Vitamins: Vitamin A Flashcards
Vitamin A
What type of vitamin is it?
Where did it get its name?
What are the active forms (vitamers) and when are they used / what order?
Where is retinol stored?
Vitamin A is a fat soluble vitamin. It was the first vitamin to be discovered (hence ‘A’)
* The active forms (vitamers) which execute the functions of vitamin A are: Retinol, retinal and retinoic acid
* Depending upon what the body requires, retinol will be oxidized to the different forms (retinol retinal retinoic acid).
* Depending upon what the body requires, retinol will be oxidized to the different forms (retinal retinal retinoic acid). If the body doesn’t need it, it will remain as retinol and be stored in the liver
What are the Forms of Vitamin A?
How can they be used in the body?
Where are they converted?
Where are they found?
Where did the word carotene come from?
The two forms of vitamin A:
- Pro-vitamin A = Converted into the active (usable) form of vitamin A (retinol) in the small intestinal epithelium and liver
o Carotenes (or carotenoids) are examples of pro-vitamin A. The most active pro-vitamin carotenes are:
- (alpha)
- (beta)
- (gamma) carotenes and cryptoxanthin
o Found in non-animal foods - Pre-formed vitamin A = this is active vitamin A the body can use as it is
o Only found in animal foods
Carotene = from the Greek Karotan for ‘carrot’ – orange pigments
Pro-vitamin = a substance that can be converted into a vitamin
Pre-formed vitamin = a Vitamin that is already formed
Food sources
Food sources:
* Pro-vitamin A is produced by plants, algae, fungi and bacteria. Rich foods sources: Dark green, yellow/orange vegetables and fruit e.g.
o Carrots
o Squash
o Mango
o Spinach
o sweet potatoes
- Pre-formed vitamin A is only found in animal foods e.g.
o Liver (calf’s liver highest) / organ meat
o Fish liver oils
o Egg yolk
o Mackerel
o Salmon
Pro vitamin A Absorption
Toxicity concerns?
Absorption rate? Where?
How to optimize absorption?
Pro vitamin A absorption:
* Dietary carotenes are converted to vitamin A only as needed, so do not have toxicity concerns
* The absorption of carotenoids in the small intestine varies between 5% and 60%
* Ensure that there are adequate healthy fats in the diet as carotenoids are fat-soluble. Drizzle with coconut oil or olive oil to optimize absorption.
* Another way to increase the bioavailability of carotenoids is to cook (slightly steam) these foods, e.g. carrots. This cooking method is thought to most effectively liberate carotenoids from cells
Carotenoids and Conversion
What are carotenoids converted to and where? What gene encodes this?
What conditions is carotenoid conversion reduced by?
What can excessive intake of carotenoids cause?
What else can cause the same presentation?
The enzyme which converts carotenoids to retinal (in the intestinal epithelium and liver) is encoded by the BCO1 gene. This enzyme activity is subject to much genetic variation
* Carotene conversion is reduced by: hyperlipidaemia, liver disorders, diabetes and hypothyroidism (consider your clients)
* Excessive intake of carotenoids can lead to a non-dangerous yellowing of the skin. However, yellowing of the skin might also be associated with disorders of the liver, gallbladder, etc.
* A sallow yellow complexion (i.e. pale or pasty), especially in the face, can indicate a Spleen Qi deficiency. In TCM, this refers to ‘weakness’ in digestive function.
Pre-formed vitamin A Absorption
Pre-formed vitamin A absorption:
* About 70-90% of dietary retinol is absorbed – this is a key reason that animal food sources of vitamin A can lead to vitamin A toxicity (liver particularly)
Recommended Dosage
Isolated supplementation concerns?
RAE conversions
- Supplemental range: Adults maximum 3000 mcg preformed A. Higher doses are appropriate in some situations but should only be used short term.
