Vitamin A

Vitamin A is an essential fat-soluble nutrient. It belongs to a group of related organic compounds known as vitamers. All these vitamers share a specific structure called a β-ionone ring.

This group includes several important molecules. These are retinol, retinyl esters, and various provitamin carotenoids. Retinol is the primary form used for transport and storage. It is vital for embryo development and immune system health. It also plays a critical role in how we see light. Without it, the body cannot function correctly.

There are two main ways we consume vitamin A through food. First, we eat retinoids from animal sources. These appear as retinol or as retinyl esters bound to fatty acids. Second, we eat carotenoids from plants. These include α-carotene, β-carotene, and γ-carotene.

Many animals use enzymes to convert these carotenoids into retinol. This process is called cleaving. However, strict carnivores like cats lack these specific enzymes. They must get their vitamin A directly from animal tissues. This makes them dependent on eating meat to survive.

Once ingested, the body begins a complex absorption process. Retinyl esters from animal foods are acted upon by enzymes in the small intestine. These enzymes release free retinol. This retinol enters cells through a process called passive diffusion. The absorption efficiency is quite high, between 70 and 90 percent.

For plant-based carotenoids, the process is different. The body uses a transporter protein called SCARB1 to take up β-carotene. Interestingly, the body increases these transporters during times of vitamin A deficiency. This helps the body grab as much nutrient as possible.

The liver serves as the body's main storage center. Retinol is stored in tiny lipid droplets within liver cells. Two types of cells manage this: hepatocytes and hepatic stellate cells. Hepatocytes take up the vitamin and pass it to stellate cells for long-term storage. Because of this high storage capacity, humans can survive for months on a deficient diet. We only see symptoms when these liver reserves are nearly empty. This is a vital survival mechanism for well-nourished people.

Inside the eye, vitamin A powers the visual cycle. Retinol is converted into a compound called 11-cis-retinal. This molecule binds with a protein called opsin. Together, they form rhodopsin, which is necessary for seeing in low light.

When light hits the eye, it changes the shape of the rhodopsin. This change creates a nervous signal sent to the brain. The body then recycles the molecule to start the process again. If vitamin A levels are low, the body cannot reform rhodopsin. This leads to night blindness, which is often the first sign of deficiency.

Vitamin A also acts as a powerful chemical messenger. Retinol can be converted into retinoic acid. This process is irreversible. Retinoic acid travels into the cell nucleus to activate hundreds of genes. This helps regulate many different biological functions. To prevent too much buildup, the body uses enzymes to break retinoic acid down. This creates a self-regulating feedback loop that keeps levels stable. This regulation is essential for healthy growth and development.

Deficiency is a major global health challenge. It is common in Sub-Saharan Africa and Southeast Asia. It affects roughly one-third of children under age five worldwide.

Severe deficiency can cause xerophthalmia, which is the drying of the eyes. This can lead to corneal ulcers and permanent blindness. It also weakens the immune system, making childhood diseases more dangerous. On the other end of the spectrum, too much vitamin A can cause toxicity. This is known as hypervitaminosis A. It can affect the nervous system, liver, and bones.

Scientists have studied these pathways for a long time. Researchers like George Wald helped us understand the visual cycle.

Understanding these mechanisms allows us to set safe dietary limits. It also helps us develop treatments for deficiency. We now know how to use retinol to regulate genes and maintain vision. This knowledge is vital for both medicine and public health globally.