Vitamin D deficiency. Way to maintain d?

I am 50 kg, and 24 yrs. On 01/01/2016 my 25-HYDROXYVITAMIN D was 5.8 ng/ml => DEFICIENT. So my dietitian recommended me Anyway nutrilite Cal mag D 2 tablets /day. This awesome supplement which helped my vitamin D level reach 60.8=> SUFFICIENT, in 5 months. I have attached the two reports. Afterwards I stopped taking the supplement. I had also gone through bone mineral density test of my entire body and the density of my spinal area was moderately low with moderate level of fracture risk which I understand because whenever I sit without back support, my spinal area starts feeling pain/fatigue very soon. SINCE I STOPPED CONSUMING CAL MAG D 11 MONTHS AGO, I ANTICIPATE THE VITAMIN D LEVEL WOULD HAVE AGAIN BECOME DEFICIENT WHICH CAN BE FURTHER SUPPORTED BY MY SPINAL FATIGUE WHICH I HAVE FOR SEVERAL YEARS. SO I WANT TO KNOW OF WHETHER I SHOULD START CONSUMING CAL MAG D AGAIN AND IF YES THEN HOW MANY TABLETS A DAY AND WHEN WOULD I KNOW TO STOP TAKING THE SAME. IS THERE A WAY TO MAINTAIN D?