“Carotenoids that can be converted in the body into vitamin A are called ‘provitamin A’. Such carotenoids, which are found primarily in dark-green leafy vegetables, such as spinach, and in orange and yellow vegetables and fruit, such as carrot, mango, and papaya, are an important source of dietary vitamin A. They provide a significant portion of daily vitamin A intake in developed countries. For example, the level is about 30% in the United States (1), where animal products that contain preformed vitamin A, such as liver and egg yolk, are not consumed in very large quantities. In low-income populations of developing countries, dietary carotenoids can provide up to 80% of daily vitamin A intake (2). While preformed dietary vitamin A is well absorbed by humans, provitamin A carotenoids from plant sources are less well absorbed and need to be converted to vitamin A in human intestinal cells.
Of the provitamin A carotenoids commonly found in foods, beta-carotene has the greatest vitamin A activity. The vitamin A equivalency ratio for beta-carotene to vitamin A (retinol) is currently estimated as 12:1, by weight (12 mg beta-carotene is equal to 1 mg retinol), for plant sources of beta-carotene in a mixed diet. The ratio is based on about 17% absorption of beta-carotene from a mixed diet (6 mg plant beta-carotene = 1 mg pure beta-carotene) and a conversion ratio to vitamin A of 2:1 (2 mg beta-carotene = 1 mg retinol) (3). However, vitamin A equivalency ratios are highly variable for both pure beta-carotene in oil and beta-carotene from plant sources and can be affected by various factors. Food- and diet-related factors that affect the bioavailability of beta-carotene include the food matrix, food preparation techniques, size of the dose of beta-carotene, and the amounts of fat, fiber, preformed vitamin A, or other carotenoids in the diet. Vitamin A equivalency ratios for plant beta-carotene range from 3.8:1 to 28:1 in humans. Vitamin A equivalency ratios for Golden Rice and biofortified maize, for example, range from 3.8:1 to 6.5:1 and are lower than those for vegetable or fruit sources of beta-carotene, which range from 10:1 to 28:1. It is encouraging that the vitamin A equivalency ratios are lower for the biofortified staple foods that are meant to be targeted to populations at risk of vitamin A deficiency in developing countries. The nutritional status and health of human populations can also affect the vitamin A equivalency of beta-carotene. Moreover, genetic variants (polymorphisms) also affect the vitamin A equivalency of beta-carotene: two common genetic polymorphisms of a gene were identified and were associated with a reduction in intestinal conversion of beta-carotene to vitamin A of approximately 32–69% in UK women (4). This finding may account for much of the observed inter-individual variability in estimates of the vitamin A equivalency of beta-carotene in human populations.
The absorption of pure beta-carotene has been reported to range from 8.7% to 65% in humans and is highly variable across studies. Most studies were carried out in healthy adults from developed countries, and although vitamin A status was not reported in all studies, it is likely that vitamin A status was adequate. The variability in absorption is likely to be related to differences in the size of the oral doses of beta-carotene and differences in measurement error associated with the methods used to assess absorption. The absorption of beta-carotene from plant sources has been reported to range from about 7% to 65% in humans. Within studies, beta-carotene tended to be better absorbed from foods that were more highly processed.
Dietary beta-carotene is a safe source of vitamin A because intestinal conversion of beta-carotene to vitamin A decreases as an oral dose of beta-carotene increases (5). Because preformed vitamin A can potentially disturb the growth and development of an embryo or foetus, pregnant women are advised to avoid consu-ming large amounts of preformed vitamin A (6) and may rely more on plant sources of beta-carotene to meet their vitamin A needs. However, there has been concern that large doses of supplemental beta-carotene may have adverse effects on human health. Trials have suggested that excessive intake of beta-carotene (20–30 mg/day) may be harmful to heavy smokers or to workers exposed to asbestos (7-9). The amounts of beta-carotene that were given to participants in the trials are much higher than the amounts that are commonly consumed in the diet. The majority of people in the United States and United Kingdom, for example, consume 1–2 mg dietary beta-carotene/day, and vegans and vegetarians consume about 4–9 mg/day (10). There are no known adverse health effects associated with the consumption of physiologic doses of beta-carotene.
Daily consumption of average portion sizes of fruit and vegetable sources of beta-carotene can increase vitamin A status in population groups at risk of vitamin A deficiency. However, the effectiveness of plant sources of beta-carotene for increasing vitamin A status is likely to be context-specific and dependent on the factors mentioned above. Dietary interventions with plant sources of beta-carotene should be considered as a concurrent strategy to existing programs for increasing vitamin A status in people at risk of deficiency. More information is needed on genetic variations that may affect beta-carotene and vitamin A metabolism and on other diet- and population-related factors that can affect the bioavailability of beta-carotene, so that appropriate dietary recommendations can be made for maintaining adequate vitamin A status in human populations.”
Based on: Haskell M. J. The challenge to reach nutritional adequacy for vitamin A: beta-carotene bioavailability and conversion – evidence in humans. Am J Clin Nutr. Published online October 2012.