Supplements with vitamin A

Three preformed compounds that show metabolic activity are found as vitamin A (retinoids). However, in supplements we can use retinol, retinol acetate, and retinol esterified with a fatty acid: retinyl palmitate.

The carotenoids found in seaweed and vegetables and which can provide retinoids when metabolized in the body are another source of vitamin A. Although there are several hundred carotenoids present naturally in foods as antioxidants, only some have significant vitamin A activity. The most important of them is ß-carotene.

Foods with vitamin A

Vitamin A as such (retinoids) is found only in certain foods of animal origin. Provitamin A carotenoids, such as beta-carotene, alpha-carotene, and beta-cryptoxanthin, are found mainly in plant-based foods and are converted to vitamin A when necessary.

The main sources of provitamin A and vitamin A are vegetables (40%) and whole dairy products (30%).

Vitamin A:

• Lamb, pork, chicken, turkey, and fish livers. Foie-gras and paté.
• Whole eggs and egg yolks.
• Butter.
• Fresh and aged cheeses.
• Ice cream.
• Caviar.

Provitamin A carotenoids:

• Margarine.
• Carrots, spinach, butternut squash, watercress, tomatoes, endives, and yams.
• Mangoes, papayas, apricots, medlars, peaches, and oranges.

Carotenoids with no provitamin A activity:

• Lycopene, in tomatoes, mainly in tomato sauce.
• Lutein and zeaxanthin, mainly in corn, avocados, and green leafy vegetables like spinach, kale, green beans, broccoli, etc.
• Astaxanthin and canthaxanthin, in salmon, trout, crustaceans, and roe.
• Capsantin, in red peppers and paprika.

Intake and deficiencies

Deficient vitamin A intake is common, and obtaining it from plants is not as easy as previously thought.

The vitamin A concentration depends on external inputs of retinol and provitamins.

• Low vitamin A levels are common in obesity, asthma, and COPD.
• In asthma and COPD, vitamin A can help improve their control.
• Vitamin A needs the “team” of basic nutrients to achieve its maximum therapeutic potential in chronic diseases.

Forms in foods. Vitamin A is found in two main forms:

• Retinol, the form present in foods of animal origin, which is yellow and lipophilic and found in tissues in ester form.
• Carotenes (alpha, beta, and gamma), and the xanthophyll beta-cryptoxanthin form, all of which are of plant origin and absorbable by herbivores and omnivores. They are A provitamins that can be converted to vitamin A.

Bioavailability. Absorption of provitamin A depends to a great extent on the amount of lipids ingested with the provitamin, as lipids increase absorption of the provitamin (Solomons and Orozco 2003). Recent studies indicate that obtaining vitamin A from plants is not as easy as previously thought; it is easier to do so from provitamins dissolved in oils and fats and those in supplements.

The conversion of carotenes to retinol varies from one person to the next, and carotene’s bioavailability in foods also varies (Borel, Drai et al. 2005; Tang, Qin et al. 2005).

Intake and deficiencies. Vitamin A intake in men falls with age, and in women, the 50–59 age group has the lowest intake. The intake of 9% of women and 15% of men is less than one third of the NRV, which can be considered high-risk (Morales Alcover, Ochoa Herrera et al. 2007).

The vitamin A intake of 60% of males and 48% of females is lower than recommended, with these levels corresponding to a risk of vitamin A deficiency (Health and Social Security Dept. of the Regional Government of Catalonia 1996).

According to the ENIDE survey, the habitual intake of this vitamin is below the Reference Dietary Intake in more than 90% of men and around 60% of women. (AESAN, 2011).

Nutrients. Zinc deficiencies can worsen vitamin A absorption, transport, and metabolism, because zinc is essential for synthesis of the vitamin’s transport proteins and oxidation of retinol to retinal. When there is a vitamin E deficiency, vitamin A is not absorbed or stored properly. Iron and vitamin A are associated in its deficiency. When there is a vitamin A deficiency, plasma levels of iron drop. Vitamin A is necessary for the release of iron stored in the liver and, likewise, an iron deficiency can reduce the mobilization of vitamin A from the liver, reducing its levels in the bloodstream.

Common medications. Orlistat, laxatives, and cholestyramine can accentuate a marginal vitamin A deficiency.

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Reference values

NRV, PRI, UL for vitamin A

The PRI (for men) in 1968 was 5,000 IU (1,500 μg of retinol). In 1974, the PRI was adjusted to 1,000 RE (1,000 μg of retinol); the dietary reference intake is currently 900 RAE (900 μg, or 3,000 IU of retinol). This is the equivalent of 1,800 μg of a β-carotene supplement (3,000 IU) or 10,800 μg of β-carotene from foods (18,000 IU).

The NRV is 800 μg of retinol or beta-carotene, or of retinol + beta-carotene.

There are no recommendations for carotenoids. The UL is 3,000 μg of retinol.

High doses of vitamin A are toxic. Signs of chronic toxicity can appear when daily intake is above 15 mg/day of retinol (50,000 IU) in adults. Daily doses (food + supplements) should not exceed 7,500 mcg of vitamin A (25,000 IU).

Excessive intake of vitamin A, particularly of retinoids, is called hypervitaminosis A and has characteristics of toxicity. The most typical effects are congenital defects, liver problems, loss of bone density potentially leading to osteoporosis, hair discoloration and loss, dry and scaly skin, etc. Acute signs include nausea and vomiting, headache, fatigue, blurry vision, and loss of muscular coordination.

Although hypervitaminosis A can occur when vitamin A-rich foods such as liver are consumed in large quantities, most cases of vitamin A toxicity are a consequence of excess vitamin A intake in the form of vitamin supplements.

NRV: Nutrient Reference Value. These are the daily amounts recommended for vitamins and minerals (micronutrients only) considered sufficient in the European Union to prevent a deficit in most healthy individuals in the population. They are the values used for labeling and claims.

PRI: Population Reference Intake. These are the reference values for a specific population group, defined as the amount of energy and/or nutrients (macronutrients and micronutrients) recommended to cover the nutritional needs of practically the entire population. They refer to healthy population groups. The values are expressed per person per day, as a 15-day average. To account for individual differences in digestive and metabolic processes, the PRI figure used for each nutrient is the average plus twice the standard deviation (except for energy, for which the average is used), thereby including almost all healthy individuals in the group (97.5%).

UL: Tolerable Upper Intake Level: the maximum level of total daily intake of a nutrient in a context of long-term or chronic consumption which does not pose a risk to health.