 How much vitamin D we should get is one of the most controversial areas in the field of nutrition. Up until recently, the Institute of Medicine, the official body that sets the recommended daily allowances, considered 200 international units a day an adequate intake for those in middle age, but just bumped it up to a recommended 600 a day. How did they come up with the original 200? Why did they go up to 600, and why are there some experts out there saying we should take thousands a day? It all started with this line, written more than a half century ago. Apparently a dosage of only 100 units of vitamin D daily was sufficient to prevent the flagrant signs and symptoms of rickets, the bow-legged bone softening disease caused by overt vitamin D deficiency. From a review last year, the fact that 100 units of vitamin D prevented overt signs of rickets led to the false security that ingesting twice this amount was more than adequate to satisfy the body's vitamin D requirement, hence the 200 recommendation, but no longer. The Institute of Medicine decided to take a more scientific approach by basing the recommendations on a specific target blood level, which they calculated to be 20 ng per mL, which should prevent rickets in children in a similar bone softening condition called osteomalacia in adults. To get most people's levels up to 20, though, the Institute of Medicine figured it would require about 600 IU's a day, and so the recommendation was officially bumped up. At the Institute of Medicine's new target of 20 in the blood, right now, 40% of Americans are vitamin D deficient. And if we use higher cut-offs, say 30, then that proportion climbs to more than 3 quarters of our population.