Vitamins A, C, D, E, and K

Vitamins A, C, D, E, and K

Vitamins are nutrients that are essential for human metabolism. Humans are able to synthesize (create) and store some vitamins, but most of them need to be introduced from foods, regularly through a balanced and varied diet.

From a chemical point of view, vitamins are groups of related organic molecules. Vitamins have many important biochemical functions that are important for the normal functioning of our bodies and overall health. Their importance can best be observed when the deprivation is present.

So far, science has recognized six groups of vitamins which are, together with minerals, certain amino acids, and fatty acids, recognized as essential nutrients. These vitamins are fat soluble (vitamins A, D, E and K) and water soluble (vitamins B and C). Fat soluble are able to be stored in fat cells, while water soluble are not.

Vitamin A

Vitamin A or retinol, as it is also called, is actually a group of organic compounds with an important nutritional value for humans. This group includes carotenoids, the most prominent one being beta-carotene, as well as retinol, retinal, and retinoic acid.

Vitamin A has many important functions in the human body. Various foods are of animal and herbal origin are a good source of this vitamin. Therefore, a well-balanced diet is usually enough to maintain the optimal levels of vitamin A in healthy individuals.

Functions of Vitamin A

Vitamin A has various metabolic functions inside the human body. The most notable ones are:

· Immune system maintenance

· Vision

· Gene transcription

Additionally, vitamin A plays a role in other processes, such as:

· Bone metabolism

· Skin and teeth health

· Embryonic development

· Formation of blood cellular components (hematopoiesis)

The Immune System

The role of vitamin A in the cell-mediated immunity is very important. This type of immune system response involves the activation of the so-called T-lymphocytes (a subtype of white blood cells). Vitamin A helps the rapid increase in the number of these T-cells. By doing so, this vitamin boosts the body’s immune response.

Vitamin A also helps the T-cells to differentiate (acquire special specialized features) and become regulatory T-cells. This is important for the prevention of an auto-immune response.

The lack of vitamin A causes the deficiency of hematopoietic stem cells or causes them to go into a dormant state. These cells have a role in the production of certain cellular components of the blood and their transition from a dormant into an active state is important for the maintenance of the immune system.


Vitamin A (in the retinal form) has a crucial role in the formation of rhodopsin inside the eye. Rhodopsin is essential for contrast and, therefore, night vision and the vision in low light. This is why one of the first symptoms of vitamin A deficiency is night blindness.

Gene Transcription

Retinoic acid, a form of vitamin A, has a big role in the process of gene transcription. Simply said, gene transcription is the copying of information from DNA to RNA. It is the first step in a bigger process called the gene expression.

Gene expression is the synthesis of functional gene products, namely proteins, which further dictate the function of various cells in the human body.

A good example of the effects of retinoic acid is its ability to activate the genes responsible for the transformation of immature skin cells (keratinocytes) into mature cells. This promotes skin healing and health. Due to such properties, vitamin A-based medications are researched as a possible therapy for acne.

Natural Sources of Vitamin A

Vitamin A can be found in many foods. For this reason, healthy individuals normally do not require supplementation.

Here is a list of foods rich in vitamin A:

· Animal liver (especially of certain fish species such as cod)

· Sweet potato

· Broccoli

· Kale

· Ghee

· Butter (especially ghee)

· Carrot

· Pumpkin

· Spinach

· Bell pepper

· Tomatoes

· Pees

· Mango

· Papaya

· Apricot

· Eggs

· Milk

· Cheese

· Fish

The foods that are extremely rich in vitamin A, such as liver (including pâté) and oily fish should not be consumed more than once a week. This is because such a diet can increase the intake of vitamin A and cause its levels to reach harmful values.

Certain groups should take extra precaution. These are women who have passed through menopause, elderly men, and people suffering from osteoporosis.

Recommended Daily Intake

The human body has the ability to store vitamin A. This means that everyday intake is not necessary. However, certain values are established as an adequate intake through diet.

These values are different for different age groups:

· Infants 0 to 6 months – 400µg/day

· Infants 7 to 12 months – 600 µg/day

· Children 300 – 400µg/day

· Females (over 14) – 700µg/day

· Males (over 14) – 900µg/day

Pregnant and lactating women should increase the daily intake of vitamin A at 770µg and 1300µg respectively.

