Ulcerative colitis (UC) is an inflammatory bowel disease that causes gastrointestinal lesions, bleeding, diarrhoea and nutritional complications. Insufficient nutrient intake can additionally deteriorate nutritional status. The present cross-sectional study aimed to determine whether UC patients adhere to national dietary guidelines and to assess their dietary habits. An online questionnaire (n 93) was used to assess health-related conditions, current nutritional knowledge, professional dietary guidance and food avoidance. A 24 h dietary recall (n 81) was used to assess nutrient intakes, which were then compared with the national recommended intake values. The results showed that the nutritional knowledge of participants was limited with unofficial sources being used, including websites. Numerous food groups, predominantly fibre-rich foods and fruit and vegetables, were largely avoided by the participants. Almost half of the study population eliminated foods such as dairy products to alleviate symptoms, possibly unnecessarily. Energy intakes were significantly (P< 0·05) lower than the national recommended intake values in women aged 18–65 years and men aged 18–60 years. Fat intake exceeded the national recommended intake values (P< 0·0001), at the expense of carbohydrate and fibre intakes, which were significantly (P< 0·005) lower than the national recommended intake values. Protein intake was significantly high in women aged 19–50 years (P< 0·00) and men aged 19–50 years (P< 0·005). Vitamin C, vitamin B12 and Ca intake levels were overachieved by all participants (P< 0·001), while women aged 19–50 years did not achieve their dietary Fe reference nutrient intake levels (P< 0·001). Osteopaenia, osteoporosis and anaemia were reported by 12, 6 and 31 % of the participants, respectively. Findings indicate that food avoidance may contribute to nutrient deficiencies in UC patients. Low intakes of these food groups, especially during remission, are preventing patients from adhering to dietary guidelines.