Nutrient Profile: Vitamin A.

“Vitamin A” is not a singular nutrient, rather a term given to variety of lipid (fat)-soluble compounds related to RETINOL.

Retinol, retinal and retinoic acid are RETINOIDS and are referred to as PRE-formed vitamin A.

Retinoids are stored in the liver of animals, they are from animal sources in the diet.

Plant sources of retinoids are CAROTENOIDS. They are fat-soluble red, orange and yellow pigments stored in plants.

Hundreds and hundreds of different carotenoids are synthesized by plants but only about 10% of them are able to convert to retinol, these are referred to as PRO-vitamin A. Beta-carotene is the most important of the pro-vitamin A carotenoids.

To re-cap: pre-formed vitamin A are the retinoids (retinol, retinal and retinoic acid), from animal sources. Pro-vitamin A are the carotenoids (major one is beta-carotene), from plant sources.

feature_vitamin_a

The main functions of vitamin A are:

  1. Vision: receptor cells in the retina contain a light-sensitive pigment called rhodopsin, which is a complex of the protein opsin plus retinal (vitamin A.)
  2. Reproduction/growth and development: Retinol and Retinoic Acid (both forms of vitamin A) are essential for embryonic development in particular the heart, eyes and ears.
  3. Cellular differentiation: Retinoic Acid exerts hormonal activity and can influence gene expression.
  4. Immune function: vitamin A is required to maintain the integrity of skin and mucus cells which are the first line of defense and it is involved in the development and differentiation of white blood cells.

The main therapeutic indications for vitamin A:

  • Dis-ease of the eye.
  • Periods of rapid growth and development.
  • Wound healing, dis-ease of the respiratory tract.
  • Dis-ease of the skin.
  • INFECTION (my second year nutrition lecturer introduced vitamin A as “THE anti-viral nutrient”.)

Deficiency signs and symptoms

  • The earliest sign of vitamin A deficiency is impaired night vision/dark adaptation.
  • A later stage sign is “dry eye” (xeropthalmia.)
  • Higher incidence of respiratory infections.
  • In developing countries, children with severe deficiency perish from viruses and diarrhea.

Susceptible individuals at risk for vitamin A deficiency:

  • Those with malabsorption syndromes such as coeliac disease.
  • Those with pancreatic disease, diabetes.
  • Those with liver, gallbladder, kidney disease.
  • Those with or having suffered, intestinal parasites.
  • Acute infections.
  • Lactating women.

Factors that enhance absorption:

  • Heating (even just gently) orange plant sources of vitamin A (carotenoids) releases the carotenoids from protein complexes and makes them more readily available.
  • Integrity of the gut, sufficient digestive enzymes and bile.
  • Eating vitamin A foods with other foods that contain fats.

Factors that inhibit absorption:

  • Insufficient dietary fat and insufficient dietary protein.
  • Processing of food.
  • Liver damage, bile duct obstruction.
  • Excessive intake of alcohol and certain prescription drugs.
  • Increased body mass.

Dosages and supplements:

  • Recommended intake for adults is between 2500IU – 3000IU daily.
  • Recommended intake for pregnant women is between 2500 – 5000IU daily (upper limit set at 5000IU.)
  • Recommended intake for children is between 1500IU (infants/babies) to 2500IU (older children) daily.
  • Therapeutic dosage starts at about 10,000IU daily.
  • Best supplement is a sustainable caught cod liver oil, providing between 950-2500IU of pre-formed vitamin A. In Australia, I recommend Nordic Naturals.

cod_liver_oil_vitamin_a

 Animal sources of vitamin A (retinoids):

  • Cow’s milk
  • Cheese
  • Cod
  • Chicken
  • Eggs
  • Halibut
  • Liver and kidneys
  • Sardines
  • Scallops
  • Shrimp
  • Tuna
  • Yoghurt

egg_vitamin_a

butter_vitamin_a

Plant sources with a higher* content of vitamin A (carotenoids):

*There are many more, orange, red, yellow and dark-green pigmented plant foods.

Notice the greens in this list are those that are pungent/bitter, this is because the carotenoids are the plants protective constituents: they deter insects and other opportunistic pathogens.

  • Apricot
  • Asparagus
  • Basil
  • Beans (green)
  • Beet greens
  • Bell peppers
  • Bok Choy
  • Broccoli
  • Brussels sprouts
  • Carrots
  • Celery
  • Chili peppers
  • Collard greens
  • Coriander
  • Grapefruit
  • Kale
  • Leeks
  • Romaine lettuce
  • Mustard greens
  • Papaya
  • Spinach
  • Sweet potato
  • Swiss chard

sweet_potato_vitamin_a

papaya_vitamin_a

Do you have to worry about not getting the “best” (pre-formed) vitamin A if you’re a vegetarian/vegan?

In short, no: providing that your gut function is optimal and you eat widely from the plant foods containing carotenoids, particularly beta-carotene as it is most abundantly converted into retinol. Also, include fats in the diet.

Five plant foods that are loaded with beta-carotene:

  1. Sweet potato
  2. Carrot
  3. Spinach
  4. Kale
  5. Bitter greens

What’s this “IU” you mention in the dosages?

IU stands for “International Units” and was the original system for measuring the various sources of  vitamin A.

A new system of measuring Vitamin A is “Retional Equivalents” (RE)  or “Retinal Activity Equivalents” (RAE) as follows:

  • One RE is 1 microgram (mcg) of dietary or supplemental (preformed) vitamin A = 1 microgram of retinol. {1:1}
  • 1RE is 3.33 international units (IU).
    • So, when I say 2500IU daily of vitamin A for “baseline” (maintenance) dose then it is 750 micrograms of vitamin A (2500/3.33 = 750.)
  • Where carotenoids (pro-vitamina A) are concerned, they differ in their RE/RAE as they have to be converted to retinol:
    • 2 micrograms of supplemental beta-carotene = 1 microgram of retinol. {2:1}
    • 12 micrograms of dietary beta-carotene = 1 microgram of retinol. {12:1}
    • One IU is equivalent to 0.3 micrograms of retinol.

Take home message for vitamin A:

  • Eat plenty of orange and yellow fruit and vegetables; and bitter green leafy vegetables.
  • Include eggs, liver oils, oily fish and high quality dairy.
  • Ensure good quality fats feature in the diet.
  • Focus on gut function.
  • Supplement or increase dietary intake during periods of infection and rapid growth.

I hope you found this post informative and easy to follow? That was my aim. I edited it 67 times!

~Katie, x

 

Bibliography:

  • Hoffer A, Prousky J. Naturopathic Nutrition. CCNM Press Inc. Toronto, Canada. 2006.
  • Higdon J. An Evidence-Based Approach to Vitamins and Minerals. Thieme New York. 2003.
  • Heneka N, Peterson M. Nutrition 2 Workbook. Nature Care College. 2008.
  • World’s Healthiest Foods. Vitamin A. Click here.
  • Mayo Clinic. Vitamin A (retinol). Click here.
  • Tang G. Bioconversion of dietary pro-vitamin A carotenoids to vitamin A in humans. Am J Clin Nutr. May 2010: vol 91 no. 5; 1468S-1473S.