Vitamin A Technical Details

Introduction

Vitamin A is a fat-soluble vitamin. It is important for vision, bone growth and cell division.

Vitamin A consists of Retinol from animal sources, and Provitamin A Carotenoids from plant sources.

Retinol is referred to as pre-formed vitamin A. This means that it can be used directly by the body.

Provitamin A Carotenoids are Vitamin A precursors. This means that they are converted to Vitamin A by the body. However, conversion of the carotenoids is less efficient than that of retinol.

The Provitamin A Carotenoids are beta-carotene, alpha-carotene, beta-cryptoxanthin, lycopein, lutein and zeaxanthin (lutein and zeaxanthin are combined in the current database). The carotenoids are responsible for the red and yellow pigments of plants.

A unit called Retinol Activity Equivalents (RAE) is used to compare the Vitamin A activity of the different forms of Vitamin A. 1 μg (microgram) of retinol is equivalent to 1 μg RAE. However it takes 12 μg beta-carotene to equal 1 μg RAE, and 24 μg of the other carotenoids to equal 1 μg RAE. The following table summarizes the conversion of vitamin A values to RAE:

Nutrient μg μg RAE
Retinol 1 1
Beta-carotene 12 1
Alpha-carotene 24 1
Beta-cryptoxanthin 24 1
Lycopene 24 1
Lutein + zeaxanthin 24 1

Historical Measures of Vitamin A

Previously, a unit called International Units (IU) was used to describe Vitamin A activity. However, at the time International Units for Vitamin A were defined (1989), it was thought that beta-carotene was half as concentrated as retinol (beta-carotene is now considered to be only 1/12 as concentrated as retinol).

Because of this, Vitamin A measurements expressed as IU tend to over-state the contribution of the provitamin A carotenoids. The following table summarizes the conversion of Vitamin A values to IU:

Nutrient μg IU
Retinol .3 1
Beta-carotene .6 1
Alpha-carotene 1.2 1
Beta-cryptoxanthin 1.2 1
Lycopene 1.2 1
Lutein + zeaxanthin 1.2 1

When it was determined that IU tended to overstate the contribution of the provitamin A carotenoids, a new unit called Retinol Equivalents (RE) was defined to describe the new values. At the time beta-carotene was thought to be 1/6 as concentrated as retinol. The following table describes the conversion of vitamin A values to RE:

Nutrient μg IU
Retinol 1 1
Beta-carotene 6 1
Alpha-carotene 12 1
Beta-cryptoxanthin 12 1
Lycopene 12 1
Lutein + zeaxanthin 12 1

In 2001, the National Academy of Sciences Institute of Medicine (NAS IOM) determined that Vitamin A activity from carotenoids is only half of what was previously believed. To account for this difference, and to avoid confusion, the new unit Retinol Activity Equivalents (RAE) was defined. Retinol Equivalents (RE) are no longer used.

The following table summarizes the relationship between the different measures:

Nutrient μg μg RE μg RAE IU
Retinol 1 1 1 3.33
Beta-carotene 12 2 1 20
Alpha-carotene 24 2 1 20
Beta-cryptoxanthin 24 2 1 20
Lycopene 24 2 1 20
Lutein + zeaxanthin 24 2 1 20

IU Details

At the time of this writing (2004), the RDI (Reference Daily Intake) of Vitamin A is 5000 IU. However, IU is an inaccurate measure of vitamin A activity because it overstates the contributions of the provitamin A carotenoids (by a factor of 6). The FDA (US Food and Drug Administration) is considering changing the RDI to express it in terms of RAE.

Update: As of July 2016, the RDI for Vitamin A was changed to 900 RAE.

Because the conversion of IU to RAE is different for retinol and the carotenoids, you cannot convert IU to RAE (or RAE to IU) for Total Vitamin A unless you know the proportion of retinol to carotenoids in the sample.

