As the name suggests, fat-soluble vitamins are a type of vitamin that is absorbed into the body through fatty tissue.
The human body requires a variety of vitamins to keep working properly. There are two types of vitamins: water-soluble and fat-soluble vitamins.
Vitamins are often obtained through regular food intake. Some people require or want additional vitamins provided through supplements.
Though both types of vitamin are important to the body, this article focuses on the types, functions, and sources of fat-soluble vitamins.
Fat-soluble vitamins will not dissolve in water. Instead, fat-soluble vitamins absorb best when taken with higher-fat foods.
Once absorbed into the body, fat-soluble vitamins are stored in fatty tissues and liver. The body can use these stores for future use. The water-soluble vitamins are vitamins B and C.
There are four types of fat-soluble vitamins:
- vitamin A
- vitamin D
- vitamin E
- vitamin K
Each type of fat-soluble vitamin promotes different functions in the body. People deficient in the fat-soluble vitamins may require supplements to boost their supply.
However, it is possible to take in too much of a fat-soluble vitamin, which could lead to toxicity and adverse reactions.
Vitamin A plays an important role in maintaining healthy vision. Without vitamin A, a person would suffer from severe vision issues.
Vitamin A does not refer to one single vitamin but is a collection of compounds known as retinoids. Retinoids can be found both in the human body and in some food sources.
Vitamin A supports several functions throughout the body. Some of the most important functions it supports include vision and the immune system.
Vitamin A can be obtained through natural sources. Some sources include:
- fish liver oil
- liver of animals
Animal sources provide the active components to help create retinols within the human body.
The recommended intake of vitamin A varies by age and gender. The following are some recommended daily allowance values:
- infants (0–12 months): 400–500 micrograms (mcg)
- children aged 1–3: 300 mcg
- children aged 4–8: 400 mcg
- children aged 9–13: 600 mcg
- adult women: 700 mcg
- adult men: 900 mcg
Vitamin A deficiency is not common in developed countries. However, vegetarians are at a higher risk of a deficiency because they do not get some kinds of vitamin A through their normal diet.
Similarly, people in developing countries with limited food sources or people whose diet is low in meat intake may also suffer from vitamin A deficiencies.
Some signs of vitamin A deficiency include:
It is possible to reach toxic levels of vitamin A. This condition is called hypervitaminosis. People who take vitamin A supplements or eat copious amounts of fish liver oils are at the highest risk.
Pregnant women should not double up on their prenatal vitamins. High levels of vitamin A are harmful to a growing fetus.
Vitamin D is produced naturally in the human body when the skin is exposed to the sun. Vitamin D aids in bone health and development.
Similar to vitamin A, vitamin D is a collective term used to describe a collection of compounds. Collectively, these are often referred to as calciferol.
There are two types found naturally:
- vitamin D-3, found in animal fats
- vitamin D-2, found in plants, such as mushrooms
Once vitamin D is absorbed into the bloodstream, the liver and kidneys change calciferol into calcitriol, the biologically active form of vitamin D.
When used in the body, vitamin D performs two major roles:
- bone maintenance
- immune system support
Vitamin D absorption is one of the only arguments for a person exposing large, unprotected areas of skin to the sun. When exposed regularly, people can actually absorb enough rays to produce vitamin D to function properly, without need for supplements.
However, many people do not spend hours in the sun. When people do, they are also often covered in sunscreen and clothing. As a result, a person is not likely to absorb as much vitamin D through sunlight alone.
Instead, people can obtain vitamin D through some food sources, including:
- fish oil
- fatty fish
- mushrooms exposed to ultraviolet light
- fortified dairy products
Recommended daily values of vitamin D vary by age, though not by much. Some general guidelines indicate the following daily values:
- infants (0–12 months): 10 mcg
- 1–70 years of age: 15 mcg
- above age 70: 20 mcg
It is not very common for a person to develop vitamin D deficiency. When it happens, most cases involve older adults or people who have been admitted to the hospital for extended amounts of time.
