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Bioavailability and the Different Forms of Vitamins

Bump Vitamins Staci Gulbin Bioavailability

Bioavailability and the Different Forms of Vitamins

By Staci Gulbin, MS, MEd, RD

Staci Gulbin is a registered dietitian with Staci is also a freelance writer, health editor, the founder of, and the author of The High-Protein Bariatric Cookbook. 

Staci has graduate degrees in Biology, Human Nutrition, and Nutrition Education from New York University and Columbia University, respectively. She has treated thousands of patients across many wellness areas such as weight management, fitness, long-term care, rehab, and bariatric nutrition. 


WHEN IT COMES to supplements, you may have noticed that some vitamins come in multiple forms. For example, you might see some labels that list Vitamin D as “D2” and others that read “D3” or “K1” versus “K2”. So how do you know which one Is best or which form your body will absorb more easily?

How well your body absorbs, digests, and breaks down a compound in the body is better known as its bioavailability (1).

When it comes to buying vitamins, you want to make sure the form you buy has the right blend of forms to achieve the greatest bioavailability, and in turn, the most value for your money. That being said, sometimes other factors come into play. For example, some of the more bioavailable forms are derived from meats like fish or dairy. Some people may have dietary restrictions that require them to take a different form that may not have as much bioavailability but is from a plant-based source that doesn’t conflict with their diet.

For this reason, companies like offer several different forms of vitamins to find a balance between bioavailability and each individual’s unique dietary needs. Read below to learn about some common vitamins and the varying forms of each.

Bioavailability and vitamins BUMP Vitamins

Vitamin A

Vitamin A plays an important role in organ function, as well as in supporting eyesight and your immune system. It is also used to treat certain hair, skin, and nail conditions. For some women, Vitamin A may also provide relief for heavy menstrual periods, PMS, and vaginal infections. *

There are two common forms of vitamin A that you can find in the human diet through food and/or supplements: preformed vitamin A (retinol or retinyl esters) and carotenoids (2). Experts report that the body can better absorb preformed vitamin A rather than carotenoids like beta-carotene (3). This is because beta-carotene must be converted to retinol and other retinoids by the body after you ingest it (3). Therefore, preformed vitamin A is the best-absorbed form, but since it is derived from animal sources, some people prefer the “Provitamin A Carotenoid” or plant-based form. In either case, both forms must be metabolized within the cell to retinal and retinoic acid.

The recommended daily allowance (RDA) for vitamin A for most adults is 700 to 900 micrograms (mcg) of retinol activity equivalents (RAE) (2,3).  Lactating women may need more at around 1200 to 1300 mcg RAE (3). Two mcg of supplemental beta-carotene are equal to one mcg of retinol (2,3). Vitamin A is a fat-soluble vitamin, so it’s vital not to overdose since the body will store excess amounts.


Vitamin B-9 (Folate)

Folate is involved in the process of methylation, which is essential to DNA and RNA repair, neurotransmitters, and homocysteine metabolism. It is also directly linked to spinal development in the very early (germinal) stages of prenatal development and may help prevent the occurrence of birth defects like NTDs and Cleft Lip/Pallet. *

Folate is a water-soluble B-vitamin that is in the tetrahydrofolate (THF) form in food and in the fully oxidized form, also known as folic acid, in fortified foods and supplements (4,5). You can also find folate in the 5-methyl-THF form in supplements. Experts report that about 85-percent of folic acid is thought to be bioavailable when taken with food, while only 50-percent of folate naturally present in food is bioavailable (4,5).

Many supplements will account for this difference in bioavailability and you will often see folate in higher doses compared to folic acid, in order to achieve the same end result. Most adults need about 400 mcg dietary folate equivalents (DFE) daily (4). However, those who are pregnant need about 600 mcg daily, and those lactating need about 500 mcg daily.


Vitamin C

Vitamin C acts as an antioxidant that helps prevent cells from being damaged by free radicals. The body requires Vitamin C to produce collagen, a protein that heals wounds and repairs the skin. It also supports the immune system and helps the body absorb iron from plant-based sources.

Vitamin C is a water-soluble vitamin found in both a natural and synthetic form in dietary supplements (6,7). Research shows in many cases that both forms of vitamin C, also known as ascorbic acid, absorb about the same in the body (7,8).

Experts suggest though that simple ascorbic acid is the preferred form of vitamin C (6). That being said, many people find the acidity of ascorbic acid can sometimes upset their stomach, especially pregnant women who are already experiencing heightened stomach sensitivity. In those cases, some people find a “buffered” form of vitamin C, like Calcium Ascorbate, to be more tolerable.

Most adults should consume 75 milligrams (mg) of vitamin C daily, while those women who breastfeed should consume closer to 120 mg daily (6).


Vitamin D

Vitamin D supports calcium absorption and bone growth. It is also needed for healthy nervous, muscular, and immune systems. When taken by itself as a supplement, it is best consumed at mealtime alongside a serving of healthy fats. Vitamin K, Magnesium, and Boron all aid in the absorption and effectiveness of Vitamin D.

Vitamin D is a fat-soluble vitamin that you can find in two common forms: vitamