The Ultimate Guide to Fat-Soluble Vitamins - Rejuvenation Therapeutics

The Ultimate Guide to Fat-Soluble Vitamins

The Ultimate Guide to Fat-Soluble Vitamins

What is the best way to take a fat-soluble vitamin? The answer depends on which vitamin you’re talking about. Each fat-soluble vitamin needs to be combined with varying levels of fats for efficient absorption. To make matters more complicated, different vitamins react differently when combined with certain food types.

Let’s take a look at the four major fat-soluble vitamins and the best ways to maximize their absorption in the body.

What are vitamins?

The word vitamin was first used by Polish biochemist Casimir Funk in 1911. At the Lister Institute in London, Funk identified a compound that prevented inflammation of the nervous system in chicken fed a diet deficient in that compound. He named the compound as ‘vitamine’ because he believed that the compound was essential for life and must be an amine. Later the last letter ‘e’ was removed from ‘vitamine’ when it was discovered that the vitamins do not necessarily have to be amines.

What are fat-soluble vitamins and how they work?

Fat-soluble vitamins are those that dissolve themselves in fats and oils. These vitamins are absorbed in fats present in food and stored in the body’s fatty tissues. Sources of fat-soluble vitamins include plants, animals, and dietary supplements. There are a total of 4 fat-soluble vitaminsA, D, E, and K. These vitamins follow the same absorption mechanism as fat and oils.1 Fat-soluble vitamins are either stored in the liver or released in the bloodstream and used by various tissues.

Among the four important fat-soluble vitamins, A, D, and K work synergistically with each other and with magnesium, an essential mineral. Vitamin D3, vitamin K, magnesium, zinc, and calcium work in tandem. If you are deficient in one of these vitamins or minerals, you should consume the others too to get optimum benefit.

How much fat is required to solubilize these vitamins?

Vitamin A
Absorption of beta carotenethe precursor of vitamin Ais sub-optimal when a fat-enriched diet is consumed. A research study suggests that 5 grams of fat in a meal is sufficient to absorb carotenoids. The researchers found no difference in vitamin A status when 5 g or 10 g of dietary fat was added to spinach, whereas 0 g of fat resulted in less improvement in plasma concentration of beta carotene. Bioavailability of beta carotene is lower in foods with complex food matrices (like dark green leafy vegetables) and higher in foods which contain a simple food matrix (such as fruits and red palm oil)2.

Vitamin D
Researchers found that vitamin D is best absorbed with low or medium fat diet in comparison to low or no fat diet. A study published in the Journal of Bone and Mineral Research in 2013 showed that vitamin D3 is best absorbed with 11 grams of fat rather than diets with no fat or 35 grams of fat.3 A large amount of fat in diet hinders vitamin D absorption. It might be because vitamin D gets too soluble and stays in the fat globule, making the globule too large and immobile to cross the intestinal lining.

Vitamin E
In a research study published in British Journal of Nutrition in 2004, it was shown that (2) H-labelled alpha-tocopherol absorption was significantly higher when a higher-fat toast and butter meal was consumed in comparison with a low-fat cereal meal or water. The results demonstrated that vitamin E absorption is influenced by both the amount of fat and the food matrix.4

Vitamin K
Like other fat-soluble vitamins, vitamin K is incorporated in mixed micelles (aggregate of molecules in the liquid colloid) with the help of bile and pancreatic enzymes and absorbed in the small intestine.

A research article published in British Journal of Nutrition in 2009 studied the absorption of free 13C-labeled phylloquinone, or vitamin K1, with three test meals formulated according to the dietary cluster identified in the UK. More phylloquinone tracer was absorbed with a heart-healthy (termed cosmopolitan) or animal-based meal than with a meal consisting of fast food (termed convenience meal). The fast-food meal contains a similar fat concentration but has a twofold higher polyunsaturated fatty acid (PUFA) content. It is evident from the research findings that vitamin K absorption depends largely on PUFA contents rather than total fat concentration.

In the same study, the bioavailability of phylloquinone from within the food matrix of 3 dietary cluster test meals relative to the 13C-labeled tracer was measured. Researchers found that the absorption of phylloquinone was more than three times better in the convenience meal (where 80% phylloquinone was present in oil phase) compared to the other meal patterns where phylloquinone is present in the vegetable matrix.5 These findings support the fact that absorption efficiency of phylloquinone from fortified oil is better than from vegetable matrix.

After reading the findings of the above research articles, it becomes obvious that we require a moderate amount of fat to absorb fat-soluble vitamins.

