Beta-carotene – benefits and limitations

In the middle of the 1990s beta-carotene became the subject of controversy when two studies reported that the intake of very large amounts (10 to 15 times the recommended daily dose) of the carotenoid over many years as supplements had led to an elevated risk of lung cancer and mortality in heavy smokers (see Beta-Carotene/Safety). Scientists who had hoped they had found a “miracle cure” for the harmful effects of smoking were disappointed. It became clear that taking high doses of micronutrients cannot compensate for an unhealthy lifestyle or smoking – the real risk factor for the development of lung cancer.

Although no harmful effects of high doses of beta-carotene were found in studies with non-smokers, and even in other studies with smokers no such effects were observed, it was decided to limit the recommended intake of beta-carotene via dietary supplements and fortified foods to 20 mg beta-carotene per day for smokers.

From this point on, beta-carotene had a bad reputation. This completely ignored the fact that, as a vitamin A precursor (‘provitamin A’), it represents a safe and for many possibly the most important source of vitamin A. Among other things vitamin A is important for maintaining the mucous membranes and in the immune defense against infectious diseases. Since the consumption of vitamin A rich foods, in particular liver, is often not sufficient to meet requirements, beta-carotene has a key role to play here.

Beta-carotene - a safe and necessary source of vitamin A

Beta-carotene holds an important position as a precursor (‘provitamin’) of vitamin A. Vitamin A is important for sight, and vitamin A deficiency leads to night blindness. Furthermore, vitamin A acts as a growth factor and plays a part in reproduction, the immune system and skin formation.

German national food consumption studies have shown that a large part of the population consumes too little pure vitamin A in the diet (1, 2, 3). Schoolchildren, older people and pregnant women are particularly affected. Liver and fatty fish are the main sources of vitamin A, but it is not possible to predict to what extent consumption of these foodstuffs can be increased in the long term. For many, beta-carotene represents the most important source of vitamin A. Germans, for example, obtain almost 50% of their intake of vitamin A through the provitamin (4).

Nutrition experts recommend a daily intake of 0.7 to 1.0 mg vitamin A (retinol) for healthy adults. At least 2–4 mg beta-carotene daily are required to reach this level. The average person’s intake falls clearly below these recommendations because still too little beta-carotene is being consumed through fruit and vegetables (5). Since foods rich in beta-carotene alone do not suffice to guarantee the desirable daily intake of vitamin A, juices such as ‘ACE’ drinks which are enriched with the carotenoid, or suitably dosed supplements offer a sensible complement to the supply (6).

As regards safety, beta-carotene in food, fortified foods and dietary supplements (e.g. vitamin tablets or pills) are considered absolutely harmless to non-smokers (7). Smokers are recommended to limit their daily intake to 20 mg beta-carotene. Nutritional health professionals view an undersupply of beta-carotene or vitamin A as posing a greater risk to the general health of the population than an oversupply.

Vitamin A deficiencies through gene variants

As provitamin A, beta-carotene guarantees the vitamin A intake of large parts of the population. As a consequence of national food consumption habits, almost 50 percent of the daily intake of vitamin A is met by beta-carotene (1). However, it has been shown that many people are not able to consume enough fruit and vegetables to ensure a sufficient supply of beta-carotene. Moreover, new research indicates that due to a gene variant many people are able to convert beta-carotene to vitamin A in the body to only a very limited extent.

After absorption in the intestine, beta-carotene is converted to active vitamin A by the monooxygenase enzyme. The extent to which beta-carotene is transformed to vitamin A is dependent on various factors such as the properties of the beta-carotene source, the concomitant intake of fat or genetic factors. Differences among healthy people can be as great as 45%.

Clinical studies have shown that women with genetic variations in beta-carotene-monooxygenase can convert up to 69% less beta-carotene to vitamin A than women who do not have this variation (2). It is estimated that around 40% of all Europeans are affected by this genetically caused limitation (3). Currently, these so-called “low responders” can only be identified in a few specialized laboratories.

Many experts doubt whether the currently recommended daily intake of 2–4 mg beta-carotene is adequate to make up for this gap in vitamin A supply. There is much to be said for increasing the recommended intake to around 7 mg per day (4). If the genetically caused restricted beta-carotene absorption is taken into account, the daily recommended intake would be even higher. Further research is needed to determine the influence of this genetic variation on vitamin A levels in the general population.

Natural skin protection

An adequate supply of beta-carotene is associated with a reduced risk of developing a number of diseases. The “antioxidant” activities of beta-carotene, other carotenoids and vitamins C and E are widely thought to be the underlying mechanisms of this protective effect. Beta-carotene protects cells against the destructive activity of aggressive oxygen molecules and other free radicals and in this way can probably reduce the risk of developing cancer. The antioxidant activity of beta-carotene is also thought to be mainly responsible for protecting the skin against harmful ultraviolet (UV) radiation.

A protective effect of beta-carotene intake has been verified for the acute effects of UV radiation, i.e. sunburn (1). This effect depends on the dose and duration of intake, and can be equivalent to sun-protection factor 4. Study data on the protective effect of beta-carotene against long-term effects of UV radiation on the skin, like premature aging or skin cancer, have yet to be finally interpreted. While there are indications of a positive influence on premature aging (2), the evidence for a reduction in the risk of skin cancer through supplementation with beta-carotene is not yet beyond doubt (3).

Despite indications that supplementation with beta-carotene and other carotenoids or antioxidants increases basic protection of the skin (4), it is not possible to “protect skin from the inside” against UV radiation just by taking these micronutrients. In combination with a sensible approach to UV radiation, using sunscreens and wearing suitable clothing, however, appropriate supplementation can help protect against the harmful effects of sunlight.

Protection against infectious diseases

A precondition for a well-functioning immune system is an adequate supply of micronutrients which can strengthen the immune response and improve the body’s defenses against infections. For example, vitamin A promotes the production of important immune cells and plays a decisive role in maintaining the mucous membranes of the mouth, nose, throat and lungs. Thus it safeguards the erection of physical barriers which are of essential importance in protection against pathogens. Consequently a good supply of beta-carotene (provitamin A) and vitamin A is a fundamental requirement, particularly during the cold season, for reducing the risk of catching colds (1).

Moreover, studies have shown that beta-carotene increases the number of lymphocytes (white blood corpuscles), which are responsible for recognizing and disposing of bacteria and viruses (2). In addition, beta-carotene increases the activity of natural killer cells that are able to recognize and destroy abnormal cells such as tumor cells and virus-infected cells (3).