We don’t judge whether you have enough vitamin D just based on what you eat anymore.  In part that's because the sources of the vitamin are both sun exposure and diet.  They aren't the same every day and they really aren't the critical issue.

Our initial understanding of vitamin D grew out of the recognition that cod liver oil and sunlight could cure rickets, a disease causing bent and weakened bones. The Vitamin D from the sun and fish oils helped absorb of calcium and magnesium and in using these minerals to form teeth and remodel bones.

But recent data are beginning to suggest that vitamin D has a far broader role in preventing high blood pressure, some autoimmune diseases, and even cancer.

We know that Vitamin D is crucial in preventing rickets in the young and osteopenia (decreased bone mineralization) and osteoporosis (a weakening of the bones) with advancing age.  But we are now learning that Vitamin D seems to have an important role in immune function, high blood pressure and preventing diabetes. The problem is that it doesn't necessarily mean that the dose need to affect bone mineralization is the same dose that's needed for these other roles.