Съдържание
Съдържание
Vitamin A is a fat-soluble vitamin needed for maintaining health. As a term, vitamin A includes a group of related compounds including active vitamin A and its precursors, the provitamin A carotenoids.
Vitamin A is found in both animal and plant foods, although in different forms. One of the active forms of the vitamin, retinol, is found in animal products like meat and eggs. The precursors, or “provitamin” forms of the vitamin, include the carotenoids beta-carotene, alpha-carotene, and beta-cryptoxanthin. Carotenoids are found in plants and often give yellow-orange coloration to different fruits and vegetables.
Vitamin A is needed for cellular growth and development. It also plays a critical role in vision, immune function, reproduction, bone health, and gene regulation. The vitamin is of crucial necessity during childhood development. In countries with poor levels of vitamin A in the diet, supplementation has been shown to reduce childhood deaths from any cause by 24%. In severely deficient children, eye damage can develop leading to blindness. Researchers have recommended that all children at risk for deficiency be supplemented with vitamin A due to the large benefits that have been found.
One of the main effects of vitamin A is the genetic regulation of cellular differentiation. When cells develop, they often differentiate. This process allows a cell to become specialized, like a muscle or nerve cell. Vitamin A is necessary for the production of differentiated cells throughout the body.
Bone health is also reliant on the vitamin, although the dose of active vitamin A is critical. Research suggests that consuming both too little and too much of the active form of the vitamin increases risks for osteoporosis. However, the risks for too much vitamin A do not appear to apply to provitamin A carotenoids from food, as they correlate with a decreased risk of osteoporosis with increasing consumption.
Provitamin A carotenoids also have been correlated, in some studies, with decreased risks for cognitive decline and dementia. However, in order to derive significant preventative effects with carotenoids, the research suggests that long-term consumption over many years is likely necessary.
Since active vitamin A and provitamin A carotenoids have different efficacy as vitamin A, the term retinol activity equivalents (RAE) was developed to allow for comparison. This simplifies nutritional labels as a food's vitamin A levels are measured in RAE.
While it may get somewhat confusing, one microgram of retinol (active vitamin A) is equal to one microgram RAE. For supplemental beta-carotene, two micrograms count as one microgram RAE. From plant sources, such as fruits and vegetables, the RAEs are much lower. It takes 12 micrograms of beta-carotene from food to count as one microgram RAE and 24 micrograms of either alpha-carotene or beta-cryptoxanthin to count as one microgram RAE.
Recommendations for daily retinol activity equivalents (RAE) based on age and sex:
As previously covered, vitamin A is crucial for developing children and deficiency can lead to increased susceptibility to infections, blindness, skin rashes, and other developmental abnormalities. Low vitamin A has also been shown to increase risks for bone loss and osteoporosis.
Too much active vitamin A, in rare cases, can be fatal. However, cases of fatal vitamin A overdose require very high levels of consumption. At lower levels, excess active vitamin A has also been associated with bone loss, especially in individuals that have vitamin D deficiency or struggle with obesity. Interestingly, provitamin A carotenoids do not appear to increase bone loss, with some research suggesting that they may actually be protective. A study out of Japan found that women with the highest blood levels of beta-carotene and beta-cryptoxanthin had a 76% and a 93% reduction in the risk of osteoporosis respectively.
As for carotenoids, excess provitamin A can cause a harmless orange coloration. When consumed in large quantities, carotenoids are deposited in the skin. While not a perfect test, you can often get a sense of a person’s carotenoid consumption by the level of orange coloration in the palms of their hand. Even with high-level consumption, carotenoids from food sources appear to be quite safe.
Multiple food sources provide vitamin A, either as active vitamin A or as the provitamin a carotenoids.
Active vitamin A, or retinol, is found in a number of animal-based food products. Meat, eggs, seafood, poultry, and dairy products all contain active vitamin A. Liver is quite high in active vitamin A and cod liver oil is a particularly rich source.
Getting vitamin A from beta-carotene and other provitamin-A, carotenoid-rich foods is likely the safest approach to vitamin A supplementation for two reasons. First, as previously mentioned, it is possible to consume too much retinol or active vitamin A. Fortunately, provitamin A carotenoids from food don’t appear to have a toxic threshold.
While food-based carotenoids have been correlated with significant health benefits, synthetic beta-carotene supplements appear to carry some risks. A study in smokers found an increased risk of cancer and death with synthetic beta-carotene supplementation. Yet diets high in beta-carotene from food have been shown to decrease the risks of heart disease and other chronic diseases.
While not proven, the problem with synthetic beta-carotene could be due to differences in the synthetic form as compared to natural beta-carotene. Natural beta-carotene comes in two forms, while synthetic beta-carotene is only composed of one. In general, the safest approach is to consume foods high in provitamin A carotenoids to avoid issues with synthetic beta-carotene supplements and too much active vitamin A.
Foods that are deep orange to yellow in color are often rich sources of provitamin A. Carrots, pumpkin, winter squash, sweet potatoes, apricots, cantaloupe, dark leafy greens, bell peppers, grapefruit, and broccoli are all good sources of provitamin A carotenoids.
A number of so-called superfoods are also rich in carotenoids. Red palm oil has extremely high levels. It contains 15 times more vitamin A than carrots and 44 times more than leafy greens. The deep red color of the oil is characteristic of high carotenoid content.
Another rich source of provitamin A is goji berries. Originally from Asia, these berries have often been touted for their anti-aging benefits in Chinese herbal medicine. The berries are quite rich in carotenoids and have about four times the beta-carotene content of carrots. They are also extremely rich in another carotenoid potentially important for eye health: zeaxanthin.
In South America, a plant in the same family as goji berries is also a good source. Ground cherries or golden berries are a tart fruit with a golden orange color, again indicative of their carotenoid content. Depending on growing conditions and the variety, they typically contain anywhere from a similar amount of beta-carotene as carrots to 10 times as much. Like goji berries, they are also very rich in zeaxanthin.
Spirulina and chlorella, two edible algae, are also very rich sources of carotenoids. By weight, spirulina has over 50 times more beta-carotene than carrots. Chlorella comes in a close second, with slightly less. Both types of algae are also rich in other vitamins, minerals, and protein.
A number of common spices are particularly rich sources of provitamin A. Paprika and cayenne pepper are both loaded with beta-carotene. Basal, parsley, marjoram, and oregano also have significant beta-carotene content. Spices can be a simple way to add flavor and additional nutrients to food.
Vitamin A comes in two main forms, active vitamin A, which comes from animal products, and provitamin A carotenoids, found in plants. Vitamin A is important for cellular differentiation, immune function, bone health, reproduction, gene regulation, and vision. While it appears that you can consume too much active vitamin A, the provitamin A carotenoids, when derived from food, appear quite safe. Research suggests that they also have significant health benefits, including protection from heart disease, osteoporosis, other chronic diseases, and dementia. Consuming foods and superfoods known to be rich in provitamin A is likely an effective strategy to help promote health.
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