Sponsors studying genetic metabolic disorders should put patients on strict diets even before clinical trials start, the FDA said in a new draft guidance.

Researchers have already developed drug treatments to complement the diets of people who suffer from such disorders as phenylketonuria, organic acidemias and urea cycle defects. But even small changes in patients’ diets can skew clinical trial results, especially when the proposed drug’s impact “is relatively modest,” the agency said.

To address those challenges, the FDA is recommending that patients in clinical trials be put on strict diets, even before trials begin, and patients should be given “clear and detailed” descriptions of their dietary plans. When monitoring patients’ diets, sponsors should remember that the two most common assessments—food frequency questionnaires and food diaries—have their limits because they “use crude measures” and patients’ memories can be unreliable.