What are Fat-Soluble Vitamins? (And Why it Matters)

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    Wondering which vitamins are fat soluble? Read on to learn about health benefits, food sources, and more from each vitamin within this family.

    Fat soluble vitamins stay within the body for extended periods of time. There are many benefits to the long-lasting nature of fat-soluble vitamins. One key benefit is that the body can store these vitamins in fatty tissues and the liver, and then rely on these vitamin reserves for future use. But there are also some key potential risks to remember. For instance, it may be easier to develop a toxicity from very high doses without guidance from a healthcare professional and even checking blood levels in some cases.

    What is the function of fat soluble vitamins?

    The function of vitamins that are fat-soluble differs for each vitamin. Each vitamin within this category plays different roles in the body to ensure optimal function.

    Vitamin A

    Vitamin A does many things for the body. Over 500 genes are regulated by retinoic acid. It is known mainly for its properties that support vision, but it also aids in skin health, and maintaining a healthy immune system. Vitamin A assists with cell division and reproduction. In addition, vitamin A helps maintain healthy membranes in the body, including the lining of mucous membranes in the nose, lungs, eyes and mouth.

    There are two main forms of vitamin A: the vitamin and the provitamin. The provitamin is a form of the vitamin that needs to be changed into an active form. For instance, the body can convert some foods high in the carotenoid called beta-carotene (known for its bright orange color) into an active form called retinol, retinoic acid, or retinal. Beta-carotene can be found in winter squash, carrots, pumpkin, and apricots. The active form of vitamin A (retinol) can be found in animal products like eggs, meat, and fish.

    The recommended daily intake of vitamin A ranges between 700-900mcg RAE for those who are 19 years old and above. The recommendation increases for those who are pregnant and breastfeeding. It is best to eat varied foods in order to get the recommended amount of vitamin A, but supplements can also help you achieve the recommended daily intake.

    Vitamin D

    Vitamin D aids the body in its absorption of calcium, which is crucial for overall bone health. Appropriate levels of vitamin D can lead to healthier and stronger bones. In addition, Vitamin D boosts the body’s immune system. It is available through exposure to the sun, dietary supplements, and from the diet. There are two types of vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). Studies have shown that vitamin D3 is more easily utilized by the body. In terms of the sun, the UV B rays help activate vitamin D production. Unfortunately there are several barriers to getting optimal vitamin D levels from the sun. Things like pollution, sunblock, distance from the equator, and skin pigment can make it more challenging to make vitamin D from sunlight exposure. Vitamin D can also be taken daily as a supplement, which can be very helpful when it is difficult to go out in the sun, like winter. The recommended daily intake of vitamin D is 600-800 IU or 15-20mcg daily. Care/of makes a vitamin D3 that is highly absorbable.

    Vitamin E

    Vitamin E functions as an antioxidant for the body while also protecting red blood cells and works synergistically with vitamins C and A. The vitamin E family has 8 different compounds but the most common one is called alpha- tocopherol. All 8 compounds are absorbed in the small intestine but only alpha-tocopherol is metabolized by the liver. The RDA is 15mg or 22 IU daily for those 14 and older.

    Vitamin K

    Vitamin K comes in two forms called phylloquinone and menaquinone. Menaquinone can be made in the body by the gut microbiome and it can be found in some animal products and fermented foods. Whereas phylloquinone can only be found in plant based foods. Vitamin K helps our blood clot and keeps bones healthy overall. The RDA for vitamin K is 90-120 mcg daily. This study suggests that menaquinone may even be beneficial for cardiovascular health based on its unique health benefits in comparison to phylloquinone. Another study demonstrated how synergistic Vitamin K and Vitamin D can be when taken together to support bone mass in post menopausal women.

    How do fat-soluble vitamins enter the bloodstream?

    Fat-soluble vitamins enter the bloodstream through lymph channels found in the wall of the intestines. Many fat-soluble vitamins are then carried throughout the body by proteins.

    To be more specific, fat-soluble vitamins follow this sequence and pathway as they move throughout the body:

    • The vitamin is ingested (either through the food or in supplement form).
    • That food or supplement is partially digested in the stomach and then moves to the small intestine for further digestion.
    • Bile is necessary for the fat-soluble vitamins to be absorbed. Bile is made in the liver and stored in the gallbladder to support digestion.
    • That bile moves into the small intestine where it breaks down fats. Vitamins are then absorbed through the intestine’s wall.
    • Once absorbed, the fat-soluble vitamin goes into the lymph vessels and then the bloodstream. Usually, the fat-soluble vitamin has to be carried throughout the body via a protein.

