Described herein is secretory IgA isolated from the intestinal luminal fluid and intestinal mucosal of animals such as pigs and cows. Also included are methods of isolating secretory IgA. The secretory IgA is useful in food compositions such as animal and human food compositions as well as pharmaceutical compositions to increase growth rate, improve feed efficiency, reduce gastrointestinal inflammation, reduce a risk of gastrointestinal infection in the animal, or a combination thereof.