We often come across the names of various vitamins in the nutrient list of food packets. Most fruit juices contain vitamin C. Milk comes enriched with vitamin D, while many breakfast cereals contain vitamin E. But do you know what vitamins are or what is their role in the body? Are you aware that there are two types of vitamins – fat-soluble and water-soluble. In this article, we will learn about the different fat-soluble vitamins, their functions, and their sources.
Types of vitamins
Vitamins are vital micronutrients that our body needs in small amounts to support a range of critical functions. The two types of vitamins are fat-soluble vitamins (A, D, E, and K) and water-soluble vitamins (B complex and C).
Water-soluble vitamins dissolve in water before the body absorbs them. They need to be replaced regularly in the body. The fat-soluble vitamins are stored in the fatty tissues of the body, and our body eliminates them much more slowly than water-soluble vitamins.
What are fat-soluble vitamins?
Vitamin A, D, E, and K are known as the fat-soluble vitamins as they dissolve in fats. They are founds most abundantly in high-fat foods and are better absorbed when eaten with fat. Excess amounts of these vitamins get stored in the liver and fatty tissues for future use. Since the body can store fat-soluble vitamins, they can accumulate to toxic levels if taken in excess.
However, it is tough to obtain an excessive amount of fat-soluble vitamins from food sources. Taking vitamin supplements that contain megadoses of these vitamins can lead to toxicity. Although most supplements do not contain more than the recommended daily value of the fat-soluble vitamins, you should always read the labels carefully before taking any supplements.
Vitamin A is integral to normal vision, immune function, and reproduction. It also helps the heart, kidneys, lungs, and other organs to work correctly. It is vital for cell growth as its deficiency may prevent growth in children. It is essential for hair growth. It helps maintain fertility and is necessary for fetal development.
Vitamin A is not a single vitamin but a collection of compounds known as retinoids. Retinoids can be found both in the body and in food sources. Retinol is the most common type of vitamin A. Retinal, and retinoic acid are found in the body but are absent in foods.
The recommended intake of vitamin A can vary according to age and gender. This table shows the recommended daily allowance (RDA) for vitamin A (1) –
|0 to 6 months||400 mcg RAE||400 mcg RAE|
|7 to 12 months||500 mcg RAE||500 mcg RAE|
|1 to 3 years||300 mcg RAE||300 mcg RAE|
|4 to 8 years||400 mcg RAE||400 mcg RAE|
|9 to 13 years||600 mcg RAE||600 mcg RAE|
|14 to 18 years||900 mcg RAE||700 mcg RAE||750 mcg RAE||1,200 mcg RAE|
|19 to 50 years||900 mcg RAE||700 mcg RAE||770 mcg RAE||1,300 mcg RAE|
|51 + years||900 mcg RAE||700 mcg RAE|
Vitamin A is naturally found in foods like beef liver and fish and is added to some foods like milk and cereal. You can get the recommended amounts of this vitamin by eating a variety of foods. Animal sources provide active components to create retinol in the human body. These include –
- Beef liver (One slice): 6,421 mcg (713% DV)
- Lamb liver (one ounce): 2,122 mcg (166% DV)
- Cod liver oil (one teaspoon): 1,350 mcg (150% DV)
- Salmon (half a fillet): 229 mcg (25% DV)
- Butter (one tablespoon): 97 mcg (11% DV)
Some plant-based sources provide pro-vitamin A compounds known as carotenoid antioxidants. The most common is beta-carotene, which is found in foods like –
- Vegetables like broccoli, carrots, and squash
- Fruits like mangos, apricots, and cantaloupe
Vitamin A deficiency is quite rare in the developed world. However, some people may have more trouble getting more vitamin A. These include –
- Premature infants
- Infants, young children, pregnant, and breastfeeding women in developing countries.
- People with cystic fibrosis.
Signs of vitamin A deficiency include –
- Dry eyes
- Hair loss
- Reduced immune function
- Skin problems
An excessive dosage of vitamin A in the form of supplements can lead to a condition known as hypervitaminosis A. Its symptoms include fatigue, headaches, irritability, joint pain, lack of appetite, vomiting, blurred vision, skin problems and inflammation of eyes and mouth (2). Extremely high dosage can even lead to death.
Pregnant women should avoid taking too much vitamin A as it may be harmful to the fetus. Hypervitaminosis in pregnant women can lead to congenital disabilities in the baby.
