Vitamin A: Beta Carotene and Retinol

Vitamin A is a group of fat-soluble vitamins that are essential for various bodily functions, including maintaining eye health, skin health, and the immune system. Interestingly, Vitamin A was the first vitamin to be successfully isolated in its pure form, a feat achieved back in 1913.

There are two main types of Vitamin A:

1. Retinol, found in animal products such as meat, fish, and dairy.
2. Provitamin A, found in plant products like fruits and vegetables. Beta-carotene, a well-known type of provitamin A, is often associated with orange vegetables and fruits, such as carrots and oranges.

Animal products like fish oil (from cod, sea bass, and halibut), liver, butter, milk, and dairy products contain retinol in ester forms. Carrots, parsley, sorrel, sea buckthorn, rowanberries, rosehips, and apricots are some of the primary plant-based sources of carotene.

There’s a popular myth that eating carrots is good for your eyes, which is only partially true. Carrots are rich in beta-carotene, which the body converts into Vitamin A – a crucial vitamin for eye health. However, eating carrots won’t improve your vision if your Vitamin A levels are already adequate. This myth originated during World War II when the British were trying to explain why their pilots had excellent night vision – they didn’t want to reveal their secret of radars, so they claimed the pilots ate lots of carrots.

From Intake to Absorption and Beyond

Retinol as medication (in the form of retinol acetate and retinol palmitate) and carotenoids are well absorbed in the digestive tract. They enter the small intestine, where, in the presence of bile acids, they are emulsified and micelles are formed. The hydrolysis of retinol esters and free fatty acids is carried out by nonspecific pancreatic lipase (cholesterol esterase).

Free retinol and beta-carotene create micelles in the intestine, which consist of bile acids, cholesterol, fatty acids, and so on. Micelle formation to some extent increases the efficiency of absorption of retinol and carotene. The absorption of retinol in the intestines amounts to 80-90% of the vitamin that comes with food. The absorption of carotene is much lower — on average 50-60%. Its absorption requires the presence of fat in the diet and a sufficient concentration of bile acids in the intestine.

The retinol that has entered the intestinal microvilli is re-esterified with fatty acids. These esters enter the lymphatic pathways and in the form of chylomicrons go to the liver. The liver is the main depot of retinol, which, as needed, enters the circulatory system. In the blood, retinol is in a bound state with plasma proteins. The transfer from the blood to the cell occurs due to the direct interaction of the retinol-retinol binding protein complex with specific membrane receptors, after which free retinol enters the cell, and the retinol-binding protein returns to the circulatory system.

The liver stores 30-50% of the retinol that comes with food. Another part of it, which is absorbed in the intestine, is bound and excreted with feces (20-30%) or oxidized and excreted with urine (10-20%). The elimination of retinol occurs slowly: over 21 days, 34% of the administered amount is eliminated.

Retinol’s action primarily occurs through genetic, membrane, and glycoprotein mechanisms. Its interactions with membrane protein-lipid complexes and impact on phospholipid and glycoprotein metabolism underline its biochemical effects. Also, retinol’s antioxidant properties are crucial in lipid peroxidation regulation.

Retinoids affect visual and systemic bodily functions. Retinol’s role in creating rhodopsin, a photosensitive pigment, is vital for visual perception, particularly in low light conditions. Systemically, retinol supports growth, reproduction, epithelial differentiation, and immunity.

Retinol deficiency can lead to night blindness, growth delay, and skin problems like dryness and rashes. Additionally, it can decrease the body’s immunological resistance, contributing to increased susceptibility to infections. It’s also involved in the immune response, and its deficiency may cause immunocompetent organ atrophy and decreased lymphocyte production.

Retinol is vital in supporting reproductive functions. Insufficient amounts in animals can halt spermatogenesis, cause testicular atrophy, keratinize epithelia in the vagina, fallopian tubes, and uterus, lead to placental resorption, and spontaneous miscarriages.

Retinol is applied in several medical contexts, including ophthalmology for conditions like keratitis, xerophthalmia, and dark adaptation impairment; dermatology for skin diseases such as dyskeratosis; and for inflammatory conditions of the respiratory and digestive tracts. It’s also used in the treatment of burns, frostbite, trophic ulcers, and other non-epithelializing wounds.

Despite its importance for health, Vitamin A can be overdosed. As Vitamin A is a fat-soluble vitamin, it can accumulate in the body and cause various health problems. One sign of Vitamin A overdose is a yellowing of the skin, which might make you resemble an Oompa-Loompa from ‘Willy Wonka and the Chocolate Factory’.

