You're thinking of doubly-labeled water. The technique doesn't directly measure calorie consumption. Rather, it's a technique to measure total calorie expenditure over a period of weeks without keeping the subject in the lab the whole time. The calorie consumption is then inferred from calorie expenditure combined with weight gain/loss. For example, if you expend 2500 calories per day while gaining weight, you're obviously eating more than that, and certainly not 1300 calories per day. This still disproves a lot of fat logic because they often claim theoretical agreement with CICO but make absurd claims about how their metabolisms respond to changes in caloric intake that allegedly make it impossible to lose weight. For example: "If I eat less than 1500 calories per day, my body will go into starvation mode and only burn 500 calories per day!" "Well fatty, fortunately you're burning 2500 calories per day (keeping useless fat cells alive takes energy). Where exactly do you think those calories are coming from? Photosynthesis?"