Vitamins are classified based on several factors. One of these is their solubility.
Most vitamins are water-soluble. This means that they can dissolve in water. Conversely, there are also fat-soluble vitamins, meaning that they are more soluble in oil than in water.
Fat-soluble vitamins are mostly found in fatty foods. The absorption of these vitamins is very high when they are eaten with fat.
The human diet contains four types of fat-soluble vitamins namely:
- Vitamin A
- Vitamin D
- Vitamin E
- Vitamin K
This article gives a detailed overview of the fat-soluble vitamins, their functions, health benefits, and primary dietary sources.
Vitamin A plays a key role in maintaining eye health. Blindness is inevitable without vitamin A.
Types of vitamin A
Here’s the thing with vitamin A – it isn’t just one compound. Vitamin A is a group of fat-soluble compounds. They are collectively known by the term retinoids.
Retinol is the most common form of vitamin A. Other forms of vitamin A include retinoic acid and retinal acid. They exist naturally in the body, but cannot be found in foods.
Another form of vitamin A is 3,4-dehydroretinal (vitamin A2). It is a less-active form of vitamin A mostly found in freshwater fish (1).
Health benefits of vitamin A
Vitamin A plays some very important functions in the body. These include:
- Maintenance of vision: Vitamin A keeps your photoreceptors healthy. It is also involved in the formation of tears (2).
- Immune function: Deficiency of vitamin A causes the weakening of the immune system. This will make you more susceptible to infections (3, 4).
- Growth of the body: Vitamin A contributes immensely to cell growth. Children that are deficient in vitamin A may experience a delay in growth (5).
- Hair growth: It is important to note that vitamin contributes to hair growth. If you are deficient in this vitamin, you may experience alopecia (6).
- Reproduction: Vitamin A enhances fertility and is very important for fetal development (7).
Dietary sources of vitamin A
Vitamin A can also be extracted from carotenoid antioxidants. Vitamins derived from these antioxidants are known as provitamin A.
What is the recommended intake of vitamin A?
The recommended daily allowance (RDA) for vitamin A is 900mcg for adult men, and 700 mcg for women. Children may take from 300-600mcg (11).
Vitamin A deficiency
One rarely hears about vitamin A deficiency in developed nations.
But then, the risk is high in vegans, because these fat-soluble vitamins are mostly found in animal-based foods.
Provitamin A may also be found in vegetables and fruits. The thing, however, is that the human body cannot convert the provitamin A in plant foods efficiently into retinol. The efficiency of this conversion is highly dependent on genetics (12, 13).
Deficiency of these fat-soluble vitamins are also common in developing countries where there are limited varieties of food. It is predominant in countries where the staple foods are cassava, white potatoes, or refined rice, and scarcity of fat, vegetables, and meat.
Night blindness is a common symptom of early vitamin A deficiency. As the disease advances, it may result in complications, like:
- Dry eyes (xerophthalmia) (2).
- Blindness (14)
- Hair loss (15)
- Skin problems including goose flesh or hyperkeratosis (16)
- Compromised immune function (3)
Vitamin A toxicity
Excessive intake of vitamin A may cause a condition called hypervitaminosis A. It is a rare condition, but very injurious to the health.
The main causes of hypervitaminosis A include high doses from supplements, fish liver oil, or liver. On the other hand, a high intake of provitamin A doesn’t trigger hypervitaminosis.
It may also cause hair loss, bone loss, and damage to the liver. Very high doses of vitamin A is lethal (17).
For adults, it is important to note that doses up to 900mcg may cause acute hypervitaminosis A. Children are prone to harmful effects even in small amounts (18).
Care should be taken by pregnant women, as high doses can harm the fetus. Doses as low as 25,000 IU per day tend to cause birth defects (19).
Benefits of supplementing with vitamin A
People who are deficient in vitamin A may benefit from supplements. However, most people are okay with what they get from their diet and therefore do not need supplements.
However, studies suggest that supplementing with vitamin A may benefit certain people even if they get enough from their diet.
Vitamin D is another classic example of fat-soluble vitamin. It is also known as the sunshine vitamin. Your skin produces vitamin A when you expose it to sunlight.
Vitamin D is very beneficial to bone health. The deficiency of vitamin A increases your bone’s susceptibility to fractures.
Types of vitamin D
The term vitamin D describes a few related fat-soluble compounds.
It is also known as calciferol. It presents in two dietary forms:
- Vitamin D2 mostly found in mushrooms and certain plants
- Vitamin D3 is found in animal foods like fish oil and eggs and also produced by the human skin when exposed to light from the sun.
What role does vitamin D play in the body?
These fat-soluble vitamins perform several functions in the body. However, studies have been done on only a few of these. They include:
- Bone health: Vitamin D controls the level of phosphorus and calcium circulating in your blood. Both minerals are very vital to bone growth and maintenance. Vitamin D enhances the absorption of these minerals from your diet.
- Regulation of the immune system: Vitamin D boosts immune function (23).
