Vitamin A: the “Eye Vitamin” and So Much More…

December 2, 2019 0 By William Morgan


Most people know that vitamin A is good for
your eyes. Already thousands of years ago, in ancient
Egypt, physicians used to prescribe raw liver to treat night blindness. Of course they didn’t know about the existence
of vitamin A, but they had indirectly observed its effects. Liver, we know today, is one of the richest
food sources of vitamin A, and night blindness is a typical symptom of its deficiency. In the retina of our eyes, where rods and
cones are located, vitamin A binds to specific structures to form rhodopsins, the key molecules
that allow vision. They receive light signals from the environment,
and convert them to electrical signals that can be forwarded and interpreted by our brain. But during this work, rodopsins are degraded
and need a constant supply of vitamin A to be regenerated. If vitamin A is deficient, the first cells
to be affected are those involved in night vision, our rods. As a result, our eyes will need more time
to readjust from bright to dim light, and will have more difficulties maintaining adequate
vision in darkness. Suppose you arrive late at the movie theater
and the movie is already on: as you go from the bright hallway to the dark theater, you
won’t be able to see anything for a few instants. Within a few seconds, however, thanks to vitamin
A your eyes will start adjusting and soon your vision will be back to normal in spite
of the dim light. And while a vitamin A deficiency in such a
situation would result, at worse, in us bumping into some legs as we grope for our seat, there
are other situations in which a vitamin A deficiency could be much more dangerous. Suppose you are driving at night and the headlights
of an oncoming car hit your eyes. Such sudden shift from dark to bright will
bleach all your rods’ rodopsins and as a result, you’ll be blind for an instant. With adequate vitamin A, you will regain your
vision within a second, but if you are deficient, it could take several seconds before you see
again, enough to risk an accident. While night blindness is an early symptom
of vitamin A deficiency, full and extended deficiency can result in much more serious
consequences, and in particular a disease called xerophthalmia, which literally means
dry eye. Without vitamin A, the cells lining the cornea
can’t secrete mucus. The eye gets dry and becomes more susceptible
to infections, which easily spread and eventually lead to blindness. Although this is not common in our post-industrialized
rich countries, hundreds of thousands of children in less developed areas of Asia and Africa
become irreversibly blind each year because of vitamin A deficiency. As vitamin A deficiency also makes them more
susceptible to infections, about two thirds of these children eventually die: an especially
unfair fate if you consider that a vitamin A supplement given a couple of time a year,
at the cost of a few cents, would be enough to save their sight, and their life. Indeed, vitamin A does a lot more than just
taking care of our night vision. And while full deficiencies are not common
in our countries, marginal deficiencies are much more widespread and can still unfavorably
impact our overall health. Vitamin A can bind to DNA and is able to regulate
gene expression in many tissues in our body, and particularly our epithelial cells, those
of our skin, our eyes and our mucous membranes such as the internal surface of our mouth,
our digestive tract, our respiratory tract and our urinary tract. Because of this role, vitamin A maintains
the health of our skin and mucous membranes, and boosts our ability to prevent and fight
infections as these tissues are our first barrier against pathogens attacks. On top of that, vitamin A also intervenes
in the regulation of our immune system and lymphocyte differentiation. Several researchers in the past repeatedly
tried to study the long-term effects of vitamin A deficiency in lab animals, but they never
succeeded for a simple reason: without vitamin A in their diet, these poor animals would
get infections and die in a matter of weeks. One last role of vitamin A that we need to
mention is its cancer preventive activity, especially for tumors of epithelial origins. Indeed, vitamin A acts by slowing down fast-growing
cells so that they have time to differentiate. Both slowing down cell cycle progression and
promoting correct differentiation, are important cancer preventive mechanisms. On top of that, vitamin A also protects our
skin from damage when we expose it to sunlight. And finally, carotenoids which are vitamin
A precursors independently exert cancer preventive functions, which we will discuss later in
this course. But now let’s recap and expand on the symptoms
that may signal a marginal deficiency of vitamin A.
