Vitamin D is a fat-soluble vitamin important for several biological processes, particularly bone formation and immune function. Vitamin D is either absorbed from food or produced in the skin after exposure to sunlight. In recent decades, researchers have discovered that a healthy liver is necessary to maintain adequate levels of vitamin D in the body. Many kinds of liver diseases that cause damage to the liver can result in low levels of vitamin D in the body.
Vitamin D Absorption
A healthy liver is required at several different points during the biological life cycle of vitamin D. Because vitamin D is fat-soluble, it must be dissolved in fat molecules to be absorbed from the digestive tract into the body. To absorb fats, and the vitamins dissolved in the those fats, the body requires a substance called bile, which contains salts and enzymes that break down large fat molecules into smaller molecules that can be absorbed by the intestines. Bile is produced by liver, then stored in the gallbladder and released into the intestines during digestion.
Cholestatic Liver Disease
Cholestatic liver disease refers to a block in the flow of bile from the liver, limiting its availability to the intestines. Several different liver ailments can result in a reduction in bile, including damage to the liver caused by alcoholism, viral hepatitis or toxic chemicals. Without sufficient bile, the body cannot breakdown and absorb fats, and deficiencies in fat-soluble vitamins such as vitamin D can occur.
Vitamin D Activation
When vitamin D is first absorbed in the intestines or produced in the skin, it is in an inactive form. In the liver, inactive vitamin D is converted into its active form, called 25-hydroxyvitamin D. Without this conversion into the active form, vitamin D cannot perform its vital functions in the body. The activity of vitamin D is further enhanced in the kidneys, where 25-hydroxyvitamin D is converted to 1,25-dihydroxyvitamin D, the most potent form of vitamin D.
Paranchymal Liver Disease
Parenchymal liver disease refers to diseases of the liver that do not affect bile production, but instead affect the other functions that occur in the liver, including the metabolism of vitamin D. When someone develops parenchymal liver disease, their liver can no longer efficiently convert vitamin D into 25-hydroxyvitamin D, leading to symptoms of vitamin D deficiency. Many factors can cause parenchymal liver disease, including alcoholism, viral hepatitis and several types of infections.