I'm glad you feel fine and stay active. Are your blood counts normal? If so, what symptoms led to your having a bone marrow biopsy?
Staying as healthy as possible, in all of the ways that any doctor would recommend, puts you in the best possible position should treatment eventually be needed. Patients who are strong, active, and healthy other than having MDS have the most treatment options available and the best chances to do well during treatment.
There are plenty of people who recommend particular diet choices, supplements, or alternative treatments to boost the immune system or maintain other aspects of your health, but I don't know of research showing that they can prevent or undo mutations to chromosomes or stop the mechanism of MDS within your bone marrow.
There are growth factors that enhance marrow function and chemotherapy drugs that stop or slow the progression MDS for many patients, but that's the type of treatment the doctors have apparently told you that you don't need, or at least don't need for now.