Specific foods MAY not DIRECTLY cause flares or any form of IBD. However, diet always matters. Taking food A may not cause a flare, but a healthy diet is instrumental for anyone that doesn't have a chronic disease much less someone who does. I can't see how it can be argued that an unhealthy diet doesn't in some way affect chronic disease, especially those of the digestive track. A specific diet may not cause flares but I will bet any amount of money that specific diets certainly reduce (or increase) the likelihood of relapse. There are plenty of studies that show people with Crohn's Disease are more likely to consume refined sugars, fast foods, high levels of unhealthy fats, low levels of quality fruits and vegetables, and the list goes on.
Example studies:
Heckers H, Melcher FW, Kamenisch W, Henneking K. Chemically prepared fats and Crohn disease. A pilot study of occurrence of trans-fatty acids in the subcutaneous tissue of patients in comparison with healthy controls as a parameter of long-term fat intake. Z Gastroenterol 1988; 26: 259–64.
Chuah SY, Jayanthi V, Lee CN, McDonald B, Probert CS, Mayberry JF. Dietary fats and inflammatory bowel disease in Asians. Ital J Gastroenterol 1992; 24: 386–8.
Persson PG, Ahlbom A, Hellers G. Diet and inflammatory bowel disease: a case-control study. Epidemiology 1992; 3: 47–52.
Geerling BJ, v Houwelingen AC, Badart-Smook A, Stockbrugger RW, Brummer RJ. Fat intake and fatty acid profile in plasma phospholipids and adipose tissue in patients with Crohn's disease, compared with controls. Am J Gastroenterol 1999; 94: 410–7.
Martini GA, Brandes JW. Increased consumption of refined carbohydrates in patients with Crohn's disease. Klin Wochenschr 1976; 54: 367–71.
Kasper H, Sommer H. Dietary fiber and nutrient intake in Crohn's disease. Am J Clin Nutr 1979; 32: 1898–901.
Silkoff K, Hallak A, Yegena L, et al. Consumption of refined carbohydrate by patients with Crohn's disease in Tel-Aviv-Yafo. Postgrad Med J 1980; 56: 842–6.
Mayberry JF, Rhodes J, Newcombe RG. Increased sugar consumption in Crohn's disease. Digestion 1980; 20: 323–6.
Mayberry JF, Rhodes J, Allan R, et al. Diet in Crohn's disease two studies of current and previous habits in newly diagnosed patients. Dig Dis Sci 1981; 26: 444–8.
Jarnerot G, Jarnmark I, Nilsson K. Consumption of refined sugar by patients with Crohn's disease, ulcerative colitis, or irritable bowel syndrome. Scand J Gastroenterol 1983; 18: 999–1002.
Katschinski B, Logan RF, Edmond M, Langman MJ. Smoking and sugar intake are separate but interactive risk factors in Crohn's disease. Gut 1988; 29: 1202–6.
Mayberry JF, Rhodes J, Allan R, et al. Breakfast and dietary aspects of Crohn's disease. Br Med J 1978; 2: 1401.
Kasper H, Sommer H. Taste thresholds in patients with Crohn's disease. J Hum Nutr 1980; 34: 455–6.
Brauer PM, Gee MI, Grace M, Thomson AB. Diet of women with Crohn's and other gastrointestinal diseases. J Am Diet Assoc 1983; 82: 659–64.
Penny WJ, Mayberry JF, Aggett PJ, Gilbert JO, Newcombe RG, Rhodes J. Relationship between trace elements, sugar consumption, and taste in Crohn's disease. Gut 1983; 24: 289–92.
Porro GN, Panze E. Smoking, sugar and inflammatory bowel disease. Br Med J 1985; 291: 971–2.
Probert CS, Bhakta P, Bhamra B, Jayanthi V, Mayberry JF. Diet of South Asians with inflammatory bowel disease. Arq Gastroenterol 1996; 33: 132–5.