Testing the validity of “negative-calorie” foods with a reptile model

A popular weight loss strategy is to consume foods deemed “negative-calorie”. These are foods, in theory, for which more energy is expended to digest and assimilate than is consumed, thereby generating an energy deficit. Although the existence of “negative-calorie” foods has been debated, no empirical study has fully addressed the validity of any food being “negative-calorie”. We conducted such a study using the omnivorous bearded dragon ( Pogona vitticeps ) and raw celery, a frequently cited “negative-calorie” food. Following their consumption of celery meals (5% of body mass), we measured postprandial metabolic rates to determine the cost of digestion and assimilation (i.e., specific dynamic action) and collected feces and urate to determine the energy lost to excretion. The specific energy of celery, feces, and urate was quantified by bomb calorimetry. Lizards lost on average 33%, 29% and 14% of the meal’s energy to specific dynamic action, feces, and urate, respectively, leaving a net gain of 24% of ingested energy. Theoretically, this gain is greater given that only a portion of fecal energy stems from the celery meal. Although refuting the validity of celery and other proposed foods as “negative-calorie”, this study identifies that such foods do contribute to a negative energy budget and thus weight loss.