具体来说，不同的维生素共同具有以下三大特征，想知道维生素有哪些共同特点的同学注意了解以下特征：
1、外源性：人体所需要的维生素大多数都不能够通过人体自行合成，大部分都需要通过外界环境摄入，目前科学已经证实，人体自身仅能合成极入量的几种维生素，而大多数维生素是通过食物摄取的，所以这种营养素才被叫做维生素。
2、微量性：第二个特征就是微量性，所谓微量有两个含义：第一是指微生素在人体代谢的过程中所需要的量非常微小。如果拿维生素和人的饮食作个对比的话，维生素只占人饮食的比例的十万分之几；第二层意思是指，微生素在人体中的含量也十分的微小，但是，人体没有微生素却是不行的，因为它是维持人生命的必要元素。
3、调节性：维生素参与人体新陈代谢过程，在人体的的生命活动过程中具有独特的生理作用。而这种生理作用是无法由其他营养元素或生命因子所代替的，举例说明，比如人体缺乏维生素a就会导致人患上夜盲症、缺乏维生素c就会得上坏血病等等。
一个国际单位相当于0.025微克的维生素D3，一微克的维生素D3有40国际单位.所以7.5微克的维生素d相当于300单位的维生素d。 1ug等于40单位的维生素d，7.5ug应该相当于300单位。我们每天维生素d的生理需要量，至少应该在400到600单位。Vitamin A Technical Details:
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:
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:
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:
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:
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.
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:
nutraCoster Details
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.
Finally, nutraCoster willrecalculate 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:
Now change the Total Vitamin A RAE to 120 µg RAE. nutraCoster performs the following steps:
Total Vitamin A RAE was the sum of the retinol and carotenoids values, so nutraCoster increases retinol and carotenoids proportionally to 90µg RAE and 30 µg RAE.
The new value of 90 µg RAE for retinol converts to 300 IU and 6 %RDI.
The new value of 30 µg RAE for carotenoids converts to 600 IU and 12 %RDI.
Using the new IU and %RDI values, Total Vitamin A is now 900 IU and 18 %RDI.
The following table shows the results after the change:
nutraCoster performs a similar series of steps when you enter IU or %RDI values for Total Vitamin A.
CarotenoidDetails
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.
为了统一计量膳食中的维生素A，而提出的一个概念。其含义是包括视黄醇和β-胡萝卜素在内的具有维生素A活性物质所相当的视黄醇量。
由于人体 维生素A 来源于动物性食物的维生素A和植物性食物中的 胡萝卜素 （ 维生素A原 ），而维生素A的常用计量单位为国际单位（IU），胡萝卜素的常用计量单位为μg（微克）或者mg（毫克），为了统一计量膳食中的维生素A， FAO /WHO（1967）提出了视黄醇当量（Retinol Equivalent, RE)概念。其含义是包括视黄醇和 β-胡萝卜素 在内的具有维生素A活性物质所相当的视黄醇量。
视黄醇当量、 维生素A 、 β-胡萝卜素 的换算关系如下：
1μg β-胡萝卜素=0.167μg视黄醇当量（理论上1分子β-胡萝卜素能形成2分子维生素A，但因为 胡萝卜素 的吸收率为1/3，而吸收后转化为维生素A的 转化率 又只有1/2，所以，1μg的胡萝卜素只能折算为0.556IU维生素A，即0.167μg视黄醇当量。1IU维生素A=0.3μg视黄醇当量）
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