The prevailing understanding of the causes of obesity is based on the disturbance of the thermodynamic law of energy balance, primarily through increased intake or decreased expenditure of energy. Alternative causes of obesity, which still respect the law but also allow for primary fat accumulation, are: virus infection of fat cells, energy supply from bacterial fermentation of indigestible food components, psychosocial factors mediated by psychoneuroendocrine pathways, sleeplessness or poor quality of sleep and early life programming, in addition to the well-known and proven genetic programming.