In total, 13 vitamins are known to the scientific world: A, B complex (containing 8 sub- vitamins), C, D, E, and K.
In healthy individuals, vitamins are effectively supplied through balanced and diverse diet. However, with different health conditions which trouble the absorption of vitamins, or with the raising popularity of diets (which strictly exclude certain foods from the menu), vitamin deficiencies are a common health threat. Some common deficiencies include Folate and Cobalamin (forms of vitamin B), as well as vitamin D and K.
In that sense, medically prescribed and controlled vitamin supplementation has shown positive results in diminishing the occurrence of health conditions related to malnutrition. However, with the raising availability of vitamin supplements on the market and the lack of control on the supplementation process, self-guided supplementation has become a serious thread that can lead to toxicity.
For that reason, knowing the types of vitamins and their purpose, deficiency symptoms, recommended dietary intake, and availability in food sources is important.
There are two main types of vitamins: fat-soluble and water-soluble vitamins.
In this article we will focus on fat-soluble vitamins, their types, characteristics, deficiency and toxicity symptoms, food sources, and recommended daily intakes.
Let’s get started!
What are fat soluble vitamins?
Fat-soluble vitamins include vitamins A, D, E, and K. They are absorbed in the same way as fats, and excess amounts of these (which are not used by the body) are stored in the liver and fatty tissues “for later use”. That being said, the risk of overdosing and toxicity caused by too much fat-soluble vitamins in the body is more common than toxicity from water-soluble vitamins. 
Even though toxicity rarely occurs if you supply your vitamins through balanced and diverse diet, self-guided or unsupervised supplementation can significantly increase the risk of storing too much fat-soluble vitamins and developing troubling toxicity symptoms.
Another important aspect of fat-soluble vitamins is that they are heat stable. In other words, when you cook foods rich in vitamins A, D, E, and K, those micronutrients are almost fully preserved and their properties are not lost. 
Now let`s see the exact purpose of each fat-soluble vitamin and in which food it can be found.
What to know about vitamin A?
Two forms of vitamin A exist in food: retinol in dairy products and meat, and alpha and beta carotene in plants. Keep in mind that the body transforms alpha and beta carotene in retinol before using it. 
- The function of vitamin A is to promote eye health, and support the immune system and proper organ functions.
- Recommended daily intake for adults (from food): 900 mcg for men and 700 mcg for women (770 during pregnancy)
- Deficiency symptoms include decreased vision (inability to see when the light is low), increased predisposition to viruses and infections, diarrhea.
- Toxicity signs include bone pain, nausea, vomiting, blurred vision, skin rashes, nose bleeds, fatigue, diarrhea, hair loss. According to NHS, you can take too much vitamin A if you consume liver or liver pate more than once a week, or if you supplement with more than 1.500 mcg of the vitamin without medical supervision.
- Foods rich in vitamin A include carrots, spinach, peppers, apricots, sweet potatoes, liver, mangos, fish, diary products, liver and liver products, fortified foods.
What to know about vitamin D?
- Vitamin D plays a key role in the formation, development, and supporting the health of bones and teeth. It is also supports the absorption of calcium, which is also important for bone health. What is more, this vitamin is responsible for maintaining healthy immune function, nervous system, skin, as well as overall cell functions.
- Recommended daily intake for adults: 600 IU (international units). Even though, in theory, the main “source” of vitamin D is the sun (the sunlight stimulates vitamin D production in your skin), relying only the sun rays to get enough of the vitamin is a common mistake. In fact, the relationship tanning and vitamin D production is still inconclusive, according to various studies.  This is also supported by the fact that vitamin D deficiency is one of the most common deficiencies worldwide. In that sense, food remains a significant source of vitamin D.
- Deficiency symptoms include deformation of bones, osteoporosis, weak and painful muscles, depression, weakened immune system, increased risk of stroke and hear attack.
- Toxicity signs include high levels of calcium in the blood, excessive thirst, headache, nausea, loss of appetite, weight loss, kidney or heart damage. Toxicity is more likely to occur due to self-guided or unsupervised supplementation with vitamin D.
- Foods sources of vitamin D include meat, fish, dairy products, fortified foods.
What to know about vitamin E?
- Vitamin E acts as an antioxidant in the body to protect the cells from free radicals damage (e.e.g, pollution, smoking, sun radiation, malnutrition) and slow down the progression of related diseases. It also playa a role in supporting the immune system, proper cell function, and healthy blood vessels. 
- Recommended daily intake for adults (from food): 15mg.
- Deficiency signs are weak immune system, loss of appetite, muscle weakness, nerve damage.
- Toxicity signs include blood thinning, increased bleeding, and augmented effects of anti-clotting medications.
- Foods rich in vitamin E are wheat germ oil, seeds, nuts, plant oils, peanut butter, green vegetables.
What to know about vitamin K?
In the form of vitamin K1 and K2.
- Vitamin K has has an enzyme function and is mainly responsible for synthesises proteins, efficient blood clotting, and bone health.
- Recommended daily intake for adults: 120 mcg for man and 90 mcg for women. Keep in mind that if you take antibiotics, bile acid sequestrants (cholesterol medications), Olistrad (weight loss drug), anticoagulants (medications that prevent blood clots), you should consult your doctor about the amounts of vitamin K that you can take. Keep in mind that as these medications may interact with balanced vitamin K levels in the blood, it’s important to seek doctor’s advice. 
- Deficiency signs include bleeding (and finding it difficult to stop the bleeding), bruises, and osteoporosis.
- Toxicity signs can include excessive blood clotting. However, vitamin K toxicity is unlikely to occur.
- Foods rich in vitamin K include dark leafy vegetables, broccoli, kale, carrots, grapes, poultry, nuts.
Why are fat-soluble vitamin deficiencies occurring?
There are different causes for Vitamin A, E, D, and K deficiencies.
Some of the reasons are related to lack of balanced and diverse diet or strict exclusion of certain foods from the menu. In the long run, this may not only lead to micronutrient deficiencies but also to malnutrition and related health issues.
Another major factor for insufficiency/deficiency of fat-soluble vitamins in the body is the development of health conditions that limit the absorption of fats. Such health condition include inflammatory bowel disease (also called IBD), cystic fibrosis, and chronic pancreatitis. Evidence suggests that these diseases may not only limit the absorption of fats, but also the absorption of fat-soluble vitamins.  This way, people who suffer from these diseases are more predisposed to vitamins A, D, E, and K deficiencies.