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Vitamin A was first discovered in butterfat and cod liver oil in 1913. While these two foods are still great sources of this vital nutrient, fruits and vegetables don’t have any of it. Many are surprised by this, and one reason millions of people don’t get enough vitamin A. The result is reduced human performance, poor aging and potential problems in virtually all areas of the body.

There are three key reasons why so many people don’t get enough vitamin A.

First, vitamin A is only found in animal foods. It’s a myth that plant foods are high in this nutrient. Instead, fruits and vegetables are high in a family of phytonutrients called carotenoids. The body must convert three of these compounds—beta-carotene, alpha-carotene, and beta-cryptoxanthin—to vitamin A. But in humans, this conversion is quite inefficient, with about 10 to 20 molecules of carotenoids needed to make one of vitamin A. In addition, 80 percent or more of natural vitamin A from animal sources is absorbed, but only three percent or less of carotenoids from plant foods are absorbed.

Second is that there are a number of genetic variants, polymorphisms similar to those described for folates in the article, “The Folate Plot,” which can significantly impair the body’s ability to convert the carotenoids to vitamin A. This genetic problem may exist in up to half of the population. Its presence appears to be associated with high blood levels of beta-carotene, alpha-carotene, beta-cryptoxanthin, and low levels of lycopene, lutein and zeaxanthin—three other carotenoids important for health but don’t convert to vitamin A.

Third, in healthy individuals, vitamin A is continuously being used for many functions throughout the body as noted below. Most vitamin A is stored in the liver, and about five percent of it is used up each day, which must be replaced by sufficient dietary sources.

Because of these three important factors, we must get our vitamin A from animal sources to avoid the risk of low levels. The best sources are meats, including liver, beef, chicken and turkey, dairy, especially cheese and butter, and egg yolks. It’s obvious this means finding healthy, organic or otherwise real foods. Likewise, dietary supplements of cod liver oil are the best.

Vitamin A is not a single nutrient but actually a group of compounds called retinoids, of which many forms exist in nature and in the body. Each form of vitamin A performs functions the others cannot. For example, retinol is the major transport and storage form of vitamin A; retinal is essential for vision, and retinoic acid has hormone actions and regulates more than 500 genes.

In addition to the poor conversion of carotenoids to vitamin A, two important factors are also necessary for the body to obtain this vitamin from plant sources. Good gut function, especially stomach, gall bladder, liver and small intestines, is necessary for optimal absorption. While vitamin A is found in foods containing fat, the carotenoids are often not, making the addition of fat in the meal important for their absorption. Conversion of carotenoids takes place in the small intestines (with some in liver and kidney), first to the retinol form of vitamin A. This process requires other nutrients including riboflavin, niacin, iron, zinc, and adequate dietary protein.

Labeling laws continue to allow the myth that beta-carotene is the same as vitamin A. Read any label on a plant-based product and it lists the amount of vitamin A. Likewise for dietary supplements—most don’t have any vitamin A even when labeled as such (except for those containing synthetic A, the most common source used in both fortification and supplementation). What the label really refers to is the vitamin A equivalent under ideal circumstances. For example, to obtain 1 mcg (microgram) of vitamin A as retinol it takes 10 mcg of beta-carotene, and 20 mcg of alpha-carotene or beta-cryptoxanthin. However, for labeling purposes, dietary supplements can claim that, to obtain 1 mcg of vitamin A, 2 mcg of beta-carotene is required—the assumption is that the pill will be taken on an empty stomach with no other food taken with or soon after it, and digestion and absorption will be ideal.

Signs and symptoms and risk factors of vitamin A insufficiency or deficiency are many. Below are 20 common ones. How many do you have?

1. Increased sun exposure

2. Reduced immunity

3. Recurrent infections, especially viral and fungal

4. Chronic intestinal problems

5. Liver dysfunction

6. Poor night vision

7. Female of childbearing age

8. Low fat diet

9. Vegetarian diet

10. Macular degeneration

11. Dry skin

12. Dry hair

13. Dry mucous membranes

14. Weak or ridged fingernails

15. Chronic inflammation

16. Allergies or asthma

17. Weak bones or teeth

18. Difficulty maintaining vitamin D levels

19. Excess body fat

20. History of cancer

In addition to acne, psoriasis and a few conditions, much of the research around vitamin A is related to cancer. Studies show that eating foods rich in vitamin A, not carotenoids, is linked to a lower risk of cancer, while others demonstrate vitamin A can also prevent normal cells from becoming cancer. Others show this nutrient can slow tumor growth, shrink tumors, and make some cancer treatments work better.

The relationship between better brain function and vitamin A is also very important, with many clinicians using this nutrient in adults and children with a wide range of neurological deficits, including those with learning and memory problems.

The recommended daily allowance (RDA) of vitamin A is between 2,310 IU (0.7 milligrams) per day for adult women (more for women who are pregnant or breastfeeding), and 3,000 IU (0.9 milligrams) per day for men, with children needing less. (Vitamin A is still listed on food and supplement labels in international units—IUs—even though scientists rarely use this measure.)

Blood tests for vitamin A as retinol, and even for carotenoid levels, are typically measured in a simple blood test. However, the value of blood tests for assessing true vitamin A status is limited because this nutrient does not decline in the blood until vitamin A levels in the liver are almost depleted.

The best source of vitamin A is cod liver oil. But if you’re taking a high dose of vitamin D due to very low levels (such as 5-10,000 IUs or more) take your dose of A in the morning and D in the evening (or the other way around) as vitamin D can reduce the absorption of vitamin A.

Low intakes of carotenoids are associated with poor health, in particular, chronic disease and disability, so continue eating your ten servings of fruits and vegetables—studies show consuming them is protective against the decline in physical performance, and overall mortality. But think of the carotenoids and vitamin A as two separate groups of nutrients. And don’t rely on beta-carotene or other phytonutrients to provide adequate vitamin A levels. Get this from animal foods, or supplements made from such, including cod liver oil.