    How long do fat-soluble vitamins stay in the body?

    Vitamins that are fat-soluble can get stored in the liver and fat tissue for later use. They release over time and as such can last weeks or even months before stores are depleted. Your body will store them until you need the nutrient, at which time the vitamin will be released gradually depending on your body’s individual needs.

    What are natural food sources of fat-soluble vitamins?

    Vitamins that are fat-soluble are available in a wide range of foods, depending on the vitamin itself. In general, this category of vitamin can be found in many animal foods including butter, fish, liver, egg yolks, certain breakfast cereals, vegetable oils, nuts, whole grains, seeds, and dark leafy green vegetables. While you can get the recommended doses through diet alone, you would need to eat a very-well balanced diet. If you aren’t able to do so, you can always use dietary supplements to complement what you are consuming through diet.

    • Vitamin A rich foods: Liver, butter, orange food have beta carotene like carrots, butternut squash
    • Vitamin D rich foods: Fatty fish, fish liver oil, mushrooms, eggs,
    • Vitamin E rich foods: Plant based oils (such as wheat germ oil, sunflower oil, and almond oil), nuts, seeds, fruits, and vegetables
    • Vitamin K rich foods: Most abundant amount of menaquinones can be found in fermented soy beans called natto. Small amounts can be found in meat, cheese, and eggs. Phylloquinones are mainly found in leafy green veggies.

    How do I know if I have a fat-soluble vitamin deficiency?

    Vitamin deficiencies can happen often in the body. The best way to know if you have a deficiency is to talk to a healthcare provider who can help you measure your blood levels. Keeping track of your food intake in an app can also give you a rough estimate of vitamin levels from your dietary intake. After tracking for a few weeks you can get a good idea of what you may be running low on and consider adding in more food based sources of the vitamin or add in a supplement.

    Another way to know if you are absorbing fat-soluble vitamins from your diet is to look at your bowel movements making sure your stools are brown, formed, and sinking. If stools are loose or light brown that can indicate some challenges with fat metabolism. Those with digestive issues may also have an increased risk for a fat soluble vitamin deficiency since absorption may be challenging.

    Vitamin A deficiency is very rare but when it does happen, it can lead to severe health issues. It can be very hard to spot a deficiency in vitamin A, but the common symptoms associated with a deficiency include challenges with night time vision, immune function, and skin health. Vitamin A levels can be measured through serum retinol. Levels less than 20 ug/nL call for supplementation.

    A deficiency of vitamin D is a bit more common than previously thought. Levels can always be measured through blood work. Lab values below 20 ng/mol are considered clinically deficient by the Institute of Medicine. Based on the Endocrine Society, vitamin D insufficiency includes levels between 21 and 29 ng/mL.

    Vitamin E deficiency is quite rare and is really only ever seen in those individuals who have trouble absorbing fats. It can also be seen in premature infants. Deficiencies can negatively impact nerve function, muscles, and immune function. When measuring, serum alpha tocopherol levels under 5 mcg/mL are considered deficient for adults.

    Likewise, it is challenging to be deficient in Vitamin K so dietary vitamin K testing is not common. That is because it is found in nearly all green vegetables and is created by bacteria in the intestines. If you were to suffer from a vitamin K deficiency, it might manifest itself as bleeding easily, bruising, or tar-like stools. Because our bodies can create Vitamin K themselves, it is far more difficult to have a deficiency. The main marker used to determine a potential vitamin K deficiency is prothrombin time.

    Overall, each fat soluble vitamin has its main role in maintaining optimal health in the body. The goal is to get most of your nutrients from foods and use supplements to fill in the gaps. Find the right vitamins to reach your health goals through our quiz.

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    Diana Morgan, MS, CISSN
    Head of Scientific & Regulatory Affairs
    Diana Morgan is an innovative and entrepreneurial strategist with over 15 years of experience in the dietary supplement industry in areas such as Regulatory Affairs, Product Development, Technical Services and Sales coupled with a personal passion for nutrition & health.
    Our Editorial Staff
    Freelance Contributor
    The Care/of Editorial Team is made up of writers, experts, and health enthusiasts, all dedicated to giving you the information you need today. Our team is here to answer your biggest wellness questions, read the studies for you, and introduce you to your new favorite product, staying up to date on the latest research, trends, and science. Each article is written by one of our experts, reviewed both for editorial standards by an editor and medical standards by one of our naturopathic doctors, and updated regularly as new information becomes available.