Inadequate exposure to sun, less intake of food or impaired intestinal absorption of Vitamin D are the reasons.
The main reason for inconsistent vitamin D levels in an individual due to inadequate exposure of body to the sun, limited oral intake of food which contains Vitamin D, or may be impaired intestinal absorption of Vitamin D.
Fish, red meat, liver, egg yolks and fortified foods are the good sources of Vitamin D so in my opinion John has inconsistent Vitamin D levels due to inadequate exposure to the sun, less intake of food that contains Vitamin D, or impaired intestinal absorption of Vitamin D.
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