Let’s start with our eyes: here we have night blindness, slower adaptation from bright
to dim light, slower vision recovery following a dazzling light, increased sensitivity to
bright light, eye fatigue after reading or watching television, often leading to headaches,
and when deficiency is more severe, a feeling of dry eyes as you first open them after sleeping. Let’s move to our skin. Our skin becomes dry and rough, it often itches,
and is prone to infections, eczema, rashes and acne. The accumulation of dead cells in our pores
can create particularly rough areas, this is called follicular hyperkeratosis, and it
is usually first observed at the elbows. Your hair can also become dry and lose its
shine. Mouth and upper airways have a tendency to
become dry as well, as vitamin A is necessary for mucus secretion in all mucous membranes. Finally, a landmark sign of vitamin A deficiency
is a higher susceptibility to infections, both systemic infections such as colds, and
localized infections of our skin, mouth, upper airways, ears, and the urinary system. Not only the frequency of infections increases,
but also their duration. Vitamin A as such, also called retinol, is
only found in foods of animal origin. It is most abundant in animals liver, the
organ that stores most of it. Beef liver, pork liver and chicken livers
are all excellent sources of this vitamin. Muscle does not store vitamin A, so meat in
general is not a very good source of it, but it has some, just like fish and eggs. Non-fortified milk and dairy are poor sources
of vitamin A, but again, they contribute some. Cod liver oil is a traditional natural supplement
of vitamin A, and it is also extremely rich in vitamin D.
Luckily for those who don’t like liver and fish liver oil, vitamin A doesn’t necessarily
need to be eaten as such. In fact, it’s a semi-essential vitamin and
can be derived in our body from some plant pigments belonging to the family of carotenoids. Beta-carotene is the most efficient vitamin
A precursor. It’s the orange-yellow pigment that colors
carrots, pumpkins, apricots, mangoes, papaya, cantaloupe and sweet potatoes, but it is also
found in green leafy vegetables such as spinach or broccoli, where its color is masked by
the green of clorophyll. Alpha-carotene and beta-cryptoxhantin also
have some provitamin A activity, although their conversion is much less efficient. All the other carotenoids, there are about
600 of them, do not have any provitamin activity, but they have other health-promoting functions
in our body. The RDA for vitamin A is between 700 and 900
micrograms of retinol activity equivalents (RAE). This unit of measure takes into account the
contribution of both pre-formed vitamin A, and its carotenoid precursors. For example, 1 microgram of pre-formed retinol
counts as 1 RAE, while 1 microgram of beta-carotene counts as 0.1 RAE. In some situations, however, a slightly higher
vitamin A intake may be beneficial, and in particular for those individuals who put a
lot of strain on their eyes because of their work, those individuals staring at the computer
screen for many hours a day, reading a lot of documents, sewing, as well as those who
work in conditions of very bright light, such as out in the sunlight, or under a fluorescent
light, or exposed to snow glare or water glare, and finally, those who work in conditions
of dim light, such as miners or warehouse workers. All these people need an extra supply of vitamin
A to efficiently regenerate their eye rodopsins. Finally, everyone during a stressful time,
when susceptibility to infections in increased, can benefit from some extra vitamin A. When it comes to using vitamin A supplements,
however, we need to be extremely careful because this is one of those tricky nutrients for
which the gap between too little and too much is quite narrow, and unfortunately, both deficiency
and toxicity can result in very serious consequences. The amounts of pre-formed vitamin A found
in regular multivitamins are safe, but the amounts contained in some over-the-counter
vitamin A supplements can be dangerously high if taken without supervision. Cod liver oil can also provide toxic amounts
of vitamin A if used inappropriately. Carotenoid precursors of vitamin A are actually
the best supplemental form of vitamin A, because they do not carry the same risks of toxicity
of pre-formed retinol. And this is for two reasons, first, the conversion
of carotenoids into vitamin A is regulated and relatively slow, second, intestinal absorption
of carotenoids significantly decreases at high dietary intakes, much more than it happens
with retinol. The only consequence of high carotenoid blood
concentrations is that the skin may turn yellow-orange: this effect is not harmful and disappears
as carotenoid intake decreases. Excess pre-formed vitamin A, in contrast,
starts being toxic at doses just three times the RDA: it is toxic for the liver, and in
the long term it can increase the risk for hip fractures. It is especially important that women in the
early months of pregnancy avoid excess vitamin A to prevent birth defects and spontaneous
abortions.