The human physiology is an electrochemical machine of astounding complexity. There are many proteins, enzymes, and hormones at play in the human body about which only the vaguest of guesses as to their function can be made. A good start at understanding the human body, however, can be made by understanding what nutrients are required for survival, and what functions they play. As the saying goes, “you are what you eat”, and no machine left long without maintenance will function at the peak of its capabilities.
In this post we’ll begin exploring the metabolic pathway of vitamin A, an essential nutrient for human life and one of the main focuses of the NIR visual perception project.
Vitamin A is a term used to describe a group of similar organic compounds which play essential roles in the human physiology. Like other organic compounds, vitamin A can be found in several different forms, collectively referred to as retinoids, and is also a natural byproduct of the digestion of provitamins called carotenoids.
Dietary vitamin A comes in two primary forms. From animal sources of food–meat, fish, poultry, dairy, etc.–it’s ingested as a retinyl ester–usually retinyl palmitate or retinyl acetate. These esters are fatty compounds that are the primary storage form of vitamin A in the body. Herbivores and omnivores (like us) can also extract vitamin A from plant food sources with a special enzymatic system known as 15-15′-dioxygenase. This enzyme system breaks down certain carotenoid compounds (alpha-, beta-, and gamma-carotene) as well as beta-cryptoxanthin in the intestine into retinol. Retinol, the alcohol form of vitamin A, is the primary transport form of vitamin A.
After ingestion, dietary vitamin A is broken down from either its esterified forms or its carotene provitamins and absorbed in the proximal (near end) small intestine with the help of pancreatic lipase enzymes. After undergoing a few more transitions, vitamin A (as retinol) is carried through the circulatory system to the liver and adipose (fatty) tissue, esterified again (retinyl ester), and stored.
Once stored, retinoid levels in the body are maintained by a homeostatic mechanism–which basically means that your body likes to have a certain amount of retinoids circulating, and will either store excess in fats or withdraw more as retinol to maintain that balance as needed. When circulating levels of retinol are high, the body esterifies and stores some. When circulating levels are low, the body synthesizes retinol binding protein (RBP) which then carries the retinol to where it is needed. It stays in its alcohol transport form until reaching its destination, where it is locally metabolized into a more useful form. The majority of vitamin A’s functions in the body are fulfilled by the form retinoic acid. Retinoic acid performs life-sustaining roles ranging from gene transcription to skin maintenance, and a severe deficiency in it will very quickly lead to illness followed by death.
Another metabolically active form of retinol, of interest to us, is retinal. Retinal is the aldehyde form of retinol, and is intraconvertible with it– meaning the body can change retinol into retinal, and vice versa, as needed. Retinal’s most significant role is played in vision. After being carried to the eyes through the circulatory system and converted into retinal, an as yet unidentified cell membrane transporter carries it into the photoreceptor cells of the retina–rods and cones. There it is bound to opsin protein complexes to form rhodopsin in the rods and photopsin in the cones, and acts as a phototransducer —a protein complex which translates radiant stimuli (light) into electrochemical signals which are passed to the brain via the optic nerve.
You now have a very basic overview of the way by which retinoids, collectively called vitamin A, are ingested, stored, transported, and utilized by the human body. In the next few posts we’ll be exploring in more detail the way vitamin A is used in the body (particularly in vision), how vitamin A2 comes into play, and what the differences in function may be between the two organic compounds.
Note: Thanks to DOG_GOD for finding this diagram of vitamin A’s metabolic pathway.