Some research suggests that a high intake of vitamin A, over a long period of time, can lead to the weakening of the bones. This is especially important for groups that are at higher risk of osteoporosis, such as women. The US Institute of Medicine and the European Food Safety Authority have set up a daily limit of 3000µg for adults.

Medical Uses of Vitamin A

Vitamin A supplements can be used to treat deficiency caused by an unvaried diet or certain medical conditions which inhibit the absorption and storage of this vitamin. Otherwise, some forms of vitamin A are used in skin creams while medications with high doses of retinoic acid are used in dermatological, HIV, and cancer treatments.

Vitamin C

Vitamin C or ascorbic acid is an essential nutrient found in various foods. It has many important roles in different processes that take place inside the human body. Vitamin C is also an antioxidant.

This vitamin is listed on the World Health Organization List of Essential Medicine among other extremely safe and highly effective medications.

Vitamin C cannot be stored inside the body. It is water-soluble, which means dissolves in water and gets expelled through urine. To maintain optimal levels of this essential nutrient, regular daily intake is necessary.

The Functions of Vitamin C

Vitamin C has a prominent role in tissue repair and wound healing. This is because it acts as a co-factor in many enzymatic reactions, most notably in the collagen synthesis. Because of this vitamin C is important for the health of various tissues such as skin, cartilage, bones, and blood vessels. Optimal levels of this vitamin also contribute to the proper functioning of the immune system.

It is a common misconception in the general public that vitamin C can be used to prevent or treat the common cold. There is no scientific evidence to support this claim.

Collagen Synthesis

Collagen is a protein present in large quantities (up to 35%) in the human body. It makes up a significant portion of many tissues including the skin, cartilage, bones, tendons, ligaments, intestines, muscles, cornea, and spinal vertebrae discs. Vitamin C has a vital role in the synthesis of collagen and its maintenance.

The so-called Type I collagen makes up around 90% of all collagen found in the human body. Vitamin C increases the synthesis of this type of collagen. This vitamin also helps the synthesis of the Type IV collagen which has an important function in the kidneys and the arterial lining.

Furthermore, Vitamin C supports the wound healing process by promoting the production of fibroblasts, cells that synthesize collagen. This also protects from the premature formation of wrinkles (aging).

Synthesis of Neurotransmitters

Vitamin C has a very direct influence on the synthesis of neurotransmitters. Therefore, its role in the brain’s ability to respond to environmental factors as well as to think and memorize is significant.

Calcium Incorporation

Vitamin C helps the calcium to become incorporated into the bone tissue, strengthening the bones in the process.

Vitamin C does this by stimulating the production of osteoblast, cells responsible for calcium incorporation, and inhibiting the formation of osteoclasts, the cells that help the dissolve of calcium.

The Immune System

Vitamin C interacts with and supports the immunes system in many ways. It has an important role in:

– Cell-mediated immune response

– Production of interferons (signaling proteins that warn the nearby cells of a virus presence)

– Production of antibodies

– Vaccination-related immune response

– Antibiotics function

There are many more important immune system-related functions of vitamin C but describing them would require a separate article.

Natural Sources of Vitamin C

Vitamin C is mostly present in fresh fruits and vegetables. Some foods of animal origin can have significant amounts of this vitamin in the raw state (e.g. chicken liver) but its amount decreases after cooking.

The foods richest in this vitamin are:

· Rosehip

· Citrus fruits such as lemons and oranges (including fresh juices)

· Guava

· Blackcurrant

· Kiwifruit

· Strawberry

· Elderberry

· Papaya

· Pineapple

· Mango

· Kale

· Red bell pepper

· Cantaloupe

· Cabbage

· Potato

· Tomato

· Onion

Some, not widely available, plant species have an extremely high content of vitamin C. Kakadu plum (up to 5300 mg/100g), native to tropical Australia, and Camu Camu from the Amazon rainforest are among these. Human breast milk contains around 5mg per 100g of vitamin C.

Vitamin C decomposes when cooked. The higher the temperature and the longer the cooking process, the more vitamin C is lost. It also dissolves in the cooking water, so soups will contain more vitamin C than those dishes after whose preparation the cooking water is poured away.

Vitamin C is widely available as a supplement and in various fortified foods. It is probably the most widely used supplement worldwide.

Recommended Daily Intake

Recommended daily intake of vitamin C is proposed by different organizations and government bodies around the world. The World Health Organization (WTO) recommends 45mg daily for adults.

