Rationale: Glucagon-Like Peptide-1 (GLP-1) receptor agonists have emerged as effective treatments for obesity and associated medical conditions. However, patients may be at risk of micronutrient deficiencies during therapy due to reduced appetite, altered gastrointestinal physiology, and weight loss. Multivitamin supplementation is commonly prescribed to mitigate these risks, but the necessity and efficacy of multivitamin use in GLP-1 users remain debatable, as some clinicians advocate for routine supplementation while others do not.

Objective: To assess the differences in micronutrient levels between patients who use multivitamin supplements while on GLP-1 therapy and those who do not, to provide evidence on the necessity and benefits of supplementation.

Study Design: Randomized, double-blind, placebo-controlled trial. Study Population: Adults with obesity (BMI ≥30 kg/m² or ≥27 kg/m² with obesity-associated medical problems) currently using GLP-1 receptor agonists for at least 3 months with stable dosing.

Intervention:

• Group A (Intervention): Multivitamin supplementation while on GLP-1 therapy
• Group B (Control): Matching placebo while on GLP-1 therapy Main Study Parameters/Endpoints: The primary outcomes are the differences in serum levels of Vitamin B12, Vitamin D, and Ferritin between the multivitamin and placebo groups after 12 months of supplementation while on GLP-1 therapy.

Nature and Extent of the Burden and Risks: Participants will undergo blood sampling at four timepoints: baseline (before starting GLP-1 therapy), at randomization (3 months of stable GLP-1 therapy), and at 6 and 12 months post-randomization. The risks associated with the study are minimal and primarily related to blood sampling and potential side effects of multivitamin supplementation. The burden includes time commitment for study visits and daily supplement intake.

Inclusion Criteria:

• Age ≥18 to 70 years.
• Start with the use of Tirzepatide GLP-1 receptor agonists and for at least 3 months with stable dosing before randomization.
• BMI ≥30 kg/m² or ≥27 kg/m² with obesity-associated medical problems (such as type 2 diabetes, hypertension, or dyslipidemia) at the start of GLP-1 therapy
• Willing and able to provide informed consent
• Able to comply with study procedures and follow-up visits
• Own vitamins should be completely discontinued 2 weeks before the start

Exclusion Criteria:

• Known allergy or hypersensitivity to any components of the multivitamin supplement
• Current use of other vitamin or mineral supplements
• Pregnancy or planning pregnancy during the study period
• Patients who follow a vegan diet.
• Active malignancy or ongoing cancer treatment
• Severe kidney disease (eGFR <30 mL/min/1.73m²)
• Severe liver disease (Child-Pugh C)
• History of metabolic bariatric surgery
• Participation in other clinical trials within the past 3 months
• Any condition that, in the investigator's opinion, would interfere with study participation or compromise patient safety

The investigator will be responsible for the data analysis. The analysis will be performed on a blinded data set after medical/scientific review has been completed, all protocol violations have been identified, and the data set has been declared complete. All data will be collected in a data management system (Castor EDC, Amsterdam, The Netherlands; https://www.castoredc.com) and performed according to Good Clinical Practice guidelines, Data Protection Directive certificate and complies with Title 21 CFR Part 11. Furthermore, the datacenter where all the research data is stored is ISO27001, ISO9001 certified and the Dutch NEN7510 certified.