By 2050, there will be 9 billion people on earth. It will be necessary to feed them to avoid the population catastrophe predicted by Malthus 150 years ago. Yet a major issue facing governments is that the current method by which food demands are estimated is widely acknowledged as completely inaccurate. An accurate method is available. It is based on measurements of isotope elimination, called the doubly labelled water (DLW) method. In 2018, the International Atomic Energy Agency (IAEA) hosted and compiled a database of measurements using this technique. It showed that the use of the DLW method has been mostly restricted to the USA and Western Europe. It has rarely been applied across Africa and Asia. The database also focused on adults, leaving much unknown about the energy balance in children and adolescents. There is a clear need to fill this gap in knowledge, providing information that governments across Low- and Middle-Income countries can use to forecast future food demands. In China, the prevalence of overweight and obesity among children and adolescents is increasing. In 2009, 22.1% of Chinese youth aged 7-17 years were either overweight or obese, according to the China Health and Nutrition Survey. The obesity rate has increased fourfold since 1995. One of the most concerning problems is the persistence of childhood obesity into adulthood. The causes of this obesity epidemic are strongly disputed, with some suggesting it is due to increasingly sedentary lifestyles (computers, cars, phones, etc.), while others suggest it is mostly due to changing food patterns (junk food and sugar-sweetened beverages). Measuring energy expenditure in children and adolescents in China will address whether declining expenditure due to reduced physical activity is a key cause of the epidemic. Most measurements come from urban populations, leaving rural populations greatly underrepresented.