CrossFit South Rockland

Wednesday, September 4, 2019

The Fat-Soluble Vitamins: A, D, E and K

The Fat-Soluble Vitamins: A, D, E and K

Vitamins can be classified based on their solubility.

Most are water-soluble, meaning they dissolve in water. In contrast, the fat-soluble vitamins are similar to oil and do not dissolve in water.

Fat-soluble vitamins are most abundant in high-fat foods and are much better absorbed into your bloodstream when you eat them with fat.

There are four fat-soluble vitamins in the human diet:
Vitamin A
Vitamin D
Vitamin E
Vitamin K

This article provides a comprehensive overview of the fat-soluble vitamins, their health benefits, functions and main dietary sources.

Vitamin A
Vitamin A plays a key role in maintaining your vision. Without it, you would go blind.

Vitamin A is not a single compound. Rather, it is a group of fat-soluble compounds collectively known as retinoids.

The most common dietary form of vitamin A is retinol. Other forms — retinal and retinoic acid — are found in the body, but absent or rare in foods.

Vitamin A2 (3,4-dehydroretinal) is an alternative, less active form found in freshwater fish (1Trusted Source).

SUMMARY:The main dietary form of vitamin A is known as retinol.
Role and Function of Vitamin A

Vitamin A supports many critical aspects of body function, including:
Vision maintenance: Vitamin A is essential for maintaining the light-sensing cells in the eyes and for the formation of tear fluid (2Trusted Source).
Immune function: Vitamin A deficiency impairs immune function, increasing susceptibility to infections (3Trusted Source, 4Trusted Source).
Body growth: Vitamin A is necessary for cell growth. Deficiency may slow or prevent growth in children (5Trusted Source).
Hair growth: It is also vital for hair growth. Deficiency leads to alopecia, or hair loss (6Trusted Source).
Reproductive function: Vitamin A maintains fertility and is vital for fetal development (7Trusted Source).
SUMMARY:Vitamin A is best known for its vital role in maintaining vision. It's also essential for body growth, immune function and reproductive health.
Dietary Sources

Vitamin A is only found in animal-sourced foods. The main natural food sources are liver, fish liver oil and butter.

The table below shows the amount of vitamin A in 3.5 ounces (100 grams) of some of its richest dietary sources (8Trusted Source):Share on Pinterest

Vitamin A can also be derived from certain carotenoid antioxidants found in plants. They are collectively known as provitamin A.

The most efficient of these is beta-carotene, which is abundant in many vegetables, such as carrots, kale and spinach (9Trusted Source, 10).
SUMMARY:The best dietary sources of vitamin A include liver and fish oil. Sufficient amounts can also be derived from provitamin A carotenoids, like beta-carotene, which are found in vegetables.
Recommended Intake

The table below shows the recommended daily allowance (RDA) for vitamin A. The RDA is the estimated amount of vitamin A that the vast majority (about 97.5%) of people need to meet their daily requirements.

This table also shows the tolerable upper intake limit (UL), which is the highest level of daily intake considered safe for 97.5% of healthy people (11Trusted Source).

SUMMARY:The RDA for vitamin A is 3,000 IU (900 mcg) for adult men and 2,333 (700 mcg) for women. For children, it ranges from 1,000 IU (300 mcg) to 2,000 IU (600 mcg).
Vitamin A Deficiency

Vitamin A deficiency is rare in developed countries.

However, vegans may be at risk, since pre-formed vitamin A is only found in animal-sourced foods.

Although provitamin A is abundant in many fruits and vegetables, it is not always efficiently converted into retinol, the active form of vitamin A. The efficiency of this conversion depends on people's genetics (12Trusted Source, 13Trusted Source).

Deficiency is also widespread in some developing countries where food variety is limited. It is common in populations whose diet is dominated by refined rice, white potatoes or cassava and lacking in meat, fat and vegetables.