Vitamin A in Food

Vitamin A is found only in animal-derived products: fish oil, milk fat, butter, cream, cottage cheese, cheese, egg yolk, liver fat, and fats from other organs like the heart and brain. However, in the human body (in the intestinal wall and liver), Vitamin A can be formed from carotene, widely found in plant products. Beta-carotene (Provitamin A) is considered the most active. It’s believed that 1 mg of beta-carotene effectively corresponds to 0.17 mg of Vitamin A (retinol). Carotene is abundant in carrots, rowan berries, apricots, rose hips, black currants, sea buckthorns, yellow pumpkins, watermelons, red peppers, spinach, cabbage, celery leaves, parsley, dill, watercress, sorrel, green onions, green peppers, nettles, dandelions, and clover. Note that the amount of vitamins changes in line with the product’s reddish-yellow coloring: the more intense the color, the more Vitamin A in the product.

Most dietary supplements contain retinol, but there are also other fully interchangeable carotenoids: Beta-carotene, Lycopene, Lutein, Alpha-carotene, Cryptoxanthin, Zeaxanthin.

Daily Vitamin A Requirement

Generally, the recommended daily allowance ranges from 400 micrograms of retinol equivalents (RE) for infants to 1300 micrograms RE for lactating women. But it’s crucial to seek personalized advice from a healthcare professional.

Therapeutic doses for treating Vitamin A deficiency or for bodybuilding can go up to 100,000 – 300,000 international units (IU) per day for adults, for short periods and under medical supervision. Given Vitamin A’s fat-solubility, it can accumulate in the body, potentially leading to hypervitaminosis symptoms, affecting skin, hair, teeth, and eye health.

Daily Consumption Estimates of Various Foods to Meet Vitamin A Requirements

The daily requirement of Vitamin A varies depending on factors such as age, gender, and whether a person is pregnant or lactating. For simplicity, let’s use the standard recommended daily allowance (RDA) for an average adult which is 900 micrograms (mcg) for men and 700 mcg for women.

Firstly, let’s convert these values into milligrams, since the food content is given in milligrams. There are 1000 micrograms in a milligram, so the RDAs become 0.9 mg for men and 0.7 mg for women.

To figure out how many milligrams of each food are needed to meet the daily Vitamin A requirement, you’d divide the RDA by the milligrams of Vitamin A per 100g of each food, and then multiply by 100 to get the weight of the food required.

Here’s how you would calculate it:

Vitamin A Content in Foods (mg/100 g of product):

• Cod liver: 1-3 mg
  • For men: 30-90g
  • For women: 23-70g
• Beef liver: 9.7 mg
  • For men: 9.3g
  • For women: 7.2g
• Caviar: 0.2-1.0 mg
  • For men: 90-450g
  • For women: 70-350g
• Butter: 0.684-0.855 mg
  • For men: 105-131g
  • For women: 82-102g
• Chicken egg: 0.14 mg
  • For men: 643g (roughly 12-13 large eggs)
  • For women: 500g (roughly 10 large eggs)
• Cheese: up to 0.33 mg
  • For men: 273g
  • For women: 212g

It’s important to remember that these are rough estimates, and actual Vitamin A content can vary based on many factors. It’s also important to get nutrients from a variety of sources and not to rely on a single food item to meet your daily requirements. Additionally, consuming high amounts of certain foods (such as liver) to meet your Vitamin A needs can lead to overconsumption of other nutrients, potentially leading to negative health effects.

Carotene is a type of Vitamin A found in plant-based foods. Your body converts it to retinol, the form of Vitamin A that it can use. The conversion is not 1:1, however, and varies from person to person. So while the foods listed are good sources of carotene, it’s not as straightforward to calculate how much of these you would need to eat to meet your Vitamin A requirements.

Vitamin A in Cosmetics

Vitamin A’s beneficial role in skin health was discovered by accident while treating acne with retinoids (forms of Vitamin A). As patients reported improved skin texture and reduced wrinkles, retinol, a type of Vitamin A, became widely used in cosmetics and skincare. It’s now recognized for promoting skin cell renewal, reducing wrinkles, and enhancing skin texture. Yet, overuse of retinol can lead to skin irritation and dryness.

Vitamin A and Bodybuilding

Vitamin A is especially important in bodybuilding, as it participates in the process of creating new muscle cells. Its deficiency can nullify post-training recovery. Vitamin A determines the rate and amount of glycogen formation in the body. Therefore, it affects not only muscle density and volume but also the athlete’s potential ability for high-intensity, energy-demanding workouts. During bodybuilding, the need for retinol increases, making it practically impossible to get the daily vitamin A requirement through food alone. This is due to Vitamin A being easily destroyed by air, sunlight, and acids. Therefore, in bodybuilding, intentional diet enrichment with high vitamin A foods such as liver, cream, cheese, and fish oil is necessary, or one can take a vitamin-mineral complex. However, care should be taken not to exceed the recommended dose.