For example, knowing that you have 40 IU of Total Vitamin A is not enough information to determine how much retinol and carotenoids you have. 40 IU of Total Vitamin A can come from 12 μg RAE of retinol, or from 2 μg RAE of carotenoids, or from 6 μg RAE retinol plus 1 μg RAE carotenoids, or from many other combinations of retinol and carotenoids. The following table summarizes:

Sample Values IU Total μg RAE
12 μg RAE Retinol 40 12
2 μg RAE Beta-carotene 40 2
6 μg RAE Retinol + μg RAE Beta-Carotene 40 7

Entering Retinol or Carotenoids Values

When you enter RAE for retinol or any of the carotenoids, nutraCoster automatically calculates the IU and %RDI values for the nutrient.

When you enter IU for retinol or any of the carotenoids, nutraCoster automatically calculates the RAE and %RDI values for the nutrient.

When you enter %RDI for retinol or any of the carotenoids, nutraCoster automatically calculates the RAE and IU values for the nutrient.

When you change any of the values for retinol or any of the carotenoids, nutraCoster checks the Total Vitamin A value. If the Total Vitamin A value was the sum of the retinol and carotene values before the change, nutraCoster recalculates the Total Vitamin A value to be the sum of the new retinol and carotenoids values.

If the Total Vitamin A value was not the sum of the retinol and carotenoids values before the change, nutraCoster does not update the Total Vitamin A value.

Entering Total Vitamin A Values

When you enter Total Vitamin A values for RAE, IU or %RDI, nutraCoster cannot calculate the values for retinol or the carotenoids unless it knows the proportions of retinol and carotenoids (see the section IU Details above for why this is so).

When you enter RAE for Total Vitamin A, nutraCoster checks to see if the Total Vitamin A RAE value was the sum of the retinol and carotenoids RAE values before the change. If so, nutraCoster recalculates the retinol and carotenoids RAE values so the sum of the values is equal to the new Total Vitamin A RAE value, while keeping the proportions the same

If nutraCoster recalculates the retinol and/or carotenoids RAE values as a result of a change to the Total Vitamin A RAE value, it will also recalculate the IU and %RDI values for the retinol and carotenoids.

Finally, nutraCoster will recalculate the IU and %RDI values of Total Vitamin A based on the new values for retinol and carotenoids.

Example: Suppose you have 80 μg RAE Total Vitamin A, which is composed of 60 μg RAE retinol and 20 μg RAE total carotenoids. The following table lists the values for the example:

Nutrient μg RAE IU %RDI
Total Vitamin A 80 600 12%
Retinol 60 200 4%
Total Carotenoids 20 400 8%

Now change the Total Vitamin A RAE to 120 μg RAE. nutraCoster performs the following steps:

  1. Total Vitamin A RAE was the sum of the retinol and carotenoids values, so nutraCoster increases retinol and carotenoids proportionally to 90 μg RAE and 30 μg RAE.
  2. The new value of 90 μg RAE for retinol converts to 300 IU and 6 %RDI.
  3. The new value of 30 μg RAE for carotenoids converts to 600 IU and 12 %RDI.
  4. Using the new IU and %RDI values, Total Vitamin A is now 900 IU and 18 %RDI.
  5. The following table shows the results after the change:
Nutrient Μg RAE IU %RDI
Total Vitamin A 120 900 18%
Retinol 90 300 6%
Total Carotenoids 30 600 12%

nutraCoster performs a similar series of steps when you enter IU or %RDI values for Total Vitamin A.

Carotenoid Details

When you enter or change a value for Total Carotenoids, nutraCoster checks to see if the Total Carotenoids value was equal to the sum of the individual Provitamin A Carotenoids before the change. If it was, nutraCoster recalculates the individual carotenoid values so the sum of the values equals the new Total Carotenoids value, while keeping the proportions the same.

When you enter or change a value for any of the individual carotenoids, nutraCoster checks to see if the Total Carotenoids value was equal to the sum of the individual Provitamin A Carotenoids before the change. If so,nutraCoster adjusts the Total Carotenoids value to equal the sum of the individual carotenoid values after the change.