There are two types of vitamin A. This entry is primarily about the active form of vitamin A — retinoids — that comes from animal products. The American Heart Association recommends obtaining antioxidants, including beta-carotene, by eating a well-balanced diet high in fruits, vegetables, and whole grains rather than from supplements until more is known about the risks and benefits of supplementation.
Introduction
Vitamin A is involved in immune function, vision, reproduction, and cellular communication [1,4,5]. Vitamin A is critical for vision as an essential component of rhodopsin, a protein that absorbs light in the retinal receptors, and because it supports the normal differentiation and functioning of the conjunctival membranes and cornea [2-4]. Two forms of vitamin A are available in the human diet: preformed vitamin A (retinol and its esterified form, retinyl ester) and provitamin A carotenoids [1-5].
Preformed vitamin A is found in foods from animal sources, including dairy products, fish, and meat (especially liver). Both provitamin A and preformed vitamin A must be metabolized intracellularly to retinal and retinoic acid, the active forms of vitamin A, to support the vitamin’s important biological functions [2,3]. Other carotenoids found in food, such as lycopene, lutein, and zeaxanthin, are not converted into vitamin A.
Retinol and carotenoid levels are typically measured in plasma, and plasma retinol levels are useful for assessing vitamin A inadequacy. However, their value for assessing marginal vitamin A status is limited because they do not decline until vitamin A levels in the liver are almost depleted [3]. Liver vitamin A reserves can be measured indirectly through the relative dose-response test, in which plasma retinol levels are measured before and after the administration of a small amount of vitamin A [5].
A plasma retinol level increase of at least 20% indicates an inadequate vitamin A level [3,5,6]. In some studies, high plasma or serum concentrations of some provitamin A carotenoids have been associated with a lower risk of various health outcomes, but these studies have not definitively demonstrated that this relationship is causal.
Contents
These include: helping your body’s natural defence against illness and infection (the immune system) work properly
helping vision in dim light
keeping skin and the lining of some parts of the body, such as the nose, healthy.
Why Do People Take Vitamin A?
Ion, a healthy immune system, and cell growth. There are two types of vitamin A.
Function
Dations for vitamin A, as well as other nutrients, are provided in the Dietary Reference Intakes (DRIs) developed by the Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine.
Dri is a term for a set of reference intakes that are used to plan and assess the nutrient intakes of healthy people. These values, which vary by age and sex, include:
Recommended Dietary Allowance (RDA): The average daily level of intake that is enough to meet the nutrient needs of nearly all (97% to 98%) healthy people. An RDA is an intake level based on scientific research evidence.
Adequate Intake (AI): This level is established when there is not enough scientific research evidence to develop an RDA. It is set at a level that is thought to ensure enough nutrition. Dietary reference intakes for vitamin A:
Infants (AI)
0 to 6 months: 400 micrograms per day (mcg/day)
7 to 12 months: 500 mcg/day
The Recommended Dietary Allowance (RDA) for vitamins is how much of each vitamin most people should get each day.
The RDA for vitamins may be used as goals for each person. Children (RDA)
1 to 3 years: 300 mcg/day
4 to 8 years: 400 mcg/day
9 to 13 years: 600 mcg/day
Adolescents and adults (RDA)
Males age 14 and older: 900 mcg/day
Females age 14 and older: 700 mcg/day (for females aged 19 to 50, 770 mcg/day during pregnancy and 1,300 mcg/day during breastfeeding)
The best way to get the daily requirement of important vitamins is to eat a wide variety of fruits, vegetables, fortified dairy foods, legumes (dried beans), lentils, and whole grain.