6 Vitamins : shocking primary sources, Bioavailability & Risks related

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Vitamins are organic substances that are generally classified as either fat-soluble or water-soluble. Fat-soluble vitamins (vitamin A, vitamin D, vitamin E, and vitamin K) dissolve in fat and tend to accumulate in the body.

Vitamin A (Retinol | Carotenoids)

Vitamin A plays a central role in our vision, skin, genes, growth, and immune system. It is especially
important during the early stages of pregnancy in supporting the developing embryo. Infections and
fevers increase the requirement for vitamin A.

Three different forms of vitamin A are active in the body: retinol, retinal, and retinoic acid. These are known as retinoids. The cells of the body can convert retinol and retinol to the other active forms of vitamin A as needed.

Each form of vitamin A performs specific tasks. Retinol supports reproduction and is the major transport form of the vitamin. Retinal is active in vision and is also an intermediate in the conversion of retinol to retinoic acid.

Retinoic acid acts like a hormone, regulating cell differentiation, growth, and embryonic development. Foods derived from animals provide retinol in a form that is easily digested and absorbed. Foods derived from plants provide carotenoids, some of which have vitamin A activity.

The body can convert carotenoids like β-carotene, α-carotene, and β-cryptoxanthin into vitamin A. The conversion rates from dietary carotene sources to vitamin A are 12:1 for β-carotene and 24: 1 for β-cryptoxanthin.

The primary sources of vitamin A

Retinol is found in liver, egg yolk, butter, whole milk, and cheese. Carotenoids are found in orange-flesh sweet potatoes, orange-flesh fruits (i.e., melon, mangoes, and persimmons), green leafy vegetables (i.e., spinach, broccoli), carrots, pumpkins, and red palm oil.

Bioavailability of vitamin A

The bioavailability of vitamin A derived from animal sources is high – about 70–90% of the vitamin A ingested is absorbed by the body. Carotenoids from plant sources are absorbed in much lower rates – between 9% and 22%  and the proportion absorbed decreases as more carotenoids are consumed.

Dietary fat enhances the absorption of vitamin A. Absorption of β-carotene is influenced by the food matrix. β-carotene from supplements is more readily absorbed than β-carotene from foods, while cooking carrots and spinach enhances the absorption of β-carotene. Diarrhea or parasite infections of the gut are associated with vitamin A malabsorption.

About 90% of vitamin A is stored in the liver. Vegetarians can meet their vitamin A requirements with sufficient intakes of deeply colored fruits and vegetables, fortified foods, or both.

Vitamin A deficiency is a major problem when diets consist of starchy staples, which are not good sources of retinol or β-carotene, and when the consumption of deeply colored fruits and vegetables, animal-source foods, or fortified foods is low.

Vitamin A plays a role in mobilizing iron from liver stores, so vitamin A deficiency may also compromise iron status. Excessive intakes of preformed vitamin A can result in high levels of the vitamin in the liver a condition known as hypervitaminosis A. No such risk has been observed with high β-carotene intakes.

Vitamin D (Calciferol)

With the help of sunlight, vitamin D is synthesized by the body from a precursor derived from cholesterol. Vitamin D is therefore not an essential micronutrient, given the right season and enough time in the sun. The active form of vitamin D is actually a hormone that targets organs most notably the intestines, kidneys, and bones.

In the intestine, vitamin D is involved in the absorption of calcium and phosphorus. In the bone, it assists in the absorption of calcium and phosphorus, helping bones grow denser and stronger as they absorb and deposit these minerals.

The primary sources of vitamin D

Sunlight exposure to ultraviolet B (UVB) rays is necessary for the body to synthesize vitamin D from the precursor in the skin. There are a few foods that are natural sources of vitamin D. These sources are oily fish,
egg yolk, veal, beef, and mushrooms.

Bioavailability of vitamin D

There is very little information on the bioavailability of vitamin D. It is assumed that the food matrix has little effect on absorption. Bioavailability also varies among individuals and depends on the level of circulating vitamin-D-binding protein.

Inadequate exposure to sunlight is the primary risk factor for poor vitamin D status. The use of sunscreen, higher levels of melanin in the skin (i.e., dark skin), skin coverings (clothes, veils), and time of day are factors that decrease exposure to UVB rays.

The distance from the equator is also a factor for UVB exposure; people living in latitudes above or below 40 degrees from the equator will be unable to form vitamin D from the skin precursor during the winter months.

Breast milk is a poor source of vitamin D. Children who are exclusively breastfed and have no or little sun exposure require vitamin D supplements to meet their vitamin D requirements.

One of the main roles of vitamin D is to facilitate the absorption of calcium and phosphorus. Consequently, a vitamin D deficiency creates a calcium deficiency, with significant consequences to bone health. Among children and adolescents, it may cause rickets and adversely affect peak bone mass. In adults, vitamin D deficiency increases the risk of osteomalacia and osteoporosis.

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Vitamin E (α-Tocopherol)

The most active form of vitamin E is α-tocopherol, which acts as an antioxidant (i.e., stops the chain reaction of free radicals producing more free radicals). Vitamin E protects cell membranes, proteins, and DNA from oxidation and thereby contributes to cellular health.

It prevents the oxidation of the polyunsaturated fatty acids and lipids in the cells. Vitamin E is stored in the liver and is safe even at high intakes.

