Definition, functions and roles
Any compound presenting similar biological characteristics to retinol is called vitamin A. Retinol is the main precursor of the active vitamin A molecule. In humans and most animal species, it can also be formed from certain carotenoids (plant pigments), mainly ß-carotene. Preformed vitamin A and vitamin A provided by the conversion of carotenoids constitute the total vitamin A intake.
The vitamin A activity of these compounds is expressed in relation to the retinol activity according to a retinol equivalent (RE) system; in humans, 6 mg of ß-carotene have the same activity as 1 mg of retinol. Note that the term "retinoids", referring to both natural and synthetic compounds (medicinal products), is not used in the food context.
Vitamin A is essential at all stages of life. Its key role in the mechanism of eyesight is now clearly established. It also plays a part in the regulation (activation, repression) of gene expression, and is thus involved in a number of bodily functions: development of the embryo, cell growth, tissue renewal (skin, intestinal mucosa), immune system, etc.
In addition to the properties of vitamin A, ß-carotene can act as an antioxidant (free-radical scavenger).
Foods that are rich in vitamin A and sensitivity of the vitamin
In food, vitamin A is present in the form of retinol and its derivatives exclusively in products of animal origin, while in the form of provitamin carotenoids it is mainly found in products of plant origin.
The livers of fish and farmed animals are richest in retinol. In contrast, vegetables are richest in ß-carotene: sweet potato, carrot, pumpkin, dandelion, mixed vegetables, parsley and other herbs, lettuce, spinach, etc.
The lists of foods that are rich in retinol and ß-carotene can be found on the CIQUAL table :
ß-carotene may lose its activity: during storage (due to the effects of light, oxygen and enzymes) and during dehydration of food. In contrast, freezing preserves its activity. Vitamin A may lose its activity and be destroyed by air oxidation (rancidity), especially in the presence of light and heat. However, it remains fairly stable during cooking (maximum loss of about 15%).
Population reference intakes
Population reference intakes (PRI) are proposed to cover the "normal" nutritional needs of a population.
The PRIs for vitamin A in adults were updated in 2016. They are 750 µg RE per day for men, and 650 µg RE for women.
For children, the previous reference intakes (French ANCs) ranged from 450 to 550 µg of RE depending on the age group. They are in the process of being re-assessed.
It is traditionally recommended that around 60% of intake come in the form of carotenoids.
Risk of deficiency and excess intake
A reduction in visual acuity, especially in twilight glow (nocturnal vision), is one of the first visible signs of vitamin A deficiency in humans. Vitamin A deficiency does not seem to exist in industrialised countries, unlike in developing countries where it poses major public health problems. However, in vulnerable groups (children and the elderly), the consequences of a moderate vitamin A deficiency, particularly due to infectious states, can be of concern.
Great caution is advised for women who are pregnant or wish to become pregnant because excess vitamin A is associated with birth defects. Pregnant women should not eat liver regularly because this food contains large quantities of vitamin A that can be directly assimilated. On the other hand, food intakes of provitamin A (colourful fruits and vegetables) are safe, even during pregnancy.
High intakes (greater than 1500 µg RE per day) increase the risk of fractures in post-menopausal women.
β-carotene supplementation in smokers increases the risk of lung cancer.