What is a fat-soluble vitamin?
Fat-soluble vitamins are those that can be dissolved only in fats. This group consists of vitamins A, D, E and K. Any extra amount consumed is stored in fatty tissues and in the liver for later use, after absorption in the small intestine. Unlike water-soluble vitamins, which are not retained in tissues and excess is easily excreted, the storage of fat-soluble vitamins offers the benefit of having a supply available for use by the body as needed; the drawback is a risk of toxicity with excess consumption. Eating a healthy and well-balanced diet does not pose a toxicity risk for reasons explained below, but caution should be used with supplementation of these vitamins.
Vitamin A – a night vision nutrient?
This nutrient may bring to mind commonly touted beliefs, such as by consuming extra you can acquire night vision superpowers or cure poor vision altogether and ditch the glasses. And while these are myths, they do have a basis in reality! They stem from the fact that severe deficiency will cause night blindness! But despite my grandma’s claims that red and orange peanut M&Ms were loaded with vitamin A and would certainly improve my eyesight, excess cannot provide superior night vision. Thanks anyway for the candy, grandma.
Why does vitamin A have so many different names?
Two main forms of vitamin A can be found in foods. Preformed vitamin A, also called retinol, is a highly bioavailable (usable) form present in organ meats, fish, dairy products, and eggs. Alternatively, provitamin A carotenoids exist in plants as pigments (primarily red and yellow colors), such as beta carotene, that can be converted into the retinol form of vitamin A in the intestines (1), (4). Carotenoids are less easily digestible and absorbable than retinol but will not cause toxicity if consumed in excess. In the United States, vitamin A toxicity is more common than deficiency due to widespread supplement use and high doses found in some supplements (6).
How does vitamin A affect pregnancy?
During pregnancy, both deficiency and excess of this vitamin can cause birth defects, with a surplus causing defects in the cardiovascular and central nervous system of the fetus (4). Vitamin A is needed beginning very early on during embryonic development for the formation of the heart, eyes, ears, lungs, limbs, and organs of the developing baby (2).
What about supplements?
The most common forms of preformed vitamin A in supplements are retinyl palmitate and retinyl acetate. Sometimes provitamin A in the form of beta carotene is used or a mixture of the two forms will be present. It’s a good idea to keep an eye on the form and dose in any supplement you are considering, as the recommended daily allowance for preformed vitamin A during pregnancy is 770 micrograms/day and excess is a risk to baby’s health. Consuming Provitamin A as beta carotene in amounts higher than 770 mcg/day is relatively safe – no recommended daily allowance or upper intake level exists for that form. One synthetic version of preformed vitamin A, isotretinoin, should be avoided during pregnancy as it is a known cause of birth defects (4). A diet rich in vitamin A remains the safest and most effective way to boost vitamin A stores. High doses of preformed vitamin A interfere with the body’s ability to utilize vitamin D, so supplementation of these two should not be taken together.
What helps dietary vitamin A absorption?
Beta carotene stands apart from other nutrients in how absorption can be supported. If you want to make the most of the beta carotene content in your food, cook it and consume it with some type of fat! Research shows that cooking carrots increases the bioavailability of beta carotene from 11% in raw carrots to 75% in stir-fried carrots! And after conversion to a form the body can use, retinol (preformed vitamin A) content had increased by a factor of 6.5, simply by cooking (5)!
Consuming fat-soluble vitamins with some type of fat, allows for better absorption, as these vitamins require fat to be absorbed in the digestive tract.
Vitamin E
This fat-soluble vitamin exists naturally in eight forms, although alpha-tocopherol is the only form useful to the human body. Together with vitamin C, this nutrient acts as an antioxidant in the body, protecting cells from damage by molecules called free radicals. Vitamin E is involved in immune function, cellular communication, regulating how genes are expressed (epigenetics!), along with other metabolic processes.
What is the role of Vitamin E during pregnancy?
Besides preventing oxidative stress in a pregnant woman which has implications for the pregnancy and baby as a result, not much is known about the specific roles of Vitamin E on fetal development. Research seems to primarily be focused on gathering statistics about the effect of supplementation during pregnancy.
Give me the low-down on Vit E supplements!
Many commonly consumed oils, such as those found in wheat, meats, nuts and seeds, are rich in vitamin E so deficiency is extremely rare and primarily seen in people with health conditions that prevent the body from absorbing fat or that affect bile secretion. Synthetic forms of alpha tocopherol, such as d-alpha-tocopherol, are less biologically active than the forms naturally occurring in foods. A review including 21 trials involving over 21,000 women determined that supplementation during pregnancy does NOT play a role in preventing stillbirth, preterm birth, preeclampsia, or low birthweight babies, but supplementation could cause abdominal pain in pregnant women (9). According to both the World Health Organization and the National Institute of Health, Vitamin E (supplementation) is not recommended for pregnant women to improve maternal and perinatal outcomes (10), (11).
