Chronic Kidney Disease and Vitamin D Deficiency

Vitamin D
Vitamin D, also known as calciferol, is a fat-soluble vitamin. It is an essential nutrient that supports strong bones by helping the body absorb calcium and phosphorus. It also plays a vital role in the functioning of the immune system, muscles, and nerves. The body produces vitamin D when exposed to sunlight, but it can also be found in certain foods such as fatty fish, eggs, and fortified milk.

In foods and dietary supplements, vitamin D has two main forms, D₂ (ergocalciferol) and D₃ (cholecalciferol), that differ chemically only in their side-chain structures. Both forms are well absorbed in the small intestine. The concurrent presence of fat in the gut enhances vitamin D absorption, but some vitamin D is absorbed even without dietary fat. Neither aging nor obesity alters vitamin D absorption from the gut.

Vitamin D Deficiency
Vitamin D deficiency is more common among people who are lactose intolerant, have a milk allergy, or follow an ovo-vegetarian or vegan diet. Deficiency may also occur in people who have limited exposure to sunlight, those whose kidneys cannot convert 25(OH)D (or 25-hydroxyvitamin D) to its active form, 1,25-dihydroxyvitamin D (calcitriol), or those who cannot absorb vitamin D efficiently in the digestive tract. Vitamin D deficiency can manifest as rickets in children and as osteomalacia in adolescents and adults.

Growing evidence has indicated that vitamin D deficiency may contribute to deteriorating kidney function, as well as increased morbidity and mortality in patients with Chronic Kidney Disease[1].

Vitamin D Deficiency and Chronic Kidney Disease
Vitamin D deficiency does not directly cause Chronic Kidney Disease, but it is strongly associated with its progression and complications[2].

Vitamin D deficiency is common in patients with Chronic Kidney Disease, because the kidneys play a key role in converting 25-hydroxyvitamin D to its active form. As kidney function declines, this conversion is impaired, leading to low vitamin D levels.

These low vitamin D levels can exacerbate Chronic Kidney Disease-related issues, such as secondary hyperparathyroidism, bone disease (renal osteodystrophy), and cardiovascular complications, by disrupting calcium and phosphorus metabolism.

Supplements
Recent studies have suggested that treatment with active vitamin D or its analogues can ameliorate kidney injury by reducing fibrosis, apoptosis, and inflammation (in animal models). This treatment also decreases proteinuria and mortality in patients with Chronic Kidney Disease[3].

[1] Andreas: Vitamin D in chronic kidney disease: a systemic role for selective vitamin D receptor activation in Kidney International - 2006
[2] Dusso, Tokumoto: Defective renal maintenance of the vitamin D endocrine system impairs vitamin D renoprotection: a downward spiral in kidney disease in Kidney International - 2011
[3] Alvarez et al: Effects of high-dose cholecalciferol on serum markers of inflammation and immunity in patients with early chronic kidney disease in Europese Journal of Clinical Nutrition - 2013