• Assess quality of life in general pre and post intervention among the groups [ Time Frame: 6 - 8 weeks ]
  12-Item Short Form Survey (SF-12) is a 12-item questionnaire used to assess generic health outcomes from the patient's perspective, covering the eight domains of health outcomes: physical functioning, role-physical, bodily pain, general health, vitality, social functioning, role-emotional & mental health. The items are weighted and summed to provide physical and mental health scores (PCS and MCS). The two composite scores are computed using the scores on the questions that range from 0 to 100, with higher score indicating better health.
• Assess quality of life with a specialized questionnaire pre and post intervention among the groups. [ Time Frame: 6 - 8 weeks ]
  IBS-QOL - a measure that assesses the degree to which IBS interfered with quality of life over the past 30 days. It is a self-report quality of life measure, which includes domains such as dysphoria, interference with activity, body image, health worry, food avoidance, social relation, and sexual and relationship issues Each of them is rated based on the Likert scale from 1 to 5, the higher the score the lower the quality of life is. The overall score ranges between 34 and 170. A decrease of 10 points or more is a significant improvement.

• Assessment of IBS related quality of life with a specialized questionnaire pre and post intervention between groups. [ Time Frame: 6 - 8 weeks ]
  IBS-QOL - a measure that assesses the degree to which IBS interfered with quality of life over the past 30 days. It is a self-report quality of life measure, which includes domains such as dysphoria, interference with activity, body image, health worry, food avoidance, social relation, sexual and relationship issues. Each of them is rated based on the Likert scale from 1 to 5, the higher the score the lower the quality of life is. The overall score ranges between 34 - 170. A decrease of 10 points or more is a significant improvement.
• Assessment of general quality of life pre and post intervention between groups [ Time Frame: 6 - 8 weeks ]
  12-Item Short Form Survey (SF-12) is a 12-item questionnaire used to assess generic health outcomes from the patient's perspective, covering the eight domains of health outcomes: physical functioning, role-physical, bodily pain, general health, vitality, social functioning, role-emotional & mental health. The items are weighted and summed to provide physical and mental health scores (PCS and MCS). The two composite scores are computed using the scores on the questions that range from 0 to 100, with higher score indicating better health.
• Adequate relief of symptoms after the intervention. [ Time Frame: Once per week ]
  Adequate relief (IBS-AR) is a single item questionnaire of pain and discomfort. It poses the question "Over the past week have you had adequate relief of your IBS symptoms?", with a positive or negative answer. Adequate relief is defined as 50% positive answers within the study period, for the purposes of the trial.
• Assessment of symptoms burden any in general pre and post intervention. [ Time Frame: 6 - 8 weeks ]
  The GSRS questionnaire was originally developed for dyspeptic patients but was later validated in patients with IBS. The GSRS is a 15-item questionnaire designed to assess common gastrointestinal symptoms. The questionnaire has five subscales, reflux, diarrhea, constipation, indigestion, and abdominal pain, with subscale scores ranging from 1 (no discomfort) to 7 (severe discomfort). Higher scores represent higher symptom burden.

• Assess any change of symptoms severity in general pre and post intervention. [ Time Frame: 6 - 8 weeks ]
  The GSRS questionnaire was originally developed for dyspeptic patients but was later validated in patients with IBS. The GSRS is a 15-item questionnaire designed to assess common gastrointestinal symptoms. The questionnaire has five subscales, reflux, diarrhea, constipation, indigestion, and abdominal pain, with subscale scores ranging from 1 (no discomfort) to 7 (severe discomfort). Higher scores represent higher symptom burden.
• Assess any change of symptoms severity with a specialized questionnaire post intervention. [ Time Frame: 6 - 8 weeks ]
  IBS- SSS contains 5 specific questions with instructions on how to score them. Each of the five questions (pain severity, pain frequency, abdominal distension severity, bowel movement satisfaction, quality of life) ranges from 0 to a maximum score of 100 using a visual analog scale (VAS) with total scores ranging from 0 to 500. with higher scores indicating more severe symptoms. Subjects can be categorized as having mild (75-175), moderate (175-300), or severe (>300) IBS. A decrease of 50 points is associated with a clinically meaningful improvement. The secondary outcome is the measure of the effects of the dietary interventions on the individual symptom items score of IBS-SSS, as well as on the characteristics of the stool habit through the administration of the diaries. Lower scores indicate improvement.
• Adequate relief of symptoms after the intervention. [ Time Frame: Through study completion, an average of 8 weeks. ]
  Adequate relief (IBS-AR) is a single item questionnaire of pain and discomfort. It poses the question "Over the past week have you had adequate relief of your IBS symptoms?", with a positive or negative answer.