A common symptom of early deficiency includes night blindness. As it progresses, it may lead to more serious conditions, such as:
Dry eyes: Severe deficiency may cause xerophthalmia, a condition characterized by dry eyes caused by reduced tear fluid formation (2Trusted Source).
Blindness: Serious vitamin A deficiency may lead to total blindness. In fact, it is among the most common preventable causes of blindness in the world (14Trusted Source).
Hair loss: If you are vitamin A deficient, you may start to lose your hair (15Trusted Source).
Skin problems: Deficiency leads to a skin condition known as hyperkeratosis or goose flesh (16Trusted Source).
Poor immune function: Poor vitamin A status or deficiency makes people prone to infections (3Trusted Source).
SUMMARY:Severe vitamin A deficiency may lead to blindness. Other symptoms may include hair loss, skin problems and an increased risk of infections.
Vitamin A Toxicity

Overdosing on vitamin A leads to an adverse condition known as hypervitaminosis A. It's rare, but may have serious health effects.

Its main causes are excessive doses of vitamin A from supplements, liver or fish liver oil. In contrast, high intake of provitamin A does not cause hypervitaminosis.

The main symptoms and consequences of toxicity include fatigue, headache, irritability, stomach pain, joint pain, lack of appetite, vomiting, blurred vision, skin problems and inflammation in the mouth and eyes.

It may also lead to liver damage, bone loss and hair loss. At extremely high doses, vitamin A can be fatal (17Trusted Source).

People are advised to avoid exceeding the upper limit for intake, which is 10,000 IU (900 mcg) per day for adults.

Higher amounts, or 300,000 IU (900 mg), may cause acute hypervitaminosis A in adults. Children can experience harmful effects at much lower amounts (18Trusted Source).

Individual tolerance varies considerably. Children and people with liver diseases like cirrhosis and hepatitis are at an increased risk and need to take extra care.

Pregnant women should also be especially careful, since high doses of vitamin A may harm the fetus. Doses as low as 25,000 IU per day have been linked with birth defects (19Trusted Source).
SUMMARY:High doses of vitamin A may lead to hypervitaminosis A, which is associated with various symptoms. Pregnant women should avoid eating high amounts of vitamin A because of the risk of birth defects.
Benefits of Vitamin A Supplements

While supplements are beneficial for those who suffer from deficiency, most people get enough vitamin A from their diet and do not need to take supplements.

Yet, controlled studies suggest that vitamin A supplements may benefit certain people even if their diet meets the basic requirements.

For instance, vitamin A supplements may help treat measles in children (20Trusted Source, 21Trusted Source).

They protect against measles-associated pneumonia and reduce the risk of death by 50–80%. Studies suggest that vitamin A acts by suppressing the measles virus (22Trusted Source).
SUMMARY:Supplements mainly benefit those who are low or deficient in vitamin A. One exception is children with measles, as studies show that supplements may help treat the disease.
Summary of Vitamin A

Vitamin A, also known as retinol, is a fat-soluble vitamin traditionally associated with vision and eye health.

The most abundant dietary sources of vitamin A are liver, fish liver oil and butter.

It can also be derived from provitamin A carotenoids found in red, yellow and orange vegetables, as well as some leafy, dark-green vegetables.

Deficiency is rare in developed countries, but is most common among people who follow diets lacking in diversity, especially those dominated by rice, white potatoes and cassava.

Early symptoms of vitamin A deficiency include night blindness, and severe deficiency may eventually lead to total blindness.

Nevertheless, while getting enough vitamin A is vital, too much may cause harm.

Pregnant women should be extra careful not to eat excessive amounts of vitamin A because of the risk of birth defects.

Vitamin E

As a powerful antioxidant, vitamin E protects your cells against premature aging and damage by free radicals.

Vitamin E is a family of eight structurally similar antioxidants that are divided into two groups:
Tocopherols: Alpha-tocopherol, beta-tocopherol, gamma-tocopherol and delta-tocopherol.
Tocotrienols: Alpha-tocotrienol, beta-tocotrienol, gamma-tocotrienol and delta-tocotrienol.