Some people are at a higher risk of developing a vitamin D deficiency. These include:
- obese people
- people with dark skin tones
- older adults
- those who get limited sun exposure
- people with chronic conditions
Some of the most common signs and symptoms of vitamin D deficiency include:
- increased bone fractures
- weakend immune system
- weakened muscles
- impaired healing
- soft bones
- hair loss
- more prone to infections
Toxic levels of vitamin D rarely occur. They are most likely to occur in people who take too many vitamin D supplements.
An overabundance of vitamin D in the body can lead to a condition called hypercalcemia. This condition is marked by excessive levels of calcium in the blood.
When hypercalcemia occurs, a person may experience:
- weight loss
- damage to heart or kidneys
- reduced appetite
- high blood pressure
Vitamin E is an antioxidant that can help the body destroy free radicals. Free radicals are unstable atoms that may cause the formation of cancer cells. As such, vitamin E could play an important part in preventing cancer.
Vitamin E is broken down into eight different types, with the two main kinds being tocopherols and tocotrienols. Tocopherol contains the most abundant form of vitamin E.
As an antioxidant, vitamin E protects fatty tissues from free radicals that can cause cancer. Some water-soluble vitamins, such as C and B, help aid vitamin E's functions.
In higher doses, vitamin E can also function as a blood thinner.
Vitamin E is most abundant in seeds, vegetable oils, and nuts. Some of the best sources of vitamin E include:
- wheat germ oil
- sunflower seeds or oil
Similar to vitamin D, recommended daily values for vitamin E vary by age.
Here are some of the breakdowns of recommended daily values:
- infants aged 0–6 months: 4 milligrams (mg)
- infants aged 7–12 months: 5 mg
- children aged 1–3 years: 6 mg
- children aged 4–8 years: 7 mg
- boys aged 9–13 years: 11 mg
- 14 years old and above: 15 mg
- during lactation: 19 mg
Vitamin E deficiency is extremely rare in otherwise healthy individuals. Those with specific illnesses that block the liver from absorbing vitamin E are most at risk.
Symptoms of deficiency include:
- trouble walking
- muscle weakness or tremors
- vision issues
It is nearly impossible for a person to overdose on vitamin E through natural sources. Most people who experience an overdose do so because of taking vitamin E supplements.
However, people taking blood thinners may be more prone to overdose. In high doses, vitamin E may actually increase the risk of a person developing cancer.
Vitamin K helps the body form blood clots. This necessary function prevents a person from bleeding out from small scratches.
Vitamin K has a variety of types. The two most common groups are:
- vitamin K-1, found in plant sources
- vitamin K-2, found in animal sources
There are additional man-made types of vitamin K.
The main role that vitamin K plays in the body is blood clotting. However, vitamin K can also help with:
- reducing risk of heart disease
- bone health
- reducing the buildup of calcium in the blood
Vitamin K-1 and K-2 are found in a variety of sources. Some of these sources include:
- egg yolks
Unlike the other vitamins mentioned, vitamin K recommended values are thought of as adequate intake.
When a supplement is measured in adequate intake, it means there is less evidence to support the specified amount.
Some recommended adequate intakes include:
- infants aged 0–6 months: 2 mcg
- Infants aged 7–12 months: 2.5 mcg
- children aged 1-3 years: 30 mcg
- children aged 4–8 years: 55 mcg
- children aged 9–13 years: 60 mcg
- children aged 14-18 years: 75 mcg
- adult women: 90 mcg
- adult men: 120 mcg
Vitamin K is not stored in as great an amount in the body as vitamin A or D. This can lead a person to experience a vitamin K deficiency very quickly.
If a person has a vitamin K deficiency, they have a greater risk of excess bleeding and reduced bone density that can lead to fractures.
Naturally occurring vitamin K has no known issues with overdose. Synthetic vitamin K-3, however, may cause overdose when taken in excess.
In general, vitamin K is considered safe to consume.
Fat-soluble vitamins play an essential role in a person's overall health.
It is important to manage the amount of each vitamin to avoid deficiencies and overdosing. Both can have adverse effects that may require medical attention.
Before starting a vitamin supplement, a person should speak to a doctor, and seek medical attention if a deficiency or overdose is expected.