Serine: An amino acid, one of the 20 building blocks of protein. A dietary nonessential amino acid, serine is present in many proteins participates in the synthesis of purines and pyrimidines, and is in the active sites of many enzymes. Symbol: Ser. QUESTION According to the USDA, there is no difference between a “portion” and a “serving.” See Answer From Nutrition and Healthy Eating Resources 9 Psoriatic Arthritis DietsPsoriasis: Foods That Help, Foods That HurtExocrine Pancreatic Insufficiency and Diet Featured Centers What Are the Best PsA Treatments for You?Understanding Biologics 10 Things People With Depression Wish You Knew Health Solutions From Our Sponsors Shot-Free MS Treatment Your Child and COVID-19