Brown sugar contains 4-6% molasses. The darker the brown sugar the more molasses. Molasses has about 1 gram of excess fructose per 100 grams. A tablespoon of brown sugar contains less than 0.1 grams (< 1/16 teaspoon) of excess fructose. Most individuals with fructose malabsorption can handle that small amount of excess fructose. I don't have problems with brown sugar but I cannot eat pure molasses. Perhaps you are more sensitive to it than me.
-------------------- STABLE: ♂, IBS-D 50+ years - Science of IBS