- It is recommended to avoid isolated beta-carotene supplementation, as it was shown to worsen the progression of lung cancer patients in a research study (recall that antioxidants do not work in isolation)
- Carotenes (provitamin A) don’t have the same vitamin A activity as pre-formed vitamin A. The amounts of vitamin A are expressed as mcg of retinol activity equivalents (RAE)
- 1 RAE = 1 mcg of retinol = 12 mcg of beta-carotene = 24 mcg of alpha-carotene or beta-cryptoxanthin from dietary sources
- Supplementations conversion is: 2mcg beta-carotene = 1 mcg retinol
- Vitamin A content using international units (IU) phased out of supplements 2021. 1 IU retinol = 0.3 mcg RAE
Mcg = micrograms
Vision and eye health - functions and therapeutic uses
- Functions:
o Required for rhodopsin – the light-sensitive protein involved in converting light into an electrical signal that goes into the brain’s visual cortex, giving the message of sight.
o Also necessary for corneal health - Therapeutic uses:
o Photosensitivity
o Visual loss
o Night-blindness
o Senile cataracts
Rhodopsin = the ‘visual purple’
Immunity - functions and therapeutic uses
- Functions:
o Enhances T-cell proliferation and interleukin-2 secretion
o Supports first line of immune defense (skin and mucous membrane barrier) - Therapeutic uses:
o Recurrent infections, e.g. respiratory tract infections
Gene expression and cell differentiation - functions and therapeutic uses
- Functions:
o Control of cell differentiation and turnover. Regulating the expression of over 500 genes, affecting the synthesis of proteins that regulate cell functioning.
o Key as a preventive measure to support healthy gene expression
o Synthesis of glycoproteins which support normal development of bones, teeth and skin - Therapeutic uses:
o Acne
o Anaemia
o Cervical dysplasia
o Hair loss in women
o Fracture repair
o Lichen planus
o Osteoarthritis
o GIT ulceration
o Vaginitis
o Periodontal disease
Reproduction: Functions and therapeutic uses
- Functions:
o Reproduction and embryonic health
o Required for spermatogenesis in men
o Required for egg (ovum) development and implantation - Therapeutic uses:
o Infertility (male and female)
o Fibrocystic breast disease
Antioxidant properties:Functions and Therapeutic uses
- Functions:
o Preformed and provitamin A have antioxidant properties
o The foods rich in carotenoids are also rich in other antioxidants, again highlighting that they don’t work in isolation - Therapeutic uses:
o Used in various therapies, including carrot juicing for cancer (Gerson therapy)
o Disease prevention
Deficiency signs and symptoms
Deficiency signs and symptoms:
* Vision impairment at night is an early sign: Loss of sensitivity to green light, unable to adapt to dim light and night blindness. Prolonged deficiency can lead to blindness
* Hyperkeratosis of skin on upper arms
* Reduced skin integrity – rough dry skin, acne, eczema, poor wound healing. Dry hair
* Poor bone growth / development
* Poor sense of taste and smell
* Lowered immunity (recurrent infections)
Hyperkeratosis = thickening of the stratum corneum (‘goose flesh’)
Factors that affect Individual requirements
Factors affecting individual requirements:
* Diabetes mellitus, thyroid and liver disease – decreased carotene conversion
* Alcoholism: Accelerates the breakdown of liver-stored retinol; absorption and carotene conversion is reduced. Increased vitamin A toxicity potential; not supplemented with preformed A
* Poor gut health (lack of absorption in small intestine) and conditions that affect fat absorption, e.g. cystic fibrosis, statins etc.
* Zinc deficiency and / or protein malnutrition: Zinc and protein are required to make Retinol Binding Protein (RBP). RBP moves vitamin A from liver storage to tissues for utilization. Without zinc, vitamin A is trapped. So, optimize intake of zinc-rich foods.
Toxicity?
What does it affect / lead to?
Vitamin A toxicity: Long-term and regular intake (roughly 5-10 times the recommended nutrient intake over many months):
* Can negatively affect gene regulation during embryological development leading to birth defects (e.g. cleft lip)
* May increase osteoclast activity and lead to bone fractures
* Can damage hepatocyte cell membranes (causing liver disease)
* Can lead to hyperlipidaemia, amenorrhoea and anorexia
* Can cause dry, red and scaling skin
