The Challenges of What To Eat For Optimum Health
Vitamin A, a fat-soluble vitamin is essential for many physiological processes, including maintaining the integrity and function of all surface tissues (epithelia): for example, the skin, the lining of the respiratory tract, the gut, the bladder, the inner ear and the eye. Vitamin A supports the daily replacement of skin cells and ensures that tissues such as the conjunctiva are able to produce mucous and provide a barrier to infection. Vitamin A is also essential for vision under conditions of poor lighting, for maintaining a healthy immune system, for growth and development (essential for growth and bone development in children) and for reproduction and gene expression. Vitamin A supports many systems in the body. This makes it a challenge to definitely define the the optimum intake levels for optimum health. Within that scope of complexity, vitamin A deficiency is now referred to as vitamin A deficiency disorders.
Vitamin A is an essential micronutrient in that our bodies cannot manufacture it and therefore it has to be included in our diet. There are two main sources of vitamin A: animal sources and plant sources. All the sources of vitamin A need some fat in the diet to aid absorption.
In animal sources, vitamin A is found as retinol, the ‘active’ form of vitamin A. Liver, including fish liver, is a very good source. Other animal sources are egg yolk (not the white) and dairy products such as milk (including human breast milk), cheese and butter. Worthwhile to note that meat, from the animal's muscles, is not a good source.
Plant sources contain vitamin A in the form of carotenoids which have to be converted during digestion into retinol before the body can use it. Carotenoids are the pigments that give plants their green colour and some fruits and vegetables their red or orange colour. Plant sources of vitamin A include: carrots, mangos, papaya, many of the squashes, sweet potatoes and maize (not the white varieties).
The amount of vitamin A one needs depends on your age and reproductive status. Recommended intakes for vitamin A for people aged 14 years and older range between 700 and 900 mcg of retinol activity equivalent (RAE) per day. Recommended intakes for women who are nursing range between 1,200 and 1,300 RAE. Lower values are recommended for infants and children younger than 14.
However, the vitamin A content of foods and dietary supplements is given on product labels in international units (IU), not mcg RAE. Converting between IU and mcg RAE is not easy. A varied diet with 900 mcg RAE of vitamin A, for example, provides between 3,000 and 36,000 IU of vitamin A depending on the foods consumed.
For adults and children aged 4 years and older, the U.S. Food and Drug Administration (FDA) has established a vitamin A Daily Value (DV) of 5,000 IU from a varied diet of both plant and animal foods. DVs are not recommended intakes; they don’t vary by age and sex, for example. But trying to reach 100% of the DV each day, on average, is useful to help you get enough vitamin A.
The upper limits for preformed vitamin A in IU are listed below. These levels do not apply to people who are taking vitamin A for medical reasons under the care of a doctor. Upper limits for beta-carotene and other forms of provitamin A have not been established.
|Life Stage||Upper Limit|
|Birth to 12 months||2,000 IU|
|Children 1-3 years||2,000 IU|
|Children 4-8 years||3,000 IU|
|Children 9-13 years||5,667 IU|
|Teens 14-18 years||9,333 IU|
|Adults 19 years and older||10,000 IU|
Going by the DV of 5000 IU /Day Vitamin A, it might appear that we are likely to meet Vitamin A requirements from inclusion of vitamin A rich foods indicated above in our daily meals. Increasing intake of foods of plant origin from the list above would be more favorable compared to foods of animal origin, as carotenoids and other provitamin A of plant origin are not known to have potential side effects from toxicity. Issues of absorption may cause a challenge and to meet the nutrient requirements for optimum health, use of beta carotene nutritional supplements may be warranted. On the other hand some animal food sources of vitamin A might such as beef liver might contain fairly high levels of retinol, for example beef liver (approximately 10 000 IU per ounce) and regular intake of beef liver might potentially cause toxic osteoporosis, particularly in the elderly. One other interesting example of high content retinol is the polar bear liver with about 35 000 IU per gram (3.5 million IU per serving), such that consumption of polar bear liver is often associated with acute vitamin A toxicity. To avoid toxicity from retinol, take foods which provide safe levels, then fill up the gape with beta carotene nutritional supplements.
Vitamin A, Fact Sheet for Consumers NIH
Linus Pauling Institute » Micronutrient information Centre, Vitamin A