What Are Fat-Soluble Vitamins?
Reading about vitamins can be like trying to make sense of alphabet soup, but it’s a bit easier if you break down vitamins into their two groups: water soluble and fat soluble. Read on to learn more about fat-soluble vitamins.
Did you know that about half of Americans take vitamin and mineral supplements on a daily basis? Vitamins and minerals, also called micronutrients, are essential for hundreds of functions in the body, including facilitating wound repair, bolstering the immune system, repairing damaged cells, converting food into energy, and strengthening bones. (1)
Vitamins break down into two groups: water soluble and fat soluble. Water-soluble vitamins dissolve in water and, apart from B12, are not stored in the body. So what are fat-soluble vitamins? These are vitamins that are dissolved in fats and enter the bloodstream through lymph channels in the intestinal wall. They are then carried along by certain proteins to their destinations. Excess fat-soluble vitamins are stored in the liver and fatty tissues. They are doled out when needed, so in a sense, they are time-release vitamins. It’s possible to go for weeks or months without needing more fat-soluble vitamins. Even so, you should always consult your doctor about which vitamins you may need to ingest more or less of through changes to your diet or through supplementation. (1, 2)
The four fat-soluble vitamins are vitamin A, vitamin D, vitamin E, and vitamin K.
Vitamin A is found naturally in animal- and plant-based foods. It is important for immune system function, eye health, and organ function, especially the heart, kidneys, and lungs. There are two types of vitamin A: preformed vitamin A (retinol) and provitamin A carotenoids. Carotenoids are fat-soluble compounds responsible for the red, orange, and yellow colors of fruits and vegetables. There are over 600 types of carotenoids, which have antioxidant properties, and the body converts some of them into vitamin A. The most common type of provitamin A carotenoid is Beta Carotene. Your body uses both preformed vitamin A and provitamin A in the same ways. (3, 4)
- Sources (5):
- Preformed vitamin A (retinol): fish oils, liver, fortified dairy products, eggs, fortified cereals.
- Provitamin A: carrots, broccoli, cantaloupe, squash, apricots, mangoes.
- Conditions caused by vitamin A deficiency (5, 6):
- Xerophthalmia (night blindness), which is the inability to see in low light or darkness. Early warning signs include dry eyes and spots on the eyes.
- Age-related macular degeneration (AMD).
- Worsening of measles.
- Groups at risk for deficiency (5):
- Young children, especially in developing countries.
- Pregnant and breastfeeding women.
- Those with cystic fibrosis.
- Toxicity (5):
- Preformed vitamin A: pain in joints and bones, increased intracranial pressure, skin irritation, dizziness, headaches, nausea, coma, and even death. Mostly occurs due to chronic excess intake of preformed vitamin A in supplements.
- Provitamin A (carotenoids like beta carotene): not associated with any detrimental effects, although a harmless condition called carotenodermia may occur in which the skin appears yellow or orange. Carotenodermia is easily remedied by lowering the amount of provitamin A ingested.
- Negative interactions with medications (5):
- Some weight-loss medications can decrease the absorption of vitamin A.
- Some skin condition medications, because they are derived from vitamin A, can increase the risk for vitamin A toxicity.
In the gut, vitamin D assists with calcium absorption, which in turn enables bone growth and repair; this process helps guard against osteoporosis, among other bone diseases. It also helps prevent involuntary muscle contractions, called hypocalcemic tetany. Other roles of vitamin D include reducing inflammation and regulating cell growth, glucose metabolism, neuromuscular function, and immune function. (7)
- Sources (7, 8):
- Egg yolks
- Beef liver
- Fatty fish, like tuna and salmon
- Foods fortified with vitamin D, like dairy products, breakfast cereals, orange juice, and soy milk
- UV light. When UV light, such as sunlight, hits the skin, vitamin D synthesis is triggered.
- Conditions caused by vitamin D deficiency (7, 9) :
- Rickets: occurs in children. The bones become misshapen or brittle.
- Osteoporosis: bones become porous and weak.
- Osteomalacia: much like rickets in children. Adult bones are not completely mineralized, resulting in weak or deformed bones, pain, seizures, and dental abnormalities.
