4.1 Vitamin A deficiency
Vitamin A, more properly known as retinol, is an important chemical intermediate in a number of biochemical processes in mammals. It is involved in vision, and is found in the rod cells of the retina of the eye. These cells are particularly important in seeing at low light levels, and night blindness is a symptom of vitamin A deficiency (VAD). Vitamin A is also involved in the proper functioning of the immune system. Children suffering from VAD are prone to serious infections, and often die from relatively minor illnesses, like diarrhoea or measles. The World Health Organisation in 2003 estimated that between 100 and 140 million children worldwide were vitamin A deficient, of whom between 250 000 and 500 000 become blind each year. Of these, half died within 12 months of losing their sight.
Many plants and bacteria can produce vitamin A from simpler molecules, but mammals cannot. Humans can either ingest vitamin A directly, or produce it by the chemical cleavage of one of a group of molecules called carotenoids. Carotenoid molecules contain 40 carbon atoms, and mammals can chemically cleave a number of them to produce either one or two molecules of the 20-carbon retinol. A number of related carotenoid molecules are found in the human diet. The ones that can be converted into vitamin A are referred to as the provitamin A carotenoids. The commonest of these is β-carotene (see Table 2).
Table 2 Estimates of the retinol provided by common dietary carotenoids using the Retinol Equivalent (RE) and Retinol Activity Equivalent (RAE) scales. (See text for explanation.)
|Colour||Mass (μg) equivalent to activity of 1 μg of retinol (RE scale)||Mass (μg) equivalent to activity of 1 μg of retinol (RAE scale)|
|Provitamin A carotenoids|
|Other common dietary carotenoids|
The bioconversion of provitamin A carotenoids into retinol is not efficient, and different provitamin A carotenoids provide different amounts of retinol when processed by the human system. Various methods have been used to gain a rough idea of how much retinol a carotenoid will produce. Until very recently, the concept of a Retinol Equivalent (RE) has been widely used. This is a measure of the mass of a given carotenoid that will be converted to 1 μg of retinol in the human body. You can see from Table 2 that according to this system, it takes 6 μg of β-carotene to produce 1 μg of retinol. In recent years a number of nutritionists have argued that RE values overestimate the level of retinol produced by a factor of two. They propose a new scale, and use the term Retinol Activity Equivalent (RAE). Some institutions, like the WHO, still use REs, but this may change over the life of this course.
Whether REs or RAEs are used, estimates of dietary requirements can only be very approximate, as the precise value will depend on the source of the food, how it is prepared, and other aspects of the diet. In particular, in order to absorb the retinol, a certain level of fat is required in the diet. The efficiency with which different individuals metabolise the provitamin A carotenoids will also differ.
Nevertheless, the concept of retinol equivalents is useful to help measure the amount of vitamin A in the diet.
According to the WHO, a woman aged 25-50 should consume a recommended dietary allowance (RDA) of 800 μg Retinol Equivalents (REs) each day. If it were the sole source of retinol in her diet, what mass of β-carotene would a woman have to consume each day to meet this requirement?
Using REs, the table above indicates that 6 μg of β-carotene is roughly equivalent to 1 μg of retinol, so the RDA in terms of β-carotene is 6 × 800 μg = 4800 μg of β-carotene.
This quantity of β-carotene could be found in approximately 40 g of raw carrots (Figure 7). If the estimate was calculated using RAEs, the figure would be 80 g of raw carrots.
The carotenoids form an important part of a balanced diet. A number of studies have suggested that they may have anti-cancer properties, perhaps resulting from their ability to act as antioxidants.
Vitamin A itself occurs in animal products, particularly in meat, liver, eggs and milk. Carotenoid compounds are found in a variety of vegetables and fruit. We have seen that β-carotene is found in carrots. Lycopene is found in relatively high concentrations in tomatoes.
What sort of diet will minimise vitamin A deficiency (VAD)?
Given the wide variety of foods that contain vitamin A and carotenes, any reasonably varied diet that contains sufficient fat will provide adequate sources of vitamin A.
VAD is a disease of poverty, found where people are unable to afford an appropriate diet. It is prevalent in countries where rice is a staple, particularly in South Asia. The rice plant itself does contain carotenes: they are found in both the leaves and the husks. Rice that has not been milled, brown rice, can therefore be an important source of both dietary fibre and carotenes.
However, white rice is often considered more palatable, and in many countries cultural issues surround the type of rice that is eaten. Unprocessed brown rice is seen as fit only for the lowest in society. Another factor is that milled rice is more easily stored. The husk contains a high proportion of oils which can degrade, causing the rice to become rancid if it is stored for a long time.