This study will test whether a plant-based diet is practical (feasible) for overweight people with monoclonal gammopathy of undetermined significance (MGUS) or smoldering multiple myeloma (SMM). The researchers will decide how practical the diet is by looking at how much weight participants lose and how well they are able to stick to the diet. The researchers will also determine whether the diet is effective in preventing multiple myeloma in participants. In addition, they will look at how safe the plant-based diet is for participants, and see if the diet affects participants' quality of life.

Inclusion Criteria:

• BMI ≥25
• Confirmed diagnosis of MGUS or SMM
• M spike (immunoglobulin) ≥0.2 g/dL or abnormal free light chain ratio with increased level of the appropriate involved light chain
• Secretory disease
• Age ≥18 years
• Willingness to comply with all study-related procedures
• ECOG performance status of 0-3
• Interest to learn to cook plant based recipes

Exclusion Criteria:

• Patients that already follow a whole foods plant based or vegan diet (ovo-lactovegetarian diets are not excluded)
• Legume allergy
• Severe allergies, such as anaphylactic shock to peanuts
• Concurrent participation in weight loss/dietary/exercise programs
• Mental impairment leading to inability to cooperate
• Enrollment onto any other therapeutic investigational study
• Concurrent pregnancy
• Patients with a known diagnosis of diabetes mellitus will not be excluded but will need to be followed regularly with an endocrinologist/primary care physician during the trial period.
• Positive HBV, HCV, HIV PCR testing
• Non English speaking
• ≥ Grade 2 electrolyte abnormalities as defined by CTCAEv5.0 (need to be resolved before enrolling on study)
• If in the opinion of the investigator there maybe any concerns regarding the ability of the patient to complete the study safely

Memorial Sloan Kettering Cancer Center supports the international committee of medical journal editors (ICMJE) and the ethical obligation of responsible sharing of data from clinical trials. The protocol summary, a statistical summary, and informed consent form will be made available on clinicaltrials.gov when required as a condition of Federal awards, other agreements supporting the research and/or as otherwise required. Requests for deidentified individual participant data can be made beginning 12 months after publication and for up to 36 months post publication. Deidentified individual participant data reported in the manuscript will be shared under the terms of a Data Use Agreement and may only be used for approved proposals. Requests may be made to: crdatashare@mskcc.org.