Introduction
Vitamin A is a name for a group of fat-soluble compounds which include retinol, retinal and retinyl esters. [1]
The two different forms of vitamin A are preformed vitamin A and provitamin A carotenoids. Both are converted in the body to retinol, which is oxidised to retinal and finally to retinoic acid. Measuring plasma retinol levels indicates the vitamin-A status in the body.[1,3]
Of the two vitamin A forms, only provitamin A is found abundantly in plant based diets and beta-carotene is one of the most important provitamin carotenoids followed by alpha-carotene and beta-cryptoxanthin. [1]
Observational studies have found a correlation between excess pre-formed vitamin A intake and increased fracture risk as well as irreversible liver damage in some cases. [1,2] However, beta-carotene is not known to be toxic, even with large supplemental doses. [1,2]
- Both vitamin A types are absorbed in the small intestine but with different ranges, the pre-formed vitamin A absorption level is high ranging from 70-90%, while the beta-carotene absorption rate ranges from 9-22%. [7]
- The absorption rate depends on food, diet-related factors and the health status of the individual. [7]
- Since vitamin A is fat-soluble, excess levels are usually stored in the body and can accumulate over a period of time.
- Approximately 70-90% of vitamin A in the body is stored in the liver and intestine in the form of retinyl esters. [1,4]
- Stored vitamin A levels in severely deficient individuals decrease to 50 per cent or less. [3,4]
- The catabolic rate for retinol or the total body vitamin A stores lost per day is expected to be 0.5 % or more for individuals on a vitamin A free diet. [3,4]
- The majority of retinol metabolites are excreted in faeces via bile, urine, and to a lesser extent in breath. [3]
Body Functions
Recommended Daily Intake
Deficiency Symptoms
Vitamin A Food Sources
Excessive Intake/ Toxicity Side Effects
Groups At Risk of Vitamin A Deficiency
Vitamin A Interaction With Other Nutrients
- Dietary fat – Dietary fat may enhance the absorption of pre-formed vitamin A and pro-vitamin A carotenoids, hence, it is advisable to eat vitamin A with fatty foods. [7]
- Iron –Vitamin A deficiency impairs iron mobilisation and vitamin A supplementation improves haemoglobin concentrations. [1,3]
- Zinc – Zinc deficiency may negatively affect vitamin A status by affecting the mobilisation of vitamin A from the liver to the circulation, however, the evidence is not conclusive. [3]
Nutrient Profiles For Food Groups
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