Yet, a child with severe acid reflux who appears to be in pain while eating may also make a caregiver hesitant to offer sufficient feedings. Inadequate caloric intake
Inadequate caloric intake indicates that an insufficient amount of food and nutrition is entering the body, whether due to lack of food, anatomical differences causing difficulty eating, or psychosocial reasons for decreased food intake. Malabsorption/caloric retention defect
Malabsorption and caloric retention defects cause the body to the unable to absorb and use nutrients from food, despite an adequate amount of food physically entering the body. Increased metabolic demand
Increased metabolic demand suggests a state of increased energy needs and caloric expenditure. This state causes greater difficulty taking in enough nutrition to meet the bodys energy needs and allow for normal growth. Epidemiology
Failure to thrive is a common presenting problem in the pediatric population in both resource-abundant and resource-poor countries. While epidemiology may vary by region, inadequate caloric intake remains the most common cause of FTT in both developed and developing countries, and poverty is the greatest risk factor for FTT worldwide. Resource-abundant regions
Failure to thrive is prevalent in developed countries, with literature from Western studies demonstrating a prevalence of about 8% among pediatric patients. Presentations of FTT comprise about 5-10% of children seen as outpatients by primary care physicians and 3-5% of hospital admissions for children. Failure to thrive is more prevalent in children of lower socioeconomic status in both rural and urban areas. FTT is also associated with lower parental education levels.
Biochemistry
The overall reaction is

4 Fe2+ – cytochrome c + 4 H+ + O2 → 4 Fe3+ – cytochrome c + 2 H2O                ΔfGo = - 218 kJ/molTwo electrons are passed from two cytochrome cs, through the CuA and cytochrome a sites to the cytochrome a3–CuB binuclear center, reducing the metals to the Fe2+ form and Cu+. The hydroxide ligand is protonated and lost as water, creating a void between the metals that is filled by O2. The oxygen is rapidly reduced, with two electrons coming from the Fe2+-cytochrome a3, which is converted to the ferryl oxo form (Fe4+=O). The oxygen atom close to CuB picks up one electron from Cu+, and a second electron and a proton from the hydroxyl of Tyr(244), which becomes a tyrosyl radical. The second oxygen is converted to a hydroxide ion by picking up two electrons and a proton. A third electron from another cytochrome c is passed through the first two electron carriers to the cytochrome a3–CuB binuclear center, and this electron and two protons convert the tyrosyl radical back to Tyr, and the hydroxide bound to CuB2+ to a water molecule.