Even though science has made a huge progress to uncover the mechanisms by which human body functions, many of the mysteries are still unrevealed. Human metabolism is also one of the obscurities. To uncover those mechanisms, many types of studies to investigate human metabolism have been developed during the last century, including the "in vivo" techniques which use stable isotopes to "follow" the kinetics of a metabolite in the body. Obesity is one of the growing issues worldwide, since it becomes "pandemic" and in the same time is one of the most important risk factors for the majority of modern illnesses, which are leading in terms of incidence, death rate, disability and economic costs. Much research has been conducted with the aim to better understand and target obesity, to determine the mechanisms underlying its genesis, maintenance and link with its comorbidities development. However, it still remains an enigma and the metabolism in human obesity continues to be one of the hot topics for the researchers. This book contains work which is one of the attempts to better understand human metabolism in obesity.