The field of medicine is much too large for doctor's to know everything. They are usually not very knowledgeable about dietary needs, following the very conservative general guidelines.

See, for instance the NIH at https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessiona...:

  In 2011, The Endocrine Society issued clinical practice 
  guidelines for vitamin D, stating that the desirable 
  serum concentration of 25(OH)D is >75 nmol/L (>30 ng/ml) 
  to maximize the effect of this vitamin on calcium, bone, 
  and muscle metabolism [37]. It also reported that to 
  consistently raise serum levels of 25(OH)D above 75 
  nmol/L (30 ng/ml), at least 1,500-2,000 IU/day of 
  supplemental vitamin D might be required in adults, and 
  at least 1,000 IU/day in children and adolescents.

1500-2000 IU/day corresponds to 37.5-50 mcg.

See the section on Cancer in the following article from the Mayo Clinic: http://www.mayoclinicproceedings.org/article/S0025-6196(13)0.... But compare to what the previously mentioned NIH page says about vitamin D and cancer.

Find more references, read them and make up your mind. There is plenty to be found. Next up: Potassium. Magnesium. Calcium.

Not my post you responded to, but during winter months in northern Europe, vitamin D in diet is recommended.

http://norden.diva-portal.org/smash/get/diva2:704251/FULLTEX... Page 351.