Vitamin A, also known as retinol, is a fat-soluble nutrient essential for various bodily functions. It plays a significant role in maintaining healthy vision, especially in low light, and supports the proper functioning of the immune system. This nutrient is also involved in cell growth, differentiation, and the maintenance of healthy skin and mucous membranes, contributing to the normal formation and upkeep of organs like the heart, lungs, and eyes. The kidneys do not produce Vitamin A.
Where Vitamin A Comes From
The human body obtains Vitamin A from dietary sources, primarily through two forms: preformed Vitamin A and provitamin A carotenoids. Preformed Vitamin A, which includes retinol and retinyl esters, is found in animal products such as dairy, eggs, fish like salmon and mackerel, and organ meats including liver. Provitamin A carotenoids, notably beta-carotene, are plant pigments abundant in yellow, red, and green leafy vegetables like carrots, sweet potatoes, and spinach, as well as yellow fruits such as mangoes and apricots. Many foods, including milk, margarine, and some cereals, are also fortified with Vitamin A.
Once consumed, Vitamin A is absorbed in the small intestine; preformed Vitamin A exhibits high bioavailability when consumed with some fat. Provitamin A carotenoids are converted into retinol within the intestine before absorption. After absorption, retinyl esters and carotenoids are packaged into chylomicrons, which then enter the lymphatic system and eventually the bloodstream. The liver serves as the main storage site for Vitamin A, holding approximately 70% to 95% of the body’s total supply, primarily as retinyl esters within specialized hepatic stellate cells. From the liver, retinol is released into circulation, bound to retinol-binding protein (RBP) and transthyretin, to be transported to various tissues throughout the body.
The Kidney’s Main Responsibilities
While the liver manages Vitamin A, the kidneys perform distinct and essential functions to maintain overall body health. Their primary responsibility is to filter waste products and excess substances from the blood. Each day, the kidneys process a large volume of blood, typically around 180 to 200 liters, to produce about 1 to 2 liters of urine containing metabolic wastes like urea, creatinine, and acids. This filtration process occurs within millions of tiny filtering units called nephrons.
Beyond waste removal, the kidneys are instrumental in regulating the body’s fluid and electrolyte balance. They manage the levels of crucial electrolytes such as sodium, potassium, chloride, calcium, and phosphate, and help maintain the body’s acid-base equilibrium. The kidneys also play a role in blood pressure regulation by releasing hormones like renin, which is part of a system that controls blood vessel constriction and fluid balance. They also produce other important hormones, including erythropoietin (EPO), which stimulates red blood cell production in the bone marrow, and calcitriol, the active form of Vitamin D, which is involved in calcium and phosphate regulation for bone health.
How Kidneys Handle Vitamin A
The kidneys do not synthesize Vitamin A; instead, they play a regulatory role in its metabolism and excretion. Vitamin A, primarily in its retinol form, circulates in the bloodstream bound to retinol-binding protein (RBP). This retinol-RBP complex typically associates with transthyretin (TTR), a larger protein, which prevents the rapid filtration and loss of Vitamin A by the kidneys. Without this association, the smaller retinol-RBP complex would be filtered out by the glomeruli, the initial filtering structures in the kidneys.
Even if filtered, more than 99% of the retinol is reabsorbed back into the bloodstream by the proximal tubules of the kidney through specific endocytic receptors. Any excess or metabolized forms of Vitamin A that are not reabsorbed are eventually excreted from the body; approximately 60% of metabolized Vitamin A is eliminated through urine, with the remaining 40% excreted in feces. In cases of impaired kidney function, such as end-stage kidney disease, the kidneys’ ability to clear the retinol-RBP complex decreases, leading to elevated Vitamin A levels in the blood. These elevated levels can sometimes contribute to complications like increased calcium levels. Beyond its processing, derivatives of Vitamin A, such as retinoic acid, are also involved in the proper development of the kidneys during embryonic stages and contribute to their function and repair in adults.