If you want to try the raw food diet while you're flaring, you'll probably need to juice alot of the veggies, otherwise the fiber and roughage will be extremely irritating to an inflamed colon.
Like you, I was "thin" all of my life - even prior to my UC diagnosis - and if you're wanting to gain weight, I wouldn't suggest going the raw food diet route. A diet high in lean proteins and healthy fats and low in carbs (starches and sugars) is what's helped me heal my UC as well as gain healthy weight. My normal weight prior to UC was 105 at 5'4". I now weigh 116 and am in the best shape of my life.
Diagnosed with ulcerative colitis spring 1999.
Maintenance dose sulfasalazine. Probiotics, l-glutamine, vitamin D and fish oil caps. George's aloe vera juice. Oregano oil antibiotic, antiviral, antifungal. Mostly grain-free and dairy-free diet. Long-term remission with only minor blips.