Other factors affecting the absorption of fat-soluble vitamins

A study published in Journal of Molecular Nutrition and Food Research demonstrated that consumption of pulses like kidney beans, white beans, chickpeas, green or brown lentils, and flageolets reduced the bioaccessibility of beta carotenea precursor of vitamin A—by up to 65%, of retinyl palmitatea synthetic form of vitamin A—by up to 69%, 45% of vitamin D, 53% of vitamin E, and 67% of vitamin K compared to the test meal containing potatoes. This is due to the binding ability of pulse fiber and other compounds such as phytates, saponin, and tannins. It is advisable to avoid eating pulses along with fat-soluble vitamin-rich foods or supplements for better bioabsorption.6

When should you take these fat-soluble vitamins?

Different fat-soluble vitamins have different roles in human physiology.7

Vitamin A
Vitamin A helps in maintaining healthy vision, maintenance of normal mucous membrane, immune function, hair and body growth, and also helps in maintaining reproductive function. The best natural dietary sources for vitamin A are liver and fish oil. This vitamin can also be taken from provitamin A carotenoids like beta carotene and cryptoxanthin.

Vitamin A overdosing is common in the United States. A study found that toxic or excess levels of vitamin A are more of a concern than its deficiency. The tolerable upper intake level of vitamin A is 3,000 mcg RAE (retinol activity equivalents).8 It will be difficult to reach that level by consuming food alone, but some multivitamin supplements contain a high amount of vitamin A in the form of retinol. Retinol is the greatest concern for toxicity. Vitamin A is stored in the liver, and the body can release it as needed. So, before swallowing a high-dose multivitamin pill, make sure it does not cross the tolerable limit. The best way to check this is with a random blood test done by your physician or at your nearest clinic.

Vitamin D
Researchers realized that the consumption of sufficient concentration of vitamin D3 in the general population can avert many chronic diseases that claim nearly 1 million lives throughout the globe each year. Vitamin D is required for the biochemical cellular mechanism of all cells and tissues in our bodies. The active form of vitamin D helps in intestinal absorption of calcium and phosphorous and promotes bone mineralization.

Vitamin D is found naturally in animal foods such as fatty fish, egg yolks, liver, and butter. Although the best source is sufficient exposure to sunlight, it depends upon the length of UV radiation, its intensity (which gets affected by pollution of the atmosphere), and skin pigmentation.

Vitamin D overdose is less common than vitamin A overdose. To keep yourself healthy, you should check your vitamin D level twice a year to maintain an optimal clinically recommended concentration in the blood. To get the maximum benefit of vitamin D, you must maintain a vitamin D level of 40 ng/mL, and to achieve that you need around 5,000-6,000 IUs of vitamin D3 per day, or an even higher dose from food, sunlight exposure, and/or vitamin D3 dietary supplements.

Vitamin E
Vitamin E is a powerful antioxidant and immunity booster. It protects polyunsaturated fatty acids from oxidation, and is helpful in obesity-related fatty acid liver problems, Alzheimer’s, and infertility issues. The natural sources include nuts, rice bran oil, seeds, whole grains, and wheat germ. To get a full spectrum of vitamin E it is recommended to consume a broader family of mixed natural tocopherols and tocotrienols to get maximum benefits.

The best way to ensure that you are getting the optimal dose of vitamin E in a form your body can absorb is to make smart dietary choices. As there are limited natural resources found for vitamin E that are also affected by seasonal variations, you may take a natural or organic dietary supplement to fulfill your requirement. An overdose of Vitamin E may occur if it consumed at a dose of more than 400 IUs/day, which may lead to hemorrhagic stroke. The best way to check that you are absorbing the Vitamin E through food or supplements correctly, and it is remaining in an active form inside your body, is to get a blood test or consult a physician.

Vitamin K
Vitamin K, or more specifically K1, is required for blood coagulation, and K2 is required for building strong bones, preventing heart disease, and many physiological processes. Like vitamin A, K2 is an important vitamin that works in tandem with vitamin D. The deficiency in any of the single vitamins, i.e. A, D, or K, will hamper the other’s activity. Vitamin K1 can be obtained from dark green vegetables, and lower levels are found in dairy, meat and cereal products, and fruits, but K2 is synthesized by bacteria. Vitamin K deficiency is common among infants or in people who take anticoagulants such as Coumadin or antibiotic drugs. People with chronic diarrhea lack the intestinal bacteria to produce vitamin K. Consulting a physician is the best way to determine if vitamin K supplementation is necessary for you.


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  2. J Nutr. 2000 Mar;130(3):503-6
  3. Journal of Bone and Mineral Research 2013 Aug;28(8):1778-83
  4. British Journal of Nutrition 2004 Oct;92(4):575-9
  5. British Journal of Nutrition 2009 Oct;102(8):1195-202
  6. Journal of Molecular Nutrition and Food Research 2019 Jun;63(11):e1801323
  7. N Acad Press. 1989: 768
  8. Colorado State University Extension. 2017 Sep; 9.315