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Nutrition and Metabolism

Vitamin A

What is vitamin A?

Vitamin A is a fat-soluble vitamin.  It is critical for vision and skin health and equally important for healthy teeth, skeletal and soft tissues. The most common symptom of vitamin A deficiency is night blindness that makes people unable to see well in dim light. Vitamin A level also plays important roles in the development of other diseases such as asthma, cardiovascular disease and cancer.

How is vitamin A used in human body?

There are two major types of vitamin A: retinol from animal fat and carotenes from plant source. Retinol is the biological active form of vitamin A.  It serves as precursor to make the light sensor molecule in the retina of human eyes. Retinol can also be converted to retinoic acid, a molecule that turn on and off genes in a variety of cell differentiation and metabolic pathways. Some carotenes (about 10% of the 600 known carotenes) can be converted into retinol in human body, therefore are also called pro-vitamin A. The three most common dietary carotenes are β-carotene, α-carotene and β-cryptoxanthin. In addition to function as pro-vitamin A, these carotenes also serve as anti-oxidants in human body.

What is the normal vitamin A level in human body?

Vitamin A level is measured by plasma retinol concentrations, which is considered adequate between 1.05 and 3 μM/L (30 and 86 μg/ml).  Plasma retinol concentrations of 0.35-0.7 μM/L (10 - 20 μg/ml) are regarded as marginal and that less than 0.35 μM/L (10 μg/ml) indicates deficiency. Plasma retinol concentrations more than 3 μM/L (86 μg/ml) are considered too high. Too much retinol can lead to increased cardiovascular diseases, prostate cancer, and hypervitaminosis A, a disease with symptoms including soft skull bone, vision damage, liver damage, skin and hair loss, headache, nausea, dizziness, drowsiness etc.

How much vitamin A do I need in my diet?

The Institute of Medicine of the National Academies recommends daily values in RAE (Retinol Activity Equivalent) and UL (Upper Intake Levels) for different human population based on age and gender (Table 1). One should consume at least the RAE but no more than the UL recommended. Over consumption of vitamin A can lead to jaundice, nausea, loss of appetite, irritability, vomiting, and even hair loss.

Table 1. RAE and UL of Vitamin A recommended by the Institute of Medicine of the National Academy of Sciences. Because of the potency difference, the RAE factor for retinol and carotenes are defined as 1 RAE = 1 μg retinol = 12 μg β-carotene = 24 μg α-carotene or β-cryptoxanthin = 0.3 IU (International Unit).

Age (years)  RAE (μg/day) UL (μg/day)
1–3 300 600
4–8 400 900
9–13 600 1,700
14-18 (male) 900 2,800
14-18 (female) 700 2,800
14-18 (pregnant women) 750 2,800
14-18 (lactating women) 1,200 2,800
19+ (male) 900 3,000
19+ (female) 700 3,000
19+ (pregnant women) 770 3,000
19+ (lactating women) 1,300 3,000

Why vitamin A is more important for people carrying certain gene variants?

Mutations in two genes result in abnormally low retinol concentrations. These two genes, RBP4 and TTR, are responsible for transporting retinol in circulation. One mutation, in the RBP4 gene, is carried by 37% of the general population.  Another mutation, related to the TTR gene is carried by 46% of the general population. People carrying these two mutations need to pay more attention to their vitamin A intake to avoid deficiency.

Which foods contain high level of vitamin A?

Animal livers contain the highest amount of retinol in all food sources. Vegetables and fruits that are red or orange in color general are rich sources of carotenes. Table 2 represents the most common vitamin A rich foods. Many fortified foods in grocery stores, such as milk and ready-to-eat cereals, also have the amount of added vitamin A listed in the Nutrition Facts on the label. They are not listed here.

Table 2. Common vitamin A-rich foods.

Animal Livers
Fish oil
Per 100 g (RAE) 30000
Per Serving (RAE) 1350
Serving Size 1 teaspoon (4.5g)
 
Goose liver pate
Per 100 g (RAE) 22600
Per Serving (RAE) 18758
Serving Size 1 liver (83 g)
Spice
Pepper, powder
Per 100 g (RAE) 2463
Per Serving (RAE) 57
Serving Size 1 teaspoon (2.3 g)
Vegetables
Sweet potato, baked
Per 100 g (RAE) 961
Per Serving (RAE) 577
Serving Size 1 small (60 g)
 
Carrots
Per 100 g (RAE) 835
Per Serving (RAE) 1069
Serving Size 1 cup (128 g)
 
Squash, Butternut
Per 100 g (RAE) 558
Per Serving (RAE) 1144
Serving Size 1 cup (205 g)
Fruits
Apricots, dried
Per 100 g (RAE) 180
Per Serving (RAE) 234
Serving Size 1 cup (130 g)
 
Cantaloupe
Per 100 g (RAE) 169
Per Serving (RAE) 270
Serving Size 1 cup (270 g)
 

How to get sufficient dietary Vitamin A intake?

First you are recommended to use the GB Food Calorie and Nutrition Calculator to get the most accurate estimate of your current vitamin A intake.

To get sufficient dietary vitamin A from diet, the best way is to eat a variety of colorful fruits and vegetables.  The color is normally a reflection of carotenes. Although not all carotenes are pro-vitamin A, they are nevertheless good for human health because of the anti-oxidant function. In general, red, yellow and orange colored fruits and vegetables, contain more pro-vitamin A carotene.

Vitamin A can be lost from foods during preparation, cooking, or storage. To prevent loss of vitamin A, you should use raw fruits and vegetables whenever possible.  Steaming, braising, baking, or broil foods instead of frying can also prevent vitamin A loss in the fat during frying.

Animal liver contain the highest RAE of vitamin A. Since normally there is more than the upper limit per serving, liver consumption need to be carefully portion controlled to avoid too much vitamin A intake. The same applies to vitamin A supplements.

Vitamin A is one of the nutrients that are required to be listed on the food labels.  Read the label when grocery shopping is another good way to ensure adequate amount of vitamin A intake.

 

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