All you need to know about fat-soluble vitamins

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.

What are fat-soluble vitamins?

Fat-soluble vitamins provide the most benefit when consumed alongside foods that contain fat.

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

Vitamin A plays an important role in maintaining healthy vision. Without vitamin A, a person would suffer from severe vision issues.
Types
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.
Function
Vitamin A supports several functions throughout the body. Some of the most important functions it supports include vision and the immune system.
Dietary sources
Vitamin A can be obtained through natural sources. Some sources include:

• fish liver oil
• liver of animals
• butter

Animal sources provide the active components to help create retinols within the human body.
Some plants also provide pro-vitamin A compounds known as carotenoid antioxidants. The most common is called beta carotene, which can be found in foods such as:

• kale
• carrots
• spinach

Deficiency
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:

• hair loss
• dry eyes
• blindness
• reduced immune function
• skin issues

Overdose
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.
If a person experiences an overdose, they may experience symptoms ranging from headaches and fatigue. In severe cases, hypervitaminosis in a pregnant woman may result in a baby with birth defects.

Vitamin D

Vitamin D is produced by the body when the skin is exposed to sunlight.

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.
Types
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

Function
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

Dietary sources
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

Deficiency
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
• tiredness

Overdose
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:

• nausea
• headache
• weight loss
• damage to heart or kidneys
• reduced appetite
• high blood pressure

Vitamin E

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.
Types
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.
Function
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.
Dietary sources
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
• hazelnuts
• almonds

Deficiency
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
• numbness

There are additional man-made types of vitamin K.
Function
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

Dietary sources
Vitamin K-1 and K-2 are found in a variety of sources. Some of these sources include:

• kale
• liver
• spinach
• parsley
• butter
• egg yolks

Deficiency
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.