Beta-carotene is the yellow or red pigment found in plants. It is often referred to as provitamin A because inside the body it is converted to vitamin A. It is one of more than 400 carotenoids that occur naturally and is by far the most active in terms of vitamin A activity.
In its natural state, one molecule of beta-carotene can be likened to two molecules of vitamin A stuck together which divide after ingestion. Both beta-carotene and vitamin A molecules have chains of double bonds that give rise to CIS and TRANS isomers. Isomers are substances that have the same molecular weight and composition but slightly different spatial structures.
Beta-carotene as such may be declared on labels simply by weight in milligrams (mg). However, if it is claimed in terms of vitamin A, the relationship defined in the Food Labelling Regulations is that
6 micrograms of beta-carotene is equivalent to 1 microgram of retinol equivalent (i.e. vitamin A) and the vitamin A must be declared in micrograms (mcg or µg).
WHERE DOES IT COME FROM?
Beta-carotene occurs in vegetables and fruit that are yellow, orange or red in colour. It is also present in dark green leafy vegetables. A rich source is Dunaliella salina, an alga found in salt lakes. Although it cannot be eaten as a vegetable, the algae can be harvested, concentrated and dried to provide a valuable supply of beta-carotene in supplement form.
Among root vegetables the richest source of beta-carotene is the carrot. Other foods containing significant quantities are parsley, tomatoes, melons, prunes, peaches, apricots, tangerines, watercress, broccoli, spring greens, turnip tops, endives, lettuce, spinach, asparagus and cabbage. The amount available from these foods varies considerably. Examples are (approximate quantities):
Dunaliella = 17000 mcg per 100g
Carrots = 12000 mcg per 100g
Broccoli = 2500 mcg per 100g
Prunes = 1000 mcg per 100g
Bananas = 200 mcg per 100g
Redcurrants = 70 mcg per 100g
WHAT DOES IT DO IN THE BODY?
Since beta-carotene consists of two forms, TRANS and CIS, it has two separate functions.
The TRANS isomer is more efficiently converted into vitamin A and depending on the body’s needs, is used in the same capacity as fat soluble vitamin A would be in its preformed state. Clinical trials have demonstrated that the CIS isomer protects against the formation of damaging free radicals which are so strongly linked with degenerative diseases such as cancer.
Beta-carotene is a micronutrient in its own right and functions as an antioxidant, similar in its effect to vitamins A, C and E and the mineral selenium. It is known as a ‘scavenger’ because, as it circulates through the body, it neutralises or ‘mops up’ the free radicals that can damage lipids in cell membranes as well as the cells’ genetic material.
Absorption takes place within the cells of the intestinal wall after it has been emulsified by bile and other digestive enzymes. The process takes 6-7 hours, although the rate of absorption varies because not all of the beta-carotene consumed is fully utilised. Studies show that as little as 12% may be absorbed from food and of this, only 60% to 70% is converted to vitamin A. The remainder of the nutrient stays in the bloodstream unchanged as beta-carotene.
It is this difference from vitamin A that makes beta-carotene unusual and also important for the following reasons:
- Unlike vitamin A, it is non-toxic and can thus be consumed in higher quantities.
- Beta-carotene that is not used to top up vitamin A levels acts as a powerful antioxidant, circulating freely via the bloodstream and working with the immune system to protect the body.
Carotenoids in foods lose some activity during storage due to exposure to light and oxygen.
Frying and boiling reduces potency to some extent, unlike freezing. In most cases, beta-carotene retains its stability even after long-term freezing.
Some drugs may interfere with beta-carotene activity and using liquid paraffin based laxatives for long periods is not advised.
Smoking and alcohol consumption both affect levels of the nutrient, as is the case with most vitamins, and those who smoke and drink heavily may require additional supplementation.
Strengthening antioxidant defences requires a healthy diet plus adequate supplies of vitamins A, C, E and minerals zinc, selenium and copper. Any of these nutrients can be linked to the body’s use of beta-carotene as a scavenger of damaging free radicals.