Nobody is disputing the first law of thermodynamics. I do question your interpretation though. Firstly because the human body is not a closed system, and second

Nobody is disputing the first law of thermodynamics. I do question your interpretation though. Firstly because the human body is not a closed system, and secondly, because there are different ways to interpret the laws as they relate to nutrition. Consider that x is the calories required to run your body at a given weight, and y is a surplus amount of calories. Traditional view: If you eat x + y calories your body will store y as fat. Non-traditional view: If your body needs x calories to function, but it's trying to store y calories as fat you need to eat x + y calories. The non-traditional view is becoming more favorable amongst obesity researchers. If you have an obligate weight gain caused by a biochemical defect, then you better eat the calories to satisfy the needs of the body.