What Causes A Vitamin D Deficiency?
Obesity

Obesity is strongly associated with lower vitamin D levels, partly because the lifestyles leading to obesity also lead, separately, to a vitamin D deficiency. For instance, overweight individuals tend to stay indoors, which prevents them from absorbing sunlight. They also tend to have poor diets, which also limits vitamin D intake. There is increasing evidence this link is also because being overweight actually causes vitamin D to be less bioavailable, leading to a deficiency, though the reasons behind this are not well understood. It might be because vitamin D is stored in fat and obese individuals have trouble burning their fat to access it. Either way, if an individual is overweight, shedding the extra weight could help them overcome chronic vitamin D deficiency. This is easier said than done, of course, but the results are well worth it. Small improvements here and there in diet and lifestyle lead to huge changes down the road.
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Kidneys Cannot Convert Vitamin D

Vitamin D2 and D3 are both inactive forms of vitamin D. The liver and kidneys convert these inactive forms into vitamin D's active form, calcitriol. If an individual's kidneys are impaired, they may become deficient in vitamin D even if they're consuming enough, simply because it cannot be converted effectively. Age can be a significant factor in the function of an individual's kidneys. As individuals age, their kidneys weaken, and they become more at risk for kidney disease. The skin also thins as individuals age, which lessens an individual's ability to produce vitamin D3.
Chronic kidney disease is extremely difficult to spot in its early stages, and is difficult to treat later on. Symptoms of advanced chronic kidney disease include fatigue, dry skin, weight loss, poor sleep, swelling, and urinary issues. If patients know they have chronic kidney disease, this will easily explain any vitamin D deficiency they may also be experiencing. If they are unsure, but are worried their kidneys cannot convert vitamin D, they should consult a doctor.
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