Individuals with gastrointestinal diseases - such as Crohn's disease, ulcerative colitis, ileostomy, or bile acid diarrhoea - are at increased risk of magnesium deficiency. Magnesium is a vital mineral that supports many essential functions in the body, including muscle contraction, nerve signalling, heart rhythm, and bone health. Deficiency may contribute to fatigue, muscle cramps, abnormal heart rhythms, and reduce the quality of life. The purpose of this study is to investigate the prevalence of magnesium deficiency in individuals with these conditions and to identify the most accurate and practical methods for assessing magnesium status in clinical care. Although plasma magnesium is commonly used in routine blood tests, it represents only about 1% of the body's total magnesium and may not reflect true magnesium levels within cells or tissues. Hence, this study compares several different ways of measuring magnesium, including: * Plasma magnesium * Magnesium levels in red and white blood cells (PBMC, RBC, and buffy coat) * Magnesium levels in muscle tissue (via biopsy) * A magnesium retention test, based on how much magnesium is excreted after an infusion The study includes four groups: 1. Patients with inflammatory bowel disease. 2. Patients with an ileostomy. 3. Patients with bile acid diarrhoea. 4. Healthy individuals (control group). All participants will provide blood and urine samples, and some may undergo optional biopsies of muscle or intestinal tissue. Participants will also complete questionnaires and undergo tests of muscle strength and body composition. The findings are expected to enhance the understanding and detection of magnesium deficiency in patients with gastrointestinal diseases and to aid in the development of more effective tools for identifying and treating this common yet often overlooked condition.