Ode to Vitamin A
Vitamin A refers to a group of molecules called retinoids, with twenty carbon atoms linked to thirty hydrogen atoms and one oxygen atom. At the back of the eye, the retina is a layer of nerve cells sensitive to light. In these cells, a form of retinoid binds to a protein, a large molecule made of strings of molecules that fold and twist into complex shapes. This protein, now called a pigment, absorbs photons of light which trigger a nerve impulse which is sent to the optic nerve and then the brain. Most of us read these words quickly, pretending they make sense, suppressing a half-panic of questions. What is a photon and how does it start a nerve impulse and what is a nerve impulse? How does a bundle of nerve impulses variously connected to aspects of vision like color, size, depth, and brightness become a buttery-brown leather couch or an angular black walnut tree topped by the unmistakable figure of a hawk?
We can’t hear ourselves think—we can’t process thought—at the speed of the cellular level. Still, we try to break things down, like cells do. A photon is what we have named the smallest unit of light. This “packet of energy” sometimes acts like a particle, sometimes a wave, a description already problematic. Entering the eye, the energy of a photon causes the electrons in the Vitamin A molecule to change orbit around their atom’s nucleus. Now hydrogen atoms rearrange into a new form, all happening in picoseconds, a few trillionths of a second. This new form of Vitamin A no longer fits into the protein to which it is bound, and in nanoseconds, billionths of a second, that protein changes into multiple new shapes. One of them activates another cellular protein which blocks the natural flow of positively-charged sodium ions into the cell, which changes the cell’s electrical charge, which creates the electric impulse, which travels—using an entirely different process—cell to cell to cell.
The words I am using are rough-shaped and also amorphous, suddenly not quite right, like that form of Vitamin A binding to that protein. And although we can pinpoint to some degree the where of what happens next—the electric impulse sent to the forebrain’s occipital lobe—we really have no clue as to that final question: how does this biochemistry become the image of a couch or a hawk? Nor do we understand the next step: how does the biochemistry of consciousness becomes a sentence? “That hawk every evening in the black walnut tree is the unwitting sentinel for my sorrow.” Or “No matter how much I love this buttery-brown leather couch, a sofa-futon would be more practical.”
It’s all happening without us. Nature as mysterious and secret as clouds racing across the sky, wild unknown landscapes that happen to be our own bodies, our very selves.
I began learning about Vitamin A because I am interested in hunger and the transformation of food into thought and story—more biochemistry turned to consciousness. Vitamins are how we describe any organic compound needed by the body in a small amount. We get Vitamin A or retinoids mostly from eating meat and meat products, and we get carotenoids—which our body converts to retinoids—mostly from eating green leafy vegetables and fruits colored orange, yellow, or red. How much Vitamin A we get from plants, however, is much lower than we once thought and varies based on season and preparation. Vitamin A is particularly important in the development of a human fetus and throughout childhood. We don’t always understand what Vitamin A is doing in the human body, but we do know that a lack of Vitamin A is the leading cause of preventable blindness, with a half million children going blind every year.
This is what happens. As well as morphing into the pigment that absorbs light, Vitamin A also helps produce mucous and maintain the surface of the eye and tear production. Without enough Vitamin A, you cannot cry. The mucosa membrane that covers the front of your eye and lines the inside of your eyelid becomes rough and dry and prone to infection. So does the cornea of the eye. Small holes 1-3 millimeters across, with steep sides, form at the edges of the cornea and then spread. Sometimes the cornea ruptures, liquid material of your eye escapes, and the result is a shrunken eyeball. Sometimes, the ulcerated areas are covered by white corneal scar tissue.
Half the children who go blind in this way die within a year, most from measles or diarrhea or malaria, because Vitamin A is also an important part of the immune system. This, again, is related to how Vitamin A produce mucous which protects the linings of the lungs and stomach and bowel. In truth, millions and millions of children die every year for lack of Vitamin A, whether they go blind first or not.
Eat a range of food and you’ll be fine. A carrot. An apricot. Butter. Scrambled eggs. Have a grilled cheese sandwich or vegetable soup or a kale salad. Drink mint tea. Vitamin A is a lovely molecule that fills the world and if you have access to these foods, eat them. You won’t need to worry about the pigments in your retina absorbing photons of light or the tears lubricating your eyes or weird changes in your skin and taste buds. If you are pregnant and understand that Vitamin A is essential for your child’s bone growth and development, you won’t wonder obsessively if you are seeing less well in dim light—the first sign of Vitamin A deficiency. Eat a sweet potato. Eat a fish. Eat a hamburger. Eat dinner with your family. Eat lunch with friends. Eat alone as you read a book or stare at the mountains in the distance.
Close your eyes. For a picosecond, in the darkness, biochemistry is a grace you understand.