Once calciferol is absorbed into your bloodstream, it is changed into calcitriol. Your kidneys and liver are responsible for this change. Calcitriol is the active form of vitamin D. Your body is capable of storing calcitriol as calcidiol.
Sources of vitamin D
Your body is capable of producing its daily need of vitamin D. The only thing you have to do is expose large parts of your skin to the light from the sun (26).
The thing is, many people do not spend much time in the sun, and even if they do, they are fully clothed. Also, many people wear sunscreens (to prevent sunburns). Wearing sunscreen is recommended. However, it limits the amount of vitamin D that your skin can produce.
And so, people have to rely on their diet to get their recommended daily allowance of this vitamin.
The dietary sources of vitamin D are few. The best sources are fish oil and fatty fish (8).
The recommended intake of vitamin D
The recommended daily allowance of vitamin D for adults and children is 15mcg. Elderly adults should take up to 20 mcg (27).
Vitamin D deficiency
Vitamin D deficiency is rare. However, mild forms of deficiency are common among hospitalized people and also among the elderly.
The consequences of vitamin A deficiency include weak muscles, soft bones, and a high risk of bone fractures. In adults, this condition is known as osteomalacia, and in children, it is known as rickets (28).
Vitamin D toxicity
Vitamin D toxicity is a rare clinical condition.
It is important to note that spending much time in the sun doesn’t cause toxicity of vitamin D. However, overdosing on supplements can cause it.
The major complication of vitamin D toxicity is hypercalcemia. Hypercalcemia is a condition characterized by high amounts of calcium in the blood.
As a rule of thumb, your vitamin D intake should not exceed the upper limit of 4,000 IU daily.
High amounts, ranging from 1,000 – 2,000 mcg (40,000 – 100, 000) per day may cause toxicity symptoms in adults when taken every day for two months. It is important to note that lower doses may be harmful to young children.
What are the health benefits of vitamin D supplements?
People who do not spend much time in the sun, or who rarely eat the liver of fatty fish will benefit immensely from vitamin D supplements.
Vitamin E is a fat-soluble vitamin as well as a very potent antioxidant. It protects against premature aging or the damaging effects of free radicals.
Types of vitamin E
The vitamin E consists of 8 members, subdivided into two groups:
Tocopherols: The members include alpha-tocopherol, beta-tocopherol, gamma-tocopherol, and delta-tocopherol.
Tocotrienols: The members includealpha-tocotrienol, beta-tocotrienol, gamma-tocotrienol and delta-tocotrienol.
The most common form of vitamin E is alpha-tocopherol. It makes up about 90% of the blood’s vitamin E content.
Functions of vitamin E
Vitamin E is an antioxidant. It prevents oxidative stress and protects your cell membranes from the damaging effects of free radicals (37).
The antioxidant properties of vitamin E are enhanced by selenium, vitamin B3, and vitamin C.
When taken in high amounts, vitamin E can function as a blood thinner, making your blood less able to clot (38).
Dietary sources of vitamin E
Vitamin E is best found in seeds, vegetable oils, and nuts (8). Other sources of vitamin E include fish liver oil, fatty fish, margarine, peanut butter, and avocados.
Vitamin E deficiency
It is worth noting that vitamin E deficiency is uncommon. It is rarely detected in healthy people.
It mostly presents in health conditions that impair vitamin E or fat absorption, like liver disease or cystic fibrosis.
Common symptoms of vitamin E deficiency are walking difficulties, muscle weakness, poor immune function, vision problems, and numbness.
Vitamin E toxicity
Vitamin E overdose is very uncommon. It is quite difficult when obtained from dietary sources. Vitamin E toxicity has been reported in people who overdosed on supplements.
However, compared to vitamins A and D, a vitamin E overdose is seemingly harmless.
It has thinning effects on the blood, and interferes with the actions of vitamin K, leading to severe bleeding. And so, people who take blood-thinning medications should avoid large doses of the E vitamin (38, 41, 42).
Also, doses exceeding 1000mg per day may have pro-oxidant effects. This means it may work as the reverse of an antioxidant, resulting in oxidative stress (43).
Benefits and risks of overdosing on vitamin E supplements
High intake of vitamin E has several benefits.
Gamma-tocopherol increases blood flow by promoting vasodilation (widening of blood vessels). This in turn reduces blood pressure as well as the risk of heart disease (44).
Supplements of gamma-tocopherol also have the blood-thinning effect, and also reduce the levels of bad LDL cholesterol (45).
The K group of fat-soluble vitamins are divided into K1 (phylloquinone) and K2 (menaquinone).
Vitamin K1 is abundant in leafy green vegetables. Conversely, vitamin K2 is sourced from animal foods, like egg yolks, butter, and liver.
The bacteria in the colon also produces small amounts of vitamin K.
Vitamin K deficiency impairs blood clotting, leading to a high risk of bleeding. There isn’t much evidence on the health benefits of vitamin K supplements among healthy people. However, some studies suggest that vitamin K supplements are beneficial to heart and bone health.