Is failure to thrive same as slow weight gaining?

Pediatric Undernu- trition, characterized by weight-for-length or BMI of < 5th %ile, is defined as "an imbalance between nutrient requirements & intake that results in cumulative deficits of energy, protein or micronutrients that may negatively affect growth, development & other relevant outcomes." Slow weight gain is having velocity of growth in weight that is less than that expected on standardized growth curves.
Well----yes! Failure to thrive means that metabolism is impaired but there are many causes for slow weight gain and failure to thrive. It is a riddle for an expert.