Margaret Hamburg (left) plans to step down as commissioner of the F.D.A. at the end of March, after which Stephen Ostroff has agreed to serve as acting commissioner.

WASHINGTON — Margaret Hamburg, M.D., plans to step down as commissioner of the Food and Drug Administration at the end of March, according to a letter she wrote to F.D.A. colleagues. She has served as commissioner since May 18, 2009. Stephen Ostroff, M.D., currently chief scientist for the F.D.A., has agreed to serve as acting commissioner after she steps down, Dr. Hamburg said in her letter.

“My tenure leading this agency has been the most rewarding of my career, and that is due in no small part to all of you, the dedicated and hardworking people that make up the heart of this agency,” Dr. Hamburg said. “While there is still work ahead (and there always will be), I know that I am leaving the agency well-positioned to fulfill its responsibilities to the American public with great success.”

The Food Safety Modernization Act became law while Dr. Hamburg led the F.D.A. A gluten-free definition also was finalized during her tenure. The F.D.A. in November 2013 proposed to drop the Generally Recognized As Safe (GRAS) status of partially hydrogenated vegetable oils, which cause trans fat, for any use in food. Early in 2014 the agency proposed changes to the Nutrition Facts Panel.

“In the foods area, we have taken critical actions that will improve the safety of the food Americans consume for years to come,” Dr. Hamburg said. “These include science-based standards developed to create a food safety system focused on preventing foodborne illness before it occurs, rather than responding after the fact.

“We have taken several significant steps to help Americans make more informed and healthful food choices. These include working to reduce trans fats in processed foods; more clearly defining when baked goods, pastas and other foods can be labeled ‘gluten-free;’ updating the iconic Nutrition Facts label; and, most recently, finalizing the rules to make calorie information available on chain restaurant menus and vending machines.”

Dr. Hamburg graduated from Harvard Medical School. Her work experience includes serving as commissioner of the New York City Department of Health and Mental Hygiene, serving as assistant secretary for policy and evaluation in the U.S. Department of Health and Human Services, and serving as the senior scientist at the Nuclear Threat Initiative, a foundation that seeks to reduce the threat to public safety from nuclear, chemical and biological weapons.

“I commend Dr. Hamburg for her years of dedicated service as F.D.A. Commissioner,” said Pamela G. Bailey, president and chief executive officer of the Grocery Manufacturers Association. “Under her leadership, the agency successfully ushered in the most sweeping set of reforms to our nation’s food safety system in a generation through the implementation of the Food Safety Modernization Act (F.S.M.A.). She was an activist commissioner in the best sense of the term: personally engaged in the important issues and always seeking the views of all stakeholders.”

Dr. Ostroff graduated from the University of Pennsylvania School of Medicine. His work experience includes serving as deputy director of the National Center for Infectious Diseases at the Centers for Disease Control and Prevention. He joined the F.D.A. in 2013 as chief medical officer in the Center for Food Safety and Applied Nutrition and senior public health advisor to the F.D.A.’s Office of Foods and Veterinary Medicine.