleep energy metabolism are essential, and I am carrying out my research with these measurements in mind.

1. Skipping breakfast and eating dinner late at night affect substrate oxidation during sleep.

Skipping breakfast (compared with larger intake for lunch and dinner and no difference in total calories for the 24-hour period) and eating dinner late at night cause elevated average blood glucose levels during sleep, resulting in increased carbohydrate oxidation and reduced fat oxidation (Obes Res Clin Pract 5: e220, 2011; 8: e249, 2014).

2. The fat-burning effectiveness of exercise varies depending on the time at which it is performed.

When the balance of total energy intake and energy expenditure for a 24-hour period is matched, although fat oxidation for the 24-hour period is increased by exercising prior to eating breakfast, 24-hour fat oxidation does not increase when performing the same exercise after eating breakfast, lunch, or dinner (J Appl Physiol 118:80, 2015; EBioMedicine 2: 2003, 2015; PLoS ONE 12: e0180472, 2017).

3. Energy metabolism during sleep is not an extension of metabolism when fasting and at rest.

After going to sleep, energy expenditure and carbohydrate oxidation both decline. However, starting from 2 - 6 hours before waking, energy expenditure and carbohydrate oxidation both begin to rise (Metabolism 69:14, 2017).

4. Intake of unsaturated fatty acids increases fat oxidation.

Unsaturated fatty acid intake has effects such as accelerating fat oxidation, increasing it in deep sleep, and advancing deep body temperature rhythms to a greater extent than saturated fatty acid intake (Yajima et al., unpublished).

At our informal event, we also introduced the limitations of the human calorimeter method (inability to conduct continuous measurement for over 24 hours, discriminate between exogenous fat oxidation and endogenous fat oxidation, estimate fat oxidation volume when meal fatty acid composition is drastically changed, etc.). I expect future developments in human studies using water or fatty acids marked with stable isotopes.