Vitamin A (also called all-trans-retinol or all-trans-retinoic acid) is a well-known fat soluble vitamin found in many foods and dietary supplements.  Vitamin A has been shown to be useful in helping with nighttime vision, but not daytime vision.

Our ability to see is dependent on two main photoreceptors that sit in the posterior aspect of the eye, the rods and cones (with the rods outnumbering the cones by a ratio of 20:1).1  To be able to see, light must first enter into and pass through the lens of the eye and then travel through the posterior segment (vitreous chamber).  Next, light must travel through ten layers of the neural retina to get to the rods and cones.1  The rods and cones are made up of an inner segment that contains the nucleus and an outer segment that is made up of discs that contain light-absorbing photopigments.1  When comparing the two photoreceptors, the rods are useful for night vision and the cones are useful for day vision.  Vitamin A is one of the required precursors for the formation of rhodopsin, the photopigment found in rods.  Rhodopsin helps us to see at night and without vitamin A, rhodopsin cannot form and "night blindness" occurs.2

Why is vitamin A needed for night vision to occur?
Vitamin A (all-trans-retinol) is a precursor to the formation of the photopigment rhodopsin, which is located in the rods.  In order for rhodopsin to be formed, vitamin A must be converted to 11-cis-retinal.  This can occur in one of two ways.  Vitamin A (all-trans-retinol) can be converted to 11-cis-retinol by isomerase.  This 11-cis-retinol can then be converted to 11-cis-retinal. Alternatively, vitamin A (all-trans-retinol) can be converted to all-trans-retinal which can then be converted to 11-cis-retinal.  Now that 11-cis-retinal has been formed by either method, it can combine with scotopsin to form the rhodopsin.  As rhodopsin absorbs light in the rods, a conformational change occurs in 11-cis-retinal to become all-trans-retinal.  A conformational change also occurs in the opsin fragment to form metarhodopsin II, which is the activated form of rhodopsin.  The metarhodopsin II then stimulates transducin, a G-coupled protein found on the surface of the disk within the outer membrane in the rod cell.  This activation of transducin causes an activation in cGMP phosphodiesterase, which will remove the cGMP mediated activation of cGMP-gated channels that are letting Na+ ions leak into the rod cytoplasm resulting in a hyperpolarization of that rod cell.  Thus, in the presence of light, the blockage of Na+ movement into the rod cell will result in a hyperpolarization of that rod cell which then allows messages about light being seen during night vision to be sent to the brain for final interpretation.

In addition to vitamin A's role in helping the rods in our eyes convert light into neuronal impulses for our brain, it is also required to maintain normal differentiation of the cornea and conjunctival membranes which helps prevent other abnormal eye conditions, such as xerophthalmia.  The Food and Nutrition Board, Institute of Medicine - National Academy of Sciences advises that the Recommended Daily Allowances (RDA; given below as retinol activity equivalents (RAE's)) and Tolerable Upper Intake Level (ULs) be the following:

•        Children 1 to 3 years of age: RDA = 300 mg/d with UL = 600mg/d. 
•        Children 4 to 8 years of age: RDA = 400 mg/d with UL = 900mg/d.
•        Males & Females 9 to 13 years of age: RDA = 600 mg/d with UL = 1,700mg/d.
•        14 to 18 years of age: RDA = 700mg/d for females and 900mg/d for males with UL = 2,800mg/d.
•        > 18 years of age: RDA = 700mg/d for females and 900mg/d for males with UL = 3,000mg/d.

If the supplement is measured in International Units (IU), there is no known direct equivalent from RAE to IU, but it can be estimated how many RAEs there are in that supplement.  First check the label on the supplement and determine the source of vitamin A (either retinol or beta-carotene).  If it is retinol, multiply the IU by 0.3 to get the micrograms of RAE or multiply the number of micrograms of RAE by 3.3 to get the number of IU.  If it is beta-carotene, multiply the number of IUs by 0.025 to get the number of micrograms of RAE or multiply the number of RAE by 39.6 to get the number of IUs. 

As with any medication or supplement, excessive doses or unnecessary exposure can be harmful.  Therefore, doses above the UL should be avoided because Vitamin A has been associated with an increase risk for hip fracture in postmenopausal women, lung cancer, cardiovascular mortality and total mortality.3-6.

References:

1. Wässle H, Boycott BB.  Functional architecture of the mammalian retina.  Physiol Rev  1991;71(2):447-80.       
   
2. Wald G.  Molecular basis of visual excitation.  Science  1968;162(850):230-9.       
   
3. Feskanich D, Singh V, Willett WC et al.  Vitamin A intake and hip fractures among postmenopausal women.  JAMA  2002;287(1):47-54.       
   
4. No Authors.  The effect of vitamin E and beta carotene on the incidence of lung cancer and other cancers in male smokers.  The Alpha-Tocopherol, Beta Carotene Cancer Prevention Study Group.  N Engl J Med   1994;330(15):1029-35.       
   
5. Omenn GS, Goodman GE, Thornquist MD et al.  Effects of a combination of beta carotene and vitamin A on lung cancer and cardiovascular disease.  N Engl J Med  1996;334(18):1150-5.       
   
6. Omenn GS, Goodman GE, Thronquist MD et al.  Risk factors for lung cancer and for itnervention effects in CARET, the Beta-Carotene and Retinol Efficacy Trial.  J Natl Cancer Inst  1996;88(21):1550-9.

Vitamin A, Vit A Supplements, All-trans-retinol, All-trans-retinoic acid, Nighttime Vision