This trial investigates the effect of fermented milk supplementation on symptoms of disease and treatment in patients with multiple myeloma. Patients with multiple myeloma may experience symptoms related to the disease and/or treatment that affect quality of life. Supplementing usual diet with a probiotic fermented milk product called kefir may contribute to reducing disease and treatment-related side effects through changing the intestinal bacteria community structure and related metabolism.

PRIMARY OBJECTIVE:

I. Determine the effect of a 3-month randomized dietary intervention with a probiotic kefir beverage in multiple myeloma (MM) patients on biomarkers of metabolism, and patient-reported pain and fatigue, gut health, and quality of life (QoL).

SECONDARY OBJECTIVE:

I. Determine the effect of kefir supplementation on gut microbial phylotype and predicted bacterial metabolic function, and assess associations with biomarkers of metabolism, and patient-reported pain and fatigue, gut health, and QoL.

EXPLORATORY OBJECTIVE:

I. Feasibility of a probiotic lifestyle intervention in MM patients.

OUTLINE: Patients are randomized to 1 of 2 arms.

ARM I: Patients consume commercial kefir beverage daily for 3 months.

ARM II: Patients maintain usual diet for 3 months.

After completion of study, patients are followed up at 30 days or until resolution of any study related toxicity.

Inclusion Criteria:

• Eastern Cooperative Oncology Group (ECOG) performance status of 0-1
• Neutrophil values > 1,000/uL
• Diagnosis of multiple myeloma: on maintenance or continued treatment
• Show no signs of comorbidities, myeloma symptoms, or treatment side effects that would put them in danger when participating in the study according to the physician's discretion
• Able to understand and comply with study instructions, including willing to purchase and consume study beverage daily if in intervention arm or, avoid regular consumption of fermented dairy foods (kefir, yogurt, lassi, etc.) if in the control arm
• Understand the investigational nature of the study and sign an Independent Ethics Committee/Institutional Review Board approved written informed consent form prior to receiving any study related procedure

Exclusion Criteria:

• Allergies to milk
• Lactose intolerance
• Current habitual (> 3 times per week) consumption of yogurt, kefir, lassi, over-the-counter probiotic dietary supplements
• Chronic inflammatory bowel disease
• Autologous stem cell transplantation or chimeric antigen receptor (CAR)-T cell therapy within the last 6 months
• Prior allogeneic stem cell transplantation
• Major comorbidities that would cause danger to the patient when participating in the study
• Pregnant or nursing female participants
• Unwilling or unable to follow protocol requirements
• Any condition which in the investigator's opinion deems the participant an unsuitable candidate to take part in study intervention
• Adults unable to consent
• Individuals who are not yet adults (infants, children, teenagers)
• Pregnant women
• Prisoners