At the start of her program, Penny's 25-hydroxy vitamin D blood level showed the usual deficiency at 22 ng/ml.
She supplemented with 8000 units of vitamin D. Another 25-hydroxy vitamin D blood level several months later showed a level of 67.8 ng/ml , right on target.
But Penny also began our diet, including the elimination of wheat, cornstarch, and sugars, and, over 6 months, lost 34 lbs.
Now a much trimmer 146 lbs (still more to go!), another vitamin D blood level: 111 ng/ml.
Penny's weight loss means that the vitamin D is distributed in a smaller total volume, particularly a lower volume of fat.
This is a common phenomenon with substantial weight loss: lose weight and the need for vitamin D is reduced. The reduction in dose is roughly proportion to the weight lost. Vitamin D should therefore be reassessed with any substantial change in weight of, say, 10 lbs or more, either up or down, because of the influence of fat on vitamin D blood levels.
Some references on this effect:
Men and women over age 65:
Adiposity in relation to vitamin D status and parathyroid hormone levels: a population-based study in older men and women.
Low 25-hydroxyvitamin D concentrations in obese women: their clinical significance and relationship with anthropometric and body composition variables
Hypovitaminosis D in obese children and adolescents: relationship with adiposity, insulin sensitivity, ethnicity, and season.
Relationship of vitamin D and parathyroid hormone to obesity and body composition in African Americans.
Although the bulk of the effect is most likely due to sequestration by fatty tissue, perhaps less sun exposure in obese people also contributes:
Body mass index determines sunbathing habits: implications on vitamin D levels.