Vitamin D is also known as the sunshine vitamin, and our body produces it in response to sun exposure. It can also be taken from food or in the form of supplements. Vitamin D helps in maintaining healthy bones and teeth, and it can protect against conditions like cancer, type 1 diabetes, and multiple sclerosis. The main functions of vitamin D are –
- Maintaining healthy bones and teeth
- Supporting a healthy brain, immune system, nervous system, and brain
- Regulating insulin levels
- Supporting lung function
- Improving cardiovascular health
Vitamin D is a term used to describe a collection of compounds. Thee compounds are also known as calciferol.
There are two types of calciferols –
- Vitamin D3 (cholecalciferol) – Found in animal fats
- Vitamin D2 (ergocalciferol) – Found in plants
According to the Food and Nutrition Board at the Institute of Medicine of the National Academies, the recommended dietary allowance(RDA) for vitamin D is (3) –
|0 to 12 months||400 IU (10 mcg)||40 IU (10 mcg)|
|1 to 13 years||600 IU (15 mcg)||600 IU (15 mcg)|
|14 to 18 years||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)|
|19 to 50 years||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)||600 IU (15 mcg)|
|51 to 70 years||600 IU (15 mcg)||600 IU (15 mcg)|
|71 + years||800 IU (20 mcg)||800 IU (20 mcg)|
Sunlight is the most common and most efficient source of vitamin D. Here’s a list of some of the best sources of vitamin D –
- Cod liver oil (1 tablespoon): 1,360 IU
- Herring (4 ounces): 1,056 IU
- Swordfish (4 ounces): 941 IU
- Maitake mushrooms (1 cup): 786 IU
- Salmon sockeye (4 ounces): 595 IU
- Canned sardines (4 ounces) 596 IU
- Fortified milk (1 cup): 120 IU
Severe deficiency of vitamin D is rare. However, older adults and people who are hospitalized for an extended period may develop mild vitamin D deficiency. People at high risk of developing a vitamin D deficiency include –
- Older adults
- People with dark skin
- Obese people
- People with limited sun exposure
- People with chronic illness
Signs of vitamin D deficiency include –
- Soft bones
- Weakened muscles
- Increased bone fractures
- Poor immune function
- Hair loss
- Impaired wound healing
Toxic levels of vitamin D are scarce. While excessive sun exposure does not result in vitamin D toxicity, long term exposure to high doses of supplements can lead to hypercalcemia. This condition leads to extreme levels of calcium in the blood.
The symptoms of hypercalcemia include headaches, nausea, weight loss, fatigue, high blood pressure, kidney, and heart damage, and fetal abnormalities. Adults should avoid taking more than 4000 IU of vitamin D per day, which is considered the upper limit.
Vitamin E is an antioxidant that helps protect your cells from damage. It is essential for vision, reproduction, and the health of blood, brain, and skin. It also plays a vital part in preventing cancer.
Vitamin E also helps in the production of hormone-like substances called prostaglandins, which are responsible for a variety to body processes like blood pressure and muscle contraction. It also acts as a blood thinner when taken in high amounts (6).
Vitamin E comes in eight forms, which are divided into two groups. These are –
- Tocopherols: Alpha-tocopherol, beta-tocopherol, gamma-tocopherol, and delta-tocopherol.
- Tocotrienols: Alpha-tocotrienol, beta-tocotrienol, gamma-tocotrienol, and delta-tocotrienol.
Our body preferentially stores alpha-tocopherol to assist in body functions. It makes up 90% of the vitamin E in the blood. The body receives the other forms of vitamin E through the topical intake, or they are actively metabolized in the body.
According to the Food and Nutrition Board at the Institute of Medicine of the National Academies, the recommended dietary allowances (RDA) for vitamin E (alpha-tocopherol)are (7) –
|0 to 6 months*||4 mg (6 IU)||4 mg (6 IU)|
|7 to 12 months*||5 mg (7.5 IU)||5 mg (7.5 IU)|
|1 to 3 years||6 mg (9 IU)||6 mg (9 IU)|
|4 to 8 years||7 mg (10.4 IU)||7 mg (10.4 IU)|
|9 to 13 years||11 mg (16.4 IU)||11 mg (16.4 IU)|
|14 + years||15 mg (22.4 IU)||15 mg (22.4 IU)||15 mg (22.4 IU)||19 mg (28.4 IU)|
* Adequate intake
The best sources of vitamin E include cooking oils, seeds, and nuts –
- What germ oil (1 tablespoon): 20 mg (135% DV)
- Sunflower seeds (1 ounce): 10 mg (66% DV)
- Almonds (1 ounce): 7.3 mg (48% DV)
- Hazelnut oil (1 tablespoon): 6.4 mg (43% DV)
- Mamey sapote (half a fruit): 5.9 mg (39% DV)
- Sunflower oil (1 tablespoon): 5.6 mg (37% DV)
Most supplements of vitamin E provide only alpha-tocopherol, although ‘mixed’ products containing other tocopherols and tocotrienols are also available.
