Trans Fat on Food Labels: Now You See It, Now You Don't - MedicineNet That's because newly implemented U.S. Food and Drug Administration rules on labeling allow foods with less than 0.5 grams of trans fats per serving to claim "zero" grams of trans fats on their labels. Under these guidelines, which went into effect on Jan. 1, a food with 0.4 grams of trans fats can be listed as having zero trans fats.