Its there but you can't have it: Vitamin B3 bioavailability

The clinical deficiency disease caused by lack of vitamin B3 (niacin) is pellagra. The diagnosis is the four D's: diarrhea, dermatitis, dementia and eventually death. This showed up in cultures that had a subsistence diet based upon corn.

Tests in the 80's showed that 27.3 micro grams of niacin per gram of corn. Although this is not a great source of niacin (broccoli has 930 micro grams per gram), it should be enough to stop pellagra. However if this corn is feed to rats only 7 micro grams were available for the rats, thus showing that somehow that the niacin was not bioavailability.

It turns out that corn gets a double whammy with vitamin B3/niacin decrease. First the level in the corn drops as the it matures and the level unavailable increases. This is shown in the graph below (day 49 the corn is deemed mature)
So combing the loss and the rise of unavailable can be combined to show the overall decrease. The graph below shows this. This decrease is rather rapid over the 30 days were measurement were taken. Starting at 50 micro grams and ending up at just under 10, this is a 80% decrease.
The reason for these changes are two fold. Vitamin B3 is used by an enzyme to produce energy, therefore when the corn is rapidly growing after pollination the vitamin B3 levels will be high. As the corn starts to slow down its growth as it is maturing the enzyme is no longer needed and thus so vitamin B3 also decreased. Also as the seed matures it starts to "shut down" getting ready for hibernation, so to speak, it modifies the vitamin to a more stable form so that it can survive until the seed starts to sprout and needs the vitamin B3 again.

Update: What this means is that you want to eat your corn as young as possible. So if you are growing your own try and pick it as soon as possible. That is not to hard as fresh season corn is one of the highlights of growing your own vegetables. As for purchasing from the store, I would suspect that the corn is picked once it is quite mature so no damage is done to the softer corn pieces, but young enough age that the kernels haven't started to dry out.

Reference: Joseph S. Wall, Michael R. Young, Kenneth J. Carpenter Transformation of niacin-containing compounds in corn during grain development: relationship to niacin nutritional availability September 1987 Volume 35, Issue 5