Alpha-tocopherol is the most common form of vitamin E. It makes up around 90% of the vitamin E in the blood.
SUMMARY:Vitamin E is a group of related compounds divided into tocopherols and tocotrienols. Alpha-tocopherol is the most common type.
Role and Function of Vitamin E

Vitamin E's main role is to act as an antioxidant, preventing oxidative stress and protecting fatty acids in your cell membranes from free radicals (37Trusted Source).

These antioxidant properties are enhanced by other nutrients, such as vitamin C, vitamin B3 and selenium.

In high amounts, vitamin E also acts as a blood thinner, reducing the blood's ability to clot (38Trusted Source).
SUMMARY:Vitamin E's key role is to serve as an antioxidant, protecting cells against free radicals and oxidative damage.
Dietary Sources

The richest dietary sources of vitamin E include certain vegetable oils, seeds and nuts. The chart below shows some of the best sources of vitamin E and the amount found in 3.5 ounces (100 grams) of these foods (8Trusted Source):Share on Pinterest

Other rich sources include avocados, peanut butter, margarine, fatty fish and fish liver oil.
SUMMARY:The best sources of vitamin E are certain vegetable oils, nuts and seeds.
Recommended Intake

The table below shows the RDA and tolerable upper limit for vitamin E intake. The values marked with an asterisk are the adequate intake, since no RDA values are available for infants.

RDA (IU / mg) UL (IU / mg)
Infants 0–6 months 6 / 4* Not known
7–12 months 8 / 5* Not known
Children 1–3 years 9 / 6 300 / 200
4–8 years 11 / 7 450 / 300
9–13 years 17 / 11 900 / 600
Adolescents 14–18 years 23 / 15 1,200 / 800
Adults 19–50 years 23 / 15 1,500 / 1,000
51+ 18 / 12 1,500 / 1,000

SUMMARY:Among adults, the RDA for vitamin E is 23 IU (15 mg). For children and adolescents, the RDA ranges from 9 IU (6 mg) to 23 IU (15 mg), depending on the age group.
Vitamin E Deficiency

Vitamin E deficiency is uncommon and is never detected in people who are otherwise healthy.

It happens most often in diseases that impair the absorption of fat or vitamin E from food, such as cystic fibrosis and liver disease.

Symptoms of vitamin E deficiency include muscle weakness, walking difficulties, tremors, vision problems, poor immune function and numbness.

Severe, long-term deficiency may lead to anemia, heart disease, serious neurological problems, blindness, dementia, poor reflexes and the inability to fully control body movements (39Trusted Source, 40Trusted Source).
SUMMARY:Vitamin E deficiency is rare, but can cause muscle weakness, susceptibility to infections, neurological problems and poor vision.
Vitamin E Toxicity

Overdosing on vitamin E is difficult when it is obtained from natural dietary sources. Cases of toxicity have only been reported after people have taken very high doses of supplements.

Yet, compared to vitamin A and D, overdosing on vitamin E appears to be relatively harmless.

It may have blood-thinning effects, counteracting the effects of vitamin K and causing excessive bleeding. Thus, people who take blood-thinning medications should avoid taking large doses of vitamin E (38Trusted Source, 41Trusted Source, 42Trusted Source).

Additionally, at high doses of more than 1,000 mg per day, vitamin E may have pro-oxidant effects. That is, it can become the opposite of an antioxidant, potentially leading to oxidative stress (43Trusted Source).
SUMMARY:Vitamin E appears to be less toxic at high doses than vitamin A and D. However, high doses may cause excessive bleeding and oxidative stress.
Benefits and Risks of High Vitamin E Intake or Supplements

High vitamin E intake from food or supplements has been linked with a number of benefits.

One form of vitamin E, gamma-tocopherol, was found to increase blood flow by promoting the dilation of blood vessels, potentially reducing blood pressure and the risk of heart disease (44Trusted Source).

