There is a difference in dietary choices for someone with ulcerative colitis who is in remission versus those people who are flaring. Symptoms of ulcerative colitis flares often includes things like diarrhea, bleeding, abdominal pain, and decreased appetite, which puts patients at risk for things like dehydration, anemia, and weight loss. For diarrhea, I usually recommend following a low-fiber diet, which is roughly about 10 to 15 grams of fiber per day, but spread throughout the day. You don't want to consume all 10 to 15 grams in one sitting.
It's also helpful to focus on a specific type of fiber called soluble fiber.
Soluble fiber can actually slow down how quickly food passes through you and acts like a sponge to soak up extra water that contributes to diarrhea. Examples of soluble fiber are things without skins and seeds. Fruit such as bananas or very ripe melon like honey dew, cantaloupe, vegetables that have been cooked and peeled, carrots, parsnips, potatoes without the skin, to name a few.
Also, if you're flaring and you have significant amounts of trigger foods, so things like caffeine, spicy foods, sugary beverages, large amounts of lactose in your diet, that's the time to maybe restrict those and see if that impacts your symptoms in any way.
With severe diarrhea, I would also be very mindful of preventing dehydration, and I suggest being mindful of electrolytes. There are a lot of things that you can do to prevent dehydration and supplement electrolytes. You can make a homemade oral rehydration solution. There are several recipes online that includes some sugar, some salt, or you can go buy a store-bought one.
The thing about the store-bought ones is you have to be careful about sugar intake. So many of them contain excessive sugar, which can be a trigger for diarrhea, but a lot of them contain artificial sweeteners or zero sugars. You want a little bit of sugar to really help you in the rehydration process. You want something that has some sugar, but not too much. If the only option are those higher sugar items, I might just dilute them. You could also use an unflavored coconut water. There are several other things that you can do.
During a flare, I would also switch your meal sizes to smaller, more frequent, nutrient-dense meals. This can help for diarrhea, but also for someone who's experiencing decreased appetite, this can help you keep your caloric intake up and prevent weight loss.
Once symptoms start to improve, you can gradually expand your diet, and it is important to do this and not remain on a low-fiber diet forever if you don't have to. There are no restrictions for a patient in remission other than personal intolerances or allergies. Then, of course, being mindful of anti-inflammatory diet principles, keeping those in mind.