These values differ in accordance with age. In the United States the recommended daily intake for different age groups is:

· Children 1 to 3 years old – 15 mg

· Children 4 to 8 years old – 25 mg

· Children 9 to 13 years old – 45 mg

· Adult females – 75 mg

· Adult males – 90 mg

Increased intake is suggested for pregnant (85 mg/a day) and lactating (120 mg/a day) women.

There is no conclusive evidence that proves adverse effects of high intake of vitamin C. Diarrhea has been reported in cases of intake higher than 300 mg a day. The US Academy of Sciences has set an upper limit at 2000 mg/day for vitamin C.

Medical Uses of Vitamin C

Vitamin C is mainly used in medicine for the prevention and treatment of scurvy, a condition caused by vitamin C deficiency. In the past, scurvy was common and often lethal among the populations deprived of vitamin C in their diet. Sailors were commonly affected because they spent months without consuming fresh fruits and vegetables.

Scurvy is characterized by the onset of symptoms such as lethargy and malaise, after a month or longer of vitamin C deprivation. If the deprivation continues, more severe symptoms appear. These include:

· Bleeding gums

· Easy bruising

· Bone pain

· Poor wound healing

· Shortness of breath

· Fever

· Convulsions

When it is not treated, scurvy results in death. In modern times scurvy is very rare and, since the disease is well understood and described, the treatment is simple and efficient. When the scurvy is treated early on, all the damage caused by the condition is reversible.

There is not enough scientific evidence to support the claim that vitamin C is beneficial for the treatment of other diseases. There is ongoing research of its role in cancer treatment, cardiovascular disease treatment, and the improvement of brain functions.

Some research results report that vitamin C supplementation can shorten the duration of certain infections (e.g. common cold).

Vitamin D

Vitamin D is a common name for a group of fat-soluble vitamins. Out of several organic compounds that make the vitamin D group, the two most significant ones for humans are the vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol).

These two vitamins are sparsely available in the diet as only a few foods contain them. The main natural source of vitamin D is through its synthesis in the skin. This synthesis is a result of a chemical reaction that occurs in cholesterol when it is exposed to UVB radiation (sunlight exposure).

Because it is not easily available in the diet, vitamin D cannot be considered an essential nutrient. However, this vitamin has many important functions in human metabolism and a vital role in the healthy development of bones, muscles, and teeth.

Functions of Vitamin D

Bone Growth and Calcium absorption

Vitamin D supports calcium absorption from foods, in the gastrointestinal tract. In this way, it makes calcium available to various tissues. Vitamin D also increases the number of osteoclasts, cells responsible for bone resorption, thus facilitating the absorption calcium and strengthening the bones. This results in proper bone development and a lowered fracture risk.


Vitamin D has a direct role in the increase of muscle strength. One of the vitamin D metabolites affects the nucleus of muscle cells and enhances their ability to contract. The increased contraction ability strengthens the muscles significantly.

Immune System

Some research results report the connection between the low levels of vitamin D in the body and seasonal flu. The levels of vitamin D are the lowest in winter when there is less exposure to sunlight which coincides with the flu season. The research showed that individuals with low vitamin D levels are at the higher risk of developing fever and other severe immune responses to the flu during winter. Vitamin D deficiency also increases the risk of other viral infections, including HIV.

Natural Sources of Vitamin D

There are not many good natural sources of vitamin D. Some fungi are able to produce vitamin D2 when they are exposed to UV light. Foods of plants and animal origin usually do not contain a significant amount of this vitamin.

Among fungi, the best sources of vitamin D2 are:

· Common mushroom (champignon mushroom)

· Raw Portobello mushroom

· Raw crimini mushroom

Vitamin D3 can naturally be found in a limited number of foods of animal origin. These are:

· Fish liver oils, especially cod liver oil

· Tuna

· Sardines

· Mackerel

· Salmon

· Egg yolks

· Animal liver (beef, pork, etc.)

When it comes to vitamin D content, the way food is prepared is very important. Cooking reduces vitamin D content in foods by 20 to 40 percent.

Recommended Daily Intake

The needs for vitamin D vary in accordance with age. Children from 0 to 12 months require 8.5 to 10 µg per day. Others, including adults, pregnant, and lactating women require 10 µg per day. People who live in temperate climate zones should consider vitamin D supplementation in the period from October to March when there is less sunlight available.

Certain groups are at an increased risk of suffering from vitamin D deficiency.