Gamma-tocopherol supplements may also have a blood-thinning effect as well as reducing levels of "bad" LDL cholesterol (45Trusted Source).

In contrast, other studies suggest that high-dose vitamin E supplements may be harmful, even when they don't cause any obvious symptoms of toxicity.

For instance, observational studies show that taking vitamin E supplements is linked with an increased risk of prostate cancer and death by all causes (46Trusted Source, 47Trusted Source, 48Trusted Source).

Given the potentially adverse effects of vitamin E supplements, they cannot be recommended at this point. High-quality studies are needed before solid conclusions can be reached about the long-term safety of these supplements.
SUMMARY:Vitamin E supplements may reduce the risk of heart disease, but the evidence is conflicting. Some studies suggest that high-dose supplements are harmful. More studies are needed.
Summary of Vitamin E

Vitamin E is a group of powerful antioxidants, the most common of which is alpha-tocopherol.

Its main function is to serve as an antioxidant and protect the body's cells against damage by free radicals.

The most abundant dietary sources of vitamin E include vegetable oils, nuts and seeds. Deficiency is very rare in healthy people.

While supplements may provide certain health benefits, not all scientists agree. The long-term safety of vitamin E supplements is a matter of debate.

Vitamin K

Vitamin K plays a key role in blood clotting. Without it, you would run the risk of bleeding to death.

Vitamin K is actually a group of fat-soluble compounds divided into two main groups:
Vitamin K1 (phylloquinone): Found in plant-sourced foods, phylloquinone is the main form of vitamin K in the diet (49Trusted Source).
Vitamin K2 (menaquinone): This variety of vitamin K is found in animal-sourced foods and fermented soy products, like natto. Vitamin K2 is also produced by gut bacteria in the colon (50Trusted Source, 51Trusted Source).

Additionally, there are at least three synthetic forms of vitamin K. These are known as vitamin K3 (menadione), vitamin K4 (menadiol diacetate) and vitamin K5.
SUMMARY:Vitamin K is a family of compounds. The main dietary forms are vitamin K1, found in plant foods, and vitamin K2, found in animal-derived foods and fermented soy products.
Role and Function of Vitamin K

Vitamin K plays an essential role in blood clotting. In fact, the "K" stands for "koagulation," the Danish word for coagulation, which means clotting.

But vitamin K has other functions as well, including supporting bone health and helping prevent the calcification of blood vessels, potentially reducing the risk of heart disease (52Trusted Source).
SUMMARY:Vitamin K is vital for blood clotting and supports bone health.
Dietary Sources

The best dietary sources of vitamin K1 (phylloquinone) are leafy green vegetables, whereas vitamin K2 (menaquinone) is mainly found in animal-sourced foods and fermented soy products.

The table below shows some of the main sources of vitamin K1 and the amounts found in 3.5 ounces (100 grams) of these foods (8Trusted Source):Share on Pinterest

In contrast to phylloquinone, menaquinone is only found in small amounts in certain high-fat, animal-sourced foods, such as egg yolks, butter and liver.

It is also found in certain soy foods, such as natto.
SUMMARY:Vitamin K1 is abundant in many leafy green vegetables, while vitamin K2 is found in low amounts in animal-sourced foods and fermented soy foods.
Recommended Intake

The table below shows the adequate intake (AI) values for vitamin K.

AI is similar to the RDA, which is a daily intake level thought to meet the requirements of 97.5% of people, but AI is based on weaker evidence than RDA.

Al (mcg)
Infants 0-6 months 2
7–12 month 2.5
Children 1–3 years 30
4–8 years 55
9–13 years 60
Adolescents 14–18 years 75
Women 18+ years 90
Men 18+ years 120

SUMMARY:The adequate intake (AI) of vitamin K is 90 mcg for women and 120 mcg for men. For children and adolescents, the AI ranges from 30–75 mcg, depending on the age group.
Vitamin K Deficiency

Unlike vitamins A and D, vitamin K isn't stored in the body in significant amounts. For this reason, consuming a diet lacking in vitamin K may lead you to become deficient in as little as a week (53Trusted Source).

