Beta-Carotene - What is Beta-Carotene?

Beta-Carotene / β-Carotene is an organic compound and classified as a terpenoid. It is a strongly-coloured red-orange pigment abundant in plants and fruits.

As a carotene with beta-rings at both ends, it is the most common form of carotene. It is a precursor (inactive form) of vitamin A.

The structure was deduced by Karrer et al. in 1930.

In nature, β-carotene is a precursor to vitamin A via the action of beta-carotene 15. β-Carotene is also the substance in carrots that colours them orange.

β-Carotene is biosynthesized from geranylgeranyl pyrophosphate. Being highly conjugated, it is deeply colored, and as a hydrocarbon lacking functional groups, it is very lipophilic.

Plant carotenoids are the primary dietary source of pro-vitamin A worldwide, with β-carotene as the most well-known pro-vitamin A carotenoid. Others inlcude α-carotene and β-cryptoxanthin.

Carotenoids are absorbed into the small intestine by passive diffusion. One molecule of β-carotene can be cleaved by a specific intestinal enzyme into two molecules of vitamin A.

Absorption efficiency is estimated to be between 9-22%. The absorption and conversion of carotenoids may depend on the form that the β-carotene is in (cooked vs. raw vegetables, in a supplement), intake of fats and oils at the same time, and the current levels of vitamin A and β-carotene.

Researchers list the following factors that determine the pro-vitamin A activity of carotenoids:

• Species of carotenoid
• Molecular linkage
• Amount in the meal
• Matrix properties
• Effectors
• Nutrient status
• Genetics
• Host specificity
• Interactions between factors

Symmetric/Asymmetric Cleavage

In the β-carotene molecule, chain between the two cyclohexyls rings cleave either symmetrically or asymmetrically. Symmetric cleavage is done by an enzyme called beta-carotene-15,15'-dioxygenase in the human body.

This symmetric cleavage gives two equivalent retinal molecules and each retinal molecule further reacts to give retinol (vitamin A) and retinoic acid. β-Carotene is also asymmetrically cleaved into two asymmetric products.

The product of asymmetric cleavage is β-apocarotenal (8',10',12'). Asymmetric cleavage reduces the level of retinoic acid significantly.

Conversion factors

Until recently, vitamin A activity in foods was expressed as international units (IU). This is still the measurement generally used on food and supplement labels. However, it is difficult to calculate the total vitamin A activity in the diet in terms of IU, because both the absorption and conversion of carotenoids, as compared with retinol, are variable.

The unit retinol equivalent (RE) was developed by the Food and Agriculture Organization of the United Nations/World Health Organization (FAO/WHO) in 1967. More recently in 2001, the US Institute of Medicine proposed retinol activity equivalents (RAE) for their Dietary Reference Intakes.

International Units

1 RE = 3.33 IU vitamin A activity from retinol

1 RE = 10 IU vitamin A activity from β-carotene

(In Canada, Health Canada sets 1 RE = 6.667 IU from β-carotene.)

Retinol Equivalents (REs)

1 RE = 1 µg retinol

1 RE = 6 µg β-carotene

(In Canada, Heath Canada sets 1 RE = 2 µg β-carotene.

The U.S. Department of Agriculture lists the following 10 foods to have the highest β-carotene content per serving.

Further Reading

• Beta-Carotene Side Effects

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