- Groups at risk for deficiency (7):
- Infants who are breastfed for long periods without the mother taking vitamin D supplements.
- Older adults; the ability to synthesize vitamin D in the skin declines with age.
- People with darker skin, as the melanin in dark skin reduces the skin’s ability to produce vitamin D.
- Those with health conditions that limit fat absorption, like cystic fibrosis, liver disease, Crohn’s disease, and celiac disease.
- Toxicity (7, 10):
- Hypercalcemia, which is too much calcium in the blood, can cause nausea and vomiting, loss of appetite, muscle weakness, excessive thirst, dehydration, kidney stones, and excessive urine production (polyuria).
- In extreme cases, vitamin D toxicity can cause kidney failure, soft tissue calcification, and cardiac arrhythmias.
- Negative interactions with medications (7):
- Some weight-loss medications can hinder the absorption of vitamin D.
- Statin drugs can reduce vitamin D synthesis.
- Steroids can impede vitamin D metabolism.
- Some diuretics, when combined with vitamin D supplements, can lead to too much calcium in the body.
The term “vitamin E” actually refers to a group of eight fat-soluble vitamins, each with unique antioxidant properties. Of these, alpha-tocopherol is the only one that meets human needs. Its antioxidant ability helps protect cells from free-radical damage. Vitamin E is also crucial for various metabolic processes and proper immune function. (11)
- Sources (12):
- Wheat germ oil
- Nuts and nut oils
- Rainbow trout
- Sweet red peppers
- Turnip greens
- A wide variety of other fruits and vegetables
- Conditions caused by vitamin E deficiency (11):
- Deficiency is rare, even if people don’t get much vitamin E in their diets.
- Groups at risk for deficiency (11):
- People with certain rare, inherited disorders.
- Premature infants with low birth weight.
- Toxicity (11):
- Very rare from food alone; occurs with excessive supplementation.
- Possibly hinders blood coagulation, increasing the risk for hemorrhages.
- Negative interactions with medications (11):
- Can increase the effect of anti-clotting medications.
- Can decrease good cholesterol when taken with some statin drugs.
- Can inhibit the effects of chemotherapy and radiation therapy.
Like “vitamin E,” the term “vitamin K” refers to a group of compounds. These compounds play a role in a variety of functions, including vascular health and blood clotting. Scientists believe that vitamin K production in the gut satisfies at least some of the body’s requirement. (13, 14)
- Sources (13):
- Vegetable oils
- Conditions caused by vitamin K deficiency (13, 14):
- Reduced bone mineralization leading to osteoporosis.
- Increased risk of bruising, bleeding, and hemorrhage.
- Groups at risk for deficiency (14):
- Newborns, because vitamin K is poorly transported across the placenta.
- People with malabsorption conditions, like Crohn’s disease, celiac disease, and cystic fibrosis.
- Toxicity (14):
- No adverse effects have been reported for excessive vitamin K
- Negative interactions with medications (14):
- Anticoagulants can decrease the compound that vitamin K needs to help the blood clot properly (potentially a very dangerous interaction).
- Some weight-loss drugs can decrease absorption of fat-soluble vitamins like vitamin K because they decrease the absorption of fat.
- Some antibiotics can kill bacteria in the gut that produce vitamin K.
The Recommended Daily Allowance for Fat-Soluble Vitamins
The Food and Nutrition Board (FNB) at the Institute of Medicine, National Academy of Sciences has developed the RDA for many nutrients. The RDA, which is determined by age and sex, is defined as “the average daily level of intake sufficient to meet the nutrient requirements of nearly all (97% to 98%) healthy individuals.” (15)
The Takeaway About Fat-Soluble Vitamins
If you are worried about whether you’re getting the right amount of any of these fat-soluble vitamins, your doctor can order blood tests to check their levels. As always, it’s important to eat a well-balanced, healthy diet. Also, be completely transparent with your healthcare team about what medications and supplements you’re taking and any symptoms you’re having. Doing so will help prevent negative interactions and help you live healthier and happier.