###### Abstract  
Inflammatory bowel disease (IBD) remains a burden for patients with increasing prevalence in industrialized countries. Phytochemicals are non-nutrient plant derived bioactive substances with antioxidant and anti-inflammatory effects that may prove beneficial to IBD patients. This review aims to overview current evidence on the application and impact of isolated phytochemicals or phytochemicals contained in plant extracts and essential oils on patients suffering from IBD. A systematic literature search was conducted for studies relating to the use of phytochemicals for the treatment of IBD. Ultimately, 37 human clinical trials and 3 systematic reviews providing human IBD patient data relevant to phytochemicals as therapeutic agents were included. Phytochemicals in the form of curcumin, _Plantago__  
IBD usually follows a lifelong pattern of remissions and flare-ups that impacts the quality of life of patients. The inflammation of the gastrointestinal tract during flare-ups is mediated by neutrophils that release cytokines, enzymes and reactive oxygen species (ROS) leading to damage and even ulceration of the mucosa [3]. Gut microbiota and its dysregulation are one of the environmental factors that also play a key role in the immune-mediated disease process of IBD. Thus, various medical therapies target these pathophysiologic pathways [3; 4].  
Phytochemicals are non-nutrient plant derived bioactive substances. As a group of heterogeneous compounds, these secondary metabolites are involved in various organic functions of plants other than growth or nutrition, such as protection from bacteria, fungi or insects [5; 6]. More than 10,000 phytochemicals have been identified up to now, a number estimated to amount to less than 10% of potentially bioactive substances contained in plant matter [7; 8]. The dietary consumption of fruits and vegetables containing phytochemicals has demonstrated anti-inflammatory and antioxidant effects while lowering cancer and cardiovascular risks [9]. Because various phytochemicals have been regarded as plant-derived toxins, their antimicrobial effect has also been investigated [10]. The scientific interest in these natural products is therefore high, and several in vitro or animal model studies focus on specific phytochemicals [11]. Some studies have demonstrated that the effects achievable by these compounds may be apparent at higher doses. The extraction and synthesis of these phytochemicals, however, can be laborious [12]. Indeed, preclinical studies are essential to isolate and focus on both medically applicable and easy to extract or manufacture substances [7]. Analyzing the constituents of plant extracts or essential oils with proven benefits can lead to the discovery of the effective phytochemical substances contained in them [13; 14].  
This analysis comprises a field of research that is vast and encompasses different fields such as biology, plant chemistry and medicine. There is in vitro and in vivo ongoing research to identify novel antioxidant and anti-inflammatory phytochemicals in animals or humans. Clinical relevance of phytochemicals and modes of delivery are also investigated.  
The objective of this review is to provide a high-level overview of current knowledge on the applied use of phytochemicals on patients suffering from IBD. We aim to review and assess the impact of various phytochemicals and plant extracts on patient outcomes by systematically reviewing clinical trial data. Focusing on phytochemicals or plant extracts under clinical trials with potential beneficial results can guide researchers to plan new ones.