Vitamin E deficiency is usually rare in healthy people. This deficiency sometimes runs in families. Diseases that reduce the absorption of fat like chronic pancreatitis, celiac disease, cholestatic liver disease, and cystic fibrosis, can lead to vitamin E deficiency. Deficiency of vitamin E an also develop in premature babies with low birth weight. The symptoms of vitamin E deficiency include –
- Walking difficulties
- Vision problems
- Poor immune function
- Heart disease
- Neurological problems
- Poor reflexes
- Lack of control on body movements
While overdosing on vitamin E is unlikely through food sources, taking high doses of this vitamin supplements can cause side effects. Overdosing on vitamin E can increase the risk of hemorrhagic stroke. Dosages should not increase 1000 IU per day if you are using vitamin E supplements.
Vitamin K is called the blood clotting vitamin. It helps the body form clots, which prevents the body from bleeding out from small scratches. Our body needs vitamin K to produce prothrombin, a protein, and clotting factor that is important in blood clotting and bone metabolism. Vitamin K plays a vital role in metabolism, regulating blood calcium levels, and reducing the risk of heart disease.
Vitamin K is divided into two groups –
- Vitamin K1 (phylloquinone): It is the main type of vitamin K and comes from plant sources
- Vitamin K2 (menaquinone): Found in animal-based and fermented foods.
According to the Food and Nutrition Board (FNB) at the Institute of Medicine of the National Academies, the adequate intake (AI) for vitamin K is (10) –
|Birth to 6 months||2 mcg||2 mcg|
|7 to 12 months||2.5 mcg||2.5 mcg|
|1 to 3 years||30 mcg||30 mcg|
|4 to 8 years||55 mcg||55 mcg|
|9 to 13 years||60 mcg||60 mcg|
|14 to 18 years||75 mcg||75 mcg||75 mcg||75 mcg|
|19 + years||120 mcg||90 mcg||90 mcg||90 mcg|
Vitamin K1 is found in green vegetables, while vitamin K2 is found in animal-based and fermented foods. The following foods are good sources of vitamin K:
- Kale, cooked (half a cup): 531 mcg (443% DV)
- Mustard greens, cooked (half a cup): 415 mcg (346% DV)
- Swiss chard (one leaf): 398 mcg (332% DV)
- Collard greens (half a cup): 386 mcg (322% DV)
- Natto (one ounce): 313 mcg (261% DV)
- Beef liver (1 slice): 72 mcg (60% DV)
While vitamin K deficiency is rare in adults, it does occur in infants. Vitamin K is not stored in the body in significant amounts, so a diet that is low in vitamin K can lead to a deficiency. The primary symptom of vitamin K deficiency is an inability to form clots, which leads to excessive bleeding. Adults are at a higher risk of vitamin K deficiency if –
- They take anticoagulants
- They take antibiotics that interfere with vitamin K absorption
- They take high doses of vitamin A or E
- They have celiac disease, cystic fibrosis, intestinal disorder, or part of their intestine removed.
Symptoms of vitamin K deficiency include –
- Excessive bleeding
- Bruising easily
- Small blood clots under the nails
- Stool that is dark or black
Unlike other fat-soluble vitamins, there are no symptoms of vitamin K overdose. Vitamin K is safe to consume.
Vitamin A, D, E, and K are known as the fat-soluble vitamins as they dissolve in fats. They are essential for good health as they play many crucial roles in the body. You can get these vitamins by taking a variety of foods like nuts, seeds, vegetables, fish, beef liver, and vegetable oils. Since the body can store fat-soluble vitamins, they can accumulate to toxic levels if taken in excess. So, always check with your doctor before you take vitamin A, D, K, and E supplements.