According to Cleveland Clinic there are a couple of main causes of Vitamin D deficiency.
1. Not getting enough vitamin D in your diet and/or through sunlight.
2. Your body isn�t properly absorbing or using vitamin D.
Moreover, there are medical conditions such as cystic fibrosis, obesity, kidney diseases,
and liver diseases that can prohibit vitamin D from being absorbed properly.
If you underwent weight loss surgery that "reduce the size of your stomach and/or
bypass part of your small intestines", the surgery can make it difficult for the body to
absorb sufficient quantities of certain nutrients, vitamins and minerals.
Plus certain medications such as laxatives, steroids, cholesterol-lowering medications, or
certain seizure preventative medications can also lower vitamin D levels.
Full story:
https://regrowthcasereports.quora.com/What-causes-a-vitamin-D-deficiency
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