· People who do not spend much time outdoors (e.g. bedbound and housebound individuals)

· People that cover most of their skin when outdoors

· People whit dark skin living in temperate climate areas

Hypercalcemia or calcium build up is a common consequence of taking very high doses (more than 100 µg per day) of vitamin D during a longer time period. Too much calcium can lead to kidney damage and the weakening of the bones.

The symptoms of Hypercalcaemia include:

– Nausea

– Vomiting

– Weakness

– Insomnia

– Anorexia

– Metastatic calcification in the kidneys

A dangerously high intake of vitamin D cannot be achieved through diet or sun exposure but only through the use of vitamin D supplements.

Medical Uses of Vitamin D

Although optimal levels of vitamin D promote bone health, there is no evidence to show that vitamin D supplementation can prevent osteoporosis or lower the risk of bone fractures.

The lowered risk of cancer, cardiovascular diseases, and diabetes is also not associated with vitamin D supplementation.

Vitamin D is used to prevent and treat its deficiency and the related conditions.

Rickets is a condition most often associated with vitamin D deficiency. It is a childhood disease that causes long bones to weaken and soften. One of the most notable characteristics of rickets are the “bow legs” that form when children affected by this disease start to walk causing the legs to bend under their weight. Rickets is rare in the developed world today but it can still be found in some low-income communities in Africa and Asia.

Osteomalacia is another disease characterized by the softening of the bones. This condition is caused by vitamin D deficiency and affects adults. It causes bending of the spine, bone fragility, muscle weakness, and bowing of the legs.

Vitamin E

Scientists do not completely agree on the crucial functions of this vitamin. This group of fat-soluble organic compounds was first discovered in 1922 and named tocopherol.

Today it is established that vitamin E group includes four tocotrienols and four tocopherols. These appear in four different forms alpha, beta, gamma, and delta.

It is also known that the deficiency of this vitamin can cause neurological disorders. Vitamin E’s antioxidant functions are recognized as well. All forms of vitamin E can penetrate cell membranes and donate a hydrogen atom to free radicals, thus reducing their numbers.

Functions of Vitamin E

Its antioxidant properties are the most important function of vitamin E. By reducing the numbers of free radicals, vitamin E protects the body from damage. Free radicals cause harm to the cells and therefore all tissues and organs. They are considered to be the number one reason for aging.

Vitamin E also has a role in the production of red blood cells. It strengthens the immune system and improves the body’s use of vitamin K.

Natural Sources of Vitamin E

Many foods contain this nutrient. Various oils are normally rich in vitamin E. Other good sources are oil-rich plants. Foods of animal origin also contain this vitamin but in smaller quantities.

Here is a list of good natural sources of vitamin E:

· Hazelnut oil

· Canola oil

· Sunflower oil

· Almond oil

· Palm oil

· Peanut butter

· Almond butter

· Margarine

· Hazelnuts

· Almonds

· Sunflower seed kernels

· Olive oil

· Pine nuts

· Pistachios

· Mayonnaise

· Avocados

· Spinach

· Broccoli

· Oily fish

· Cheese

· Eggs

· Beef

· Pork

· Chicken

· milk

Many foods are also fortified with vitamin E. These include baby formulas, liquid meals, and cereals. Supplements are also available, usually in the form of soft capsules. Vitamin E taken in through diet is absorbed in the intestinal tract and transferred to the liver via the portal vein where it is further metabolized. The absorption of vitamin E from food can be as high as 85%.

Recommended Daily Intake

There are estimated daily recommendations for vitamin E intake by many health organizations and authorities around the world. The United States National Academy of Medicine recommends the following:

· Infants 0 to 6 months – 4 mg per day

· Children 7 to 12 months – 5 mg per day

· Children 1 to 3 years – 6 mg per day

· Children 4 to 8 years – 7 mg per day

· Children 9 to 13 years – 11 mg per day

Above the age of 14, the recommended daily intake for both men and women is 15 mg per day. The same amount is recommended for pregnant women. The recommended daily value for lactating women is 19 mg per day.

This organization sets the upper limit at 1000 mg per day although no relevant evidence exists to prove the adverse effects of excessive intake of vitamin E.

Medical Uses of Vitamin E

Vitamin E supplements can be used to treat the deficiency of this nutrient. However, vitamin E deficiency is extremely rare in healthy individuals. It is usually a consequence of a genetic disorder or an underlying condition that inhibits the absorption of this vitamin.