Vitamin K
This fat-soluble vitamin is involved in blood clotting and bone building. Although fat-soluble, this particular nutrient is rapidly broken down and excreted so it doesn’t pose the same toxicity risk as other vitamins of its type (12). Green leafy and cruciferous vegetables contain vitamin K in the form of phylloquinone. Its other naturally occurring form, menaquinones, can be found in animal sources, fermented foods, and it can also be created by intestinal bacteria in the body. Deficiency of vitamin K is rare, except in cases of extended antibiotic use when destruction of necessary bacteria in the gut prevents production of this nutrient.
Why are newborns routinely given vitamin K shots?
Not much is known about the transport of vitamin K from a pregnant woman to a fetus, but it appears to be minimal. A significant amount of evidence exists that newborns are vitamin K deficient based both on testing of vitamin K levels as well as the occurrence of abnormal clotting, which is why routine vitamin K injection is recommended by professionals.
Vitamin D
Vitamin D is both a fat-soluble vitamin that we consume as well as a hormone our bodies create from exposure to ultraviolet B rays from the sun. This nutrient can be obtained from diet, direct sunlight, or supplementation. The two forms of this vitamin that can be found in supplements are D2 (ergocalciferol), a form produced in plants and fungi, and D3 (cholecalciferol), which is produced in animals, including humans. D3 is generally the preferred supplemental form due to better bioavailability. (13) Because it’s difficult to acquire adequate amounts through diet and, in some locations, through sunlight, supplementation is often recommended by medical professionals.
It’s easy to guess that vitamin D plays a role in baby’s bone and teeth development during pregnancy, but maternal vitamin D levels also affect a baby’s immune function, with deficiency leading to a significant increase in risk of RSV infection (14). Both past and recent epidemiologic and case-control studies highlight a correlation between vitamin D deficiency and preeclampsia (14).
Noteworthy highlights of the fat-soluble group
In summary, this group of vitamins requires significant caution if supplemented due to retention in the body and the resulting risk of toxicity. Fortunately, with a typical diet these nutrients are not a deficiency risk. Consuming some fat with each meal will support the body’s ability to utilize the fat-soluble vitamins consumed in meals.
This blog is intended to be informational and should not be used as medical advice. As always, your doctor is the best source for health advice.
- S. Department of Health and Human Services. (n.d.-c). Office of dietary supplements – vitamin A and carotenoids. NIH Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
- Youness, R. A., Dawoud, A., ElTahtawy, O., & Farag, M. A. (2022, September 5). Fat-soluble vitamins: Updated review of their role and orchestration in human nutrition throughout life cycle with sex differences – nutrition & metabolism. BioMed Central. https://nutritionandmetabolism.biomedcentral.com/articles/10.1186/s12986-022-00696-y
- Reddy, P. (2022, September 19). Biochemistry, fat-soluble vitamins. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK534869/
- Vitamin A. Linus Pauling Institute. (2024, March 7). https://lpi.oregonstate.edu/mic/vitamins/vitamin-A#:~:text=Yet%2C%20while%20preformed%20vitamin%20A,uptake%20into%20the%20small%20intestine.
- LJ;, G. A. W. (n.d.). The effect of food preparation on the bioavailability of carotenoids from carrots using intrinsic labeling. The British journal of nutrition. https://pubmed.ncbi.nlm.nih.gov/21923982/#:~:text=From%20kinetic%20data%2C%20it%20was,of%206.5%20by%20stir%2Dfrying.
- Vitamin A. The Nutrition Source. (2023c, March 7). https://www.hsph.harvard.edu/nutritionsource/vitamin-a/
- S. Department of Health and Human Services. (n.d.-e). Office of dietary supplements – vitamin E. NIH Office of Dietary Supplements. https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/
- Rumbold, A., Ota, E., Hori, H., Miyazaki, C., & Crowther, C. A. (2015, September 7). Vitamin E supplementation in pregnancy. The Cochrane database of systematic reviews. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8406700/
- Vitamin E supplementation in pregnancy. (n.d.). https://www.cochrane.org/CD004069/PREG_vitamin-e-supplementation-pregnancy
- Institute of Medicine (US) Committee on Nutritional Status During Pregnancy and Lactation. (1990, January 1). Vitamins A, E, and K. Nutrition During Pregnancy: Part I Weight Gain: Part II Nutrient Supplements. https://www.ncbi.nlm.nih.gov/books/NBK235251/
- World Health Organization. (n.d.). Vitamin E and C supplementation during pregnancy. World Health Organization. https://www.who.int/tools/elena/interventions/vitaminsec-pregnancy
- Vitamin K. The Nutrition Source. (2023d, March 7). https://www.hsph.harvard.edu/nutritionsource/vitamin-k/
- Alayed Albarri, E. M., Sameer Alnuaimi, A., & Abdelghani, D. (2022a, August 4). Effectiveness of vitamin D2 compared with vitamin D3 replacement therapy in a primary healthcare setting: A retrospective cohort study. Qatar Medical Journal. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9372493/
- Wagner, C. L., Taylor, S. N., Johnson, D. D., & Hollis, B. W. (2012, May). The role of vitamin D in pregnancy and lactation: Emerging concepts. Women’s health (London, England). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4365424/