The primary sources of vitamin E

Vitamin E in the α-tocopherol form is found in edible vegetable oils, especially wheat germ, sunflower, and rapeseed oil. Other good sources of vitamin E are leafy green vegetables (i.e., spinach, chard), nuts (almonds, peanuts) and nut spread, avocados, sunflower seeds, mango, and kiwifruit.

Bioavailability of vitamin E

Vitamin E is a fat-soluble nutrient. As such, absorption of this vitamin is enhanced in the presence of fat in a meal.

Individuals whose diets consist mostly of starchy staples – with an inconsistent intake of edible oils or other vegetable sources of vitamin E – are at a higher risk of inadequate vitamin E intake. Vitamin E deficiency leads to red blood cell breakage and nerve damage.

Recent studies from Bangladesh link low vitamin E blood levels to an increased risk of miscarriage. In other studies vitamin E supplementation has been successfully used for the treatment of non-alcoholic fatty liver disease, a condition widespread in overweight and obese people. Excessive intake of vitamin E from food is very rare.


Vitamin K (Phylloquinone | Menaquinones)

Vitamin K acts primarily in blood clotting, where its presence can make the difference between life and death. More than a dozen different proteins and the mineral calcium are involved in making a blood clot. Vitamin K is essential for the activation of several of these proteins.

When any of the blood clotting factors is lacking, hemorrhagic disease (uncontrolled bleeding) results. Vitamin K also participates in the metabolism of bone proteins, most notably osteocalcin. Without vitamin K, osteocalcin cannot bind to the minerals that normally form bones, resulting in poor bone mineralization. Vitamin K is stored in the liver.

The primary sources of vitamin K

Vitamin K is found in plant foods such as phylloquinone (K1). Bacteria in the lower intestine can synthesize vitamin K as menaquinone (K2), which is absorbed by the body. Sources of phylloquinone are green leafy vegetables (i.e., parsley, spinach, collard greens, and salad greens), cabbage, and vegetable oils (soybean, canola, olive).

Menaquinones are also found in fermented foods such as fermented cheese, curds, and natto (fermented soybeans).

Bioavailability of vitamin K

Absorption of vitamin K from food sources is about 20%, and dietary fat enhances absorption.

Vitamin K is poorly transferred via the placenta and is not found in significant quantities in breast milk, so newborn infants are especially at risk for bleeding. This innate vitamin K deficiency is treated with intramuscular injection or oral administration of phylloquinone.

Supplementation with vitamin K has been found to be beneficial for improving bone density among adults with osteoporosis because it drives the synthesis of a special protein called matrix Gla protein. Primary natural sources
Animal products, Dairy products | Meat | Liver, Vegetable products, Leafy vegetables | Cabbage | Vegetable oils

Vitamin C (Ascorbic Acid)

Vitamin C parts a company with the B vitamins in its mode of action. It acts as an antioxidant or as a cofactor, helping a specific enzyme perform its job. High levels of vitamin C are found in the pituitary and adrenal glands, eyes, white blood cells, and the brain.

Vitamin C has multiple roles – in the synthesis of collagen, absorption of iron, free radical scavenging, and defense against infections and inflammation

The primary sources of vitamin C

Fruits (especially citrus fruits), cabbage-type vegetables, green leafy vegetables, lettuce, tomatoes, potatoes, and liver (ox /calf).

Bioavailability of vitamin C

Levels of vitamin C in foods depend on the growing conditions, season, stage of maturity, cooking practices, and storage time prior to consumption. Vitamin C is easily destroyed by heat and oxygen.

Absorption levels depend on the amounts consumed. About 70–90% of vitamin C is absorbed. If intakes exceed 1000 mg/day, absorption levels drop to 50%.

Individuals who do not consume sufficient quantities of fruits and vegetables are at risk for inadequate intakes of vitamin C. Because smoking generates free radicals, individuals who smoke have elevated requirements for vitamin C.

Vitamin C deficiency can cause scurvy; signs of scurvy are bleeding gums, small hemorrhages below the skin, fatigue, loss of appetite and weight, and lowered resistance to infections.


Strictly speaking, choline is not a vitamin, but an essential nutrient that is often grouped under the B vitamins. Although the body can make choline, dietary intake of choline is necessary to meet the body’s needs for this nutrient. Choline also acts as a methyl donor.

Choline has several functions, including fat and cholesterol metabolism, cell structure and cell integrity, cellular signaling, neurotransmission, and gene expression. In pregnancy, choline is essential for the brain development of
the growing fetus.

The primary sources of choline

Choline can be found in many foods, mainly in milk, eggs, and peanuts. It is also part of lecithin, which is used as an emulsifier in food processing.

A varied diet should provide enough choline for most people, but strict vegetarians (who consume no milk or eggs) may be at risk of inadequate choline intake. Insufficient intake of choline can lead to liver dysfunction and muscle damage.

During pregnancy, choline is especially important as it is involved in fetal brain development. There is some data to suggest that maternal choline status might be related to neural tube defects. Choline biosynthesis declines in women during menopause.

Recent research has linked low choline blood levels to an increased risk of stunting (short-for-age) in children from Malawi.

Choline and folate interact at the level where homocysteine is converted to methionine. If the metabolism of one of these methyl donors is disturbed, it disrupts the metabolism of choline. Excess intake of choline is rare but can result in a fishy body odor, vomiting, salivation, hypotension, and liver toxicity.

Primary natural sources Animal products, Dairy products | Eggs, Fruit | Vegetable products, Peanuts, and Water-soluble nutrients, Water-soluble.


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