Ataxia is a genetic movement disorder that causes vitamin E deficiency. Vitamin E supplements are used as a part of the treatment for ataxia.

There is a significant number of conditions for whose treatment vitamin E is presumed to be effective. However, more research is needed in this area.

Some of these conditions include:

· Anemia

· Alzheimer’s disease

· Bladder cancer

· Beta-thalassemia

· Dysmenorrhea

· Male infertility

· Huntington’s disease

· Non-alcoholic Steatohepatitis

· Parkinson’s disease

· Premenstrual syndrome

· Rheumatoid arthritis

· Sunburn

· Dementia

Vitamin K

Vitamin K is also a group of fat-soluble vitamins that can be stored in by the human body. It has an important role in blood coagulation and the resorption of calcium in various tissues.

The two major compounds of the vitamin K group are the vitamin K1 and the vitamin K2. Vitamin K1 is the form usually found in commercially available dietary supplements.

There are many foods, of both plant and animal origin, which are rich in this vitamin. Therefore, vitamin K deficiency is rare in healthy adults. However, it is common in newborn children. For that reason, it is common for babies to be injected with a single dose of this vitamin shortly after birth.

Functions of Vitamin K

Blood Coagulation

Vitamin K’s most important function is the prevention of uncontrolled bleeding through its crucial role in blood coagulation, a process severely impaired by the deficiency of this vitamin.

Bone Health

Due to its role in the process of calcium binding to various tissues, optimal levels of vitamin K support the maintenance of healthy bones. Whether vitamin K lowers the risk of fractures and contributes to the increase in bone density is still a matter of scientific research. The same applies to its potential benefits in osteoporosis prevention and treatment.

Cardiovascular Health

Vitamin K also helps the regulation of the calcium levels in the blood and prevents calcification of the arteries. In this way, optimal levels of vitamin K lower the risk of various cardiovascular diseases.

Natural Sources of Vitamin K

There are many good natural sources of both vitamin K forms which are important for humans. Vitamin K1 can mostly be found in green leafy vegetables and some fruits, while a good source of vitamin K2 is the animal liver and some fermented foods.

Here is a list of best natural sources for vitamin K1:

· Kale

· Broccoli

· Brussel sprouts

· Spinach

· Asparagus

· Cabbage

· Collards

· Mustard

· Turnip

· Swiss chard

· Parsley

· Lettuce

Vitamin K2 can be found in the following foods of animal origin:

· Goose liver pate

· Chicken liver

· Beef

· Bacon

· Pork

· Duck breast

· Venison

· Salmon

· Mackerel

· Egg yolk

· Butter

Cooking does not significantly affect the amount of vitamin K in most foods. However, it may decrease the amount of some other essential nutrients.

Recommended Daily Intake

The necessary intake of vitamin K can normally be achieved through a balanced diet and supplementation is rarely needed.

The recommended daily intake depends on the weight of the individual. One microgram (1 µg) is recommended for every kilogram of weight. For example, if a person weighs 70 kg, the recommended daily intake of vitamin K would be 70 µg. Pregnant and lactating women can increase the recommended dose slightly.

This vitamin can be stored in the liver, so a regular daily intake is not necessary.

It is a common practice to inject a preventive dose of vitamin K (usually between 0.5 and 1 µg) to all newborns during the first 24h following the birth.

Vitamin K deficiency is extremely rare. Some groups are at a higher risk though. These include alcoholics, people with liver disease, malnourished people, and those suffering from inflammatory bowel disease or cystic fibrosis.

Common symptoms of vitamin K1 deficiency include:

· Anemia

· Bruising

· Bleeding gums

· Frequent nosebleeds

· Severe menstrual bleeding

Vitamin K2 deficiency is associated with an increased risk or aortic calcification, cardiovascular disease, and osteoporosis.

Medical Uses of Vitamin K

The most important medical use of vitamin K is the prevention and treatment of its deficiency and, therefore, facilitating a normal blood coagulation process. Vitamin K can also be used to counter the effects of high doses of certain anti-coagulant medications.

Supplements are used to maintain the optimal levels of vitamin K in people suffering from conditions which affect normal absorption of this vitamin, such as Crohn’s and celiac disease.

Vitamin K is also used in topical creams to improve the appearance of dark circles around the eyes, capillary burst, and to reduce bleeding after cosmetic procedures (e.g. Botox injections).

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