People who do not efficiently digest and absorb fat are at the greatest risk of developing vitamin K deficiency. This includes those who suffer from celiac disease, inflammatory bowel disease and cystic fibrosis.

Use of broad-spectrum antibiotics may also raise the risk of deficiency, as well as very high doses of vitamin A, which seem to reduce vitamin K absorption.

Mega-doses of vitamin E may also counteract the effects of vitamin K on blood clotting (41Trusted Source, 54Trusted Source).

Without vitamin K, your blood wouldn't clot and even a small wound could cause unstoppable bleeding. Fortunately, vitamin K deficiency is rare, since the body only needs small amounts to maintain blood clotting.

Low levels of vitamin K have also been linked with reduced bone density and increased risk of fractures in women (55Trusted Source).
SUMMARY:Deficiency in vitamin K may lead to excessive bleeding. Diseases that interfere with fat absorption increase the risk of deficiency.
Vitamin K Toxicity

Unlike the other fat-soluble vitamins, natural forms of vitamin K have no known symptoms of toxicity.

As a result, scientists have not been able to establish a tolerable upper intake level for vitamin K. Further studies are needed.

In contrast, a synthetic form of vitamin K, known as menadione or vitamin K3, may have some adverse effects when consumed in high amounts (56Trusted Source, 57Trusted Source).
SUMMARY:The maximum safe dosage of vitamin K is unknown and no symptoms of toxicity have been identified.
Benefits of Vitamin K Supplements

Several controlled studies have examined the effects of vitamin K supplements in humans. These studies show that vitamin K supplements — vitamin K1 and vitamin K2 — may decrease bone loss and reduce the risk of bone fractures (58Trusted Source, 59Trusted Source).

Additionally, taking vitamin K2 supplements at 45–90 mg per day slightly increased the survival of patients with liver cancer (60Trusted Source).

Observational studies also suggest that a high intake of vitamin K2 may lower the risk of heart disease. However, the evidence from controlled studies is limited and inconclusive (61Trusted Source, 62Trusted Source).

Finally, vitamin K1 supplements taken at 0.5 mg every day for three years slowed the development of insulin resistance in older men, compared to a placebo. No significant differences were detected in women (63Trusted Source).
SUMMARY:Limited evidence suggests that vitamin K supplements may improve bone health, reduce the risk of heart disease and increase survival among liver cancer patients.
Summary of Vitamin K

Vitamin K is a group of fat-soluble compounds divided into vitamin K1 (phylloquinone) and vitamin K2 (menaquinone).

Vitamin K1 is mainly found in leafy green vegetables, whereas vitamin K2 comes from animal-sourced foods, such as liver, butter and egg yolks.

Small amounts are also produced by gut bacteria in the colon.

Deficiency impairs the blood's ability to clot, causing a risk of excessive bleeding.

There is limited evidence on the health benefits of supplements among people who aren't deficient. However, a few controlled studies suggest that vitamin K supplements benefit bone and heart health.

The Bottom Line

There are four fat-soluble vitamins in the human diet: A, D, E and K. They are essential for health and play many important roles in the body.

With the exception of vitamin D, most of them are easy to get from a diverse diet, especially if you eat plenty of nuts, seeds, vegetables, fish and eggs.

These vitamins tend to be abundant in fatty foods and you can enhance their absorption by adding fat or oil to an otherwise low-fat meal.

Few foods are naturally rich in vitamin D. It is abundant in fatty fish and fish oil, but also formed by your skin when you're exposed to sunlight.

For this reason, vitamin D deficiency is a problem for people who follow an inadequate diet and spend most of their time indoors.

While you generally do not need to supplement with vitamin A, E and K, taking vitamin D supplements is widely recommended.

For optimal health, make sure you get all of the fat-soluble vitamins in adequate amounts.

No comments:

Post a Comment