 3. Relevance: Our research group has recently developed a new simplified equation to measure VA pool size in 4-5 days, highly correlated with compartmental model-predicted value and estimate VA pool size in adults with high precision. However, it is not clear whether this new simplified equation also works in infants and children. In this proposed study, we validate the method in healthy infants and infants with inflammatory condition as well as in children with adequate and high dietary VA intake. 