Irritable Bowel Syndrome (IBS) is a functional gastrointestinal disorder. Patients associate their symptoms with specific foods consumption, creating the need for developing a new therapeutic approach based on altering the dietary intake of these patients. Low FODMAP Diet is often used as a first-line treatment based on observations showing that short-chain carbohydrates are not adequatetly absorbed in the small intestine and exacerbate the symptoms of IBS. Additionally, the Mediterranean diet is a cultural heritage representative of all the countries surrounded by the Mediterranean Sea, described as a diet rich in herbal foods (cereals, fruits, vegetables, legumes, nuts, olives), with olive oil as the main consumption of fats, high to moderate consumption of fish and seafood, moderate consumption of eggs, poultry, dairy products and alcohol and low consumption of red meat. On the other hand, the nutritional recommendations of British National Institute for Health and Care Excellence (mNICE) managing IBS were created on the grounds of systematic reviews and are based on dietary and lifestyle modifications about caffeine, alcohol, fizzy drinks, fibre, resistant starch, fruits, sorbitol, oat and linseed consumption, meal patterns, probiotics and physical activity.

The direct aim of the study is to compare the efficacy of two dietary patterns, the MED-LFD and the nutritional recommendations of mNICE managing IBS. Efficacy will be assessed based on the severity of symptoms. Additionally, possible improvements in quality of life and relief of symptoms post intervention between the two groups will be evaluated.

The sample was calculated to 108 participants with prespecified statistical power 80%, level of significance α=0.05 and 10% adjustment for non-compliance in each group to detect an increase in the primary outcome measure from 59 in the mNICE group to 100 in the MED-LFD group (standard deviation of outcome=60). The above hypothesis of the means was based in the meta-analysis of Peter Varju, et al. 2017. Patients will be recruited by the Department of Clinical Nutrition, Attikon University General Hospital, where the nutritional intervention will be carried out.

• Other: Med-LFD
  All participants of this group at phase 1 (elimination phase), will initially follow the Med- LFD (2-6 weeks). At phase 2 (reintroduction phase), patients will gradually reintroduce foods rich in FODMAPs (6-8 weeks) and test their tolerance. At phase 3 (maintenance phase), following the reintroduction of foods rich in FODMAPs, patients will follow an individualized diet based on their personal tolerance (a combination of high and low FODMAPs).
• Other: Nutritional Guidelines of the British National Institute for Health and Care Excellence (NICE) Managing IBS
  All participants of this group will be informed to follow a diet based on the nutritional guidelines of the British National Institute for Health and Care Excellence (NICE) for Managing IBS for 8 weeks.

• Active Comparator: Group A
  MED-LFD Diet (diet A) for 2 - 6 weeks. After this period there will be a reintroduction phase protocol that will last 4-6 weeks.
  Intervention: Other: Med-LFD
• Active Comparator: Group B
  Diet according to guidelines from the National Institute for Health and Care Excellent (mNICE) Managing IBS (diet B) for 14 weeks.
  Intervention: Other: Nutritional Guidelines of the British National Institute for Health and Care Excellence (NICE) Managing IBS

• Varjú P, Farkas N, Hegyi P, Garami A, Szabó I, Illés A, Solymár M, Vincze Á, Balaskó M, Pár G, Bajor J, Szűcs Á, Huszár O, Pécsi D, Czimmer J. Low fermentable oligosaccharides, disaccharides, monosaccharides and polyols (FODMAP) diet improves symptoms in adults suffering from irritable bowel syndrome (IBS) compared to standard IBS diet: A meta-analysis of clinical studies. PLoS One. 2017 Aug 14;12(8):e0182942. doi: 10.1371/journal.pone.0182942. eCollection 2017.
• Dalrymple J, Bullock I. Diagnosis and management of irritable bowel syndrome in adults in primary care: summary of NICE guidance. BMJ. 2008 Mar 8;336(7643):556-8. doi: 10.1136/bmj.39484.712616.AD. Review. Erratum in: BMJ. 2015;350:h1216.

Inclusion Criteria:

• Fulfillment of the Rome IV criteria for IBS (IBS-D, IBS-M, IBS-U)
• Provision of written informed consent.
• Commitment of availability throughout the study period.
• IBS-SSS > 175

Exclusion Criteria:

• Any concomitant disease requiring specialized nutrition (e.g. renal failure, diabetes, celiac disease, cerebrovascular disease of the central nervous system, major surgical cavity).
• Pregnancy.
• Breastfeeding.
• IBS-C (Bristol Scale type 1 - 2)