Inflammatory bowel disease (IBD) is a chronic illness characterized by inflammation of the intestine. Many individuals with IBD suffer from depressive symptoms and anxiety which can lead to a decreased quality of life, poor treatment compliance, and higher morbidity and mortality. The object of this clinical trial is to investigate the effects of a fasting mimicking diet in IBD patients who are suffering with symptoms of depression. Participants will carry out 3 cycles of a 5-day period of a plant-based low caloric diet or a plant-based caloric sufficient diet following by 3 weeks of eating normally. Effects of the dietary intervention on microbes in the gut, immune and metabolic function, and depressive symptoms will be measured. The overall goal is to develop a safe and effective treatment to improve mental health in patients with IBD by targeting the gut microbiome through dietary interventions.

The primary objective of this proof of principle study is to investigate the effects of a fasting mimicking diet on depressive symptoms in patients with inflammatory bowel disease.

The specific aims of this project include:

1. Determine whether a fasting-mimicking diet is effective in ameliorating depressive symptoms in patients with Crohn's disease or ulcerative colitis
2. Determine if beneficial effects seen during the interventional period are sustained following cessation of fasting
3. Examine effects of the intervention on the gut microbiome and metabolome

Inclusion Criteria:

• Age ≥ 18 and ≤ 65 years at the time of screening
• Diagnosis of Crohn's disease in clinical remission or with mild-moderate disease with Harvey Bradshaw Index (HBI<8) or diagnosis of ulcerative colitis in clinical remission with partial Mayo (pMayo<7)
• Suffering from mild-moderate depression with PHQ-9 ≥5 and ≤ 19

Exclusion Criteria:

• Subjects who are reliant on partial or total parenteral nutrition
• Subjects with prior gastrointestinal surgery and consequences such as short bowel syndrome, ostomy of small or large intestine, total colectomy, proctocolectomy, or ileoanal pouch
• Subjects with diabetes or celiac disease
• Subjects with a body mass index (BMI) lower than 18
• Subjects suffering from malnutrition or at high risk of malnutrition assessed by a score of ≥6 on the abridged patient generated subjective global assessment (PGSGA)
• Subjects allergic to nuts
• Subjects currently on a fasting/intermittent type caloric restricted diet
• History of psychotic or bipolar disorders or experiencing suicidal thoughts
• Pregnant women, women who are breast feeding, or women planning on becoming pregnant
• Subjects with immune-compromised condition other than inflammatory bowel disease (e.g. AIDS, lymphoma)
• Subjects with severe uncontrolled cardiovascular or respiratory disease or active malignancy
• Unable to read English or provide informed consent

Investigator and lab personnel involved in assessment of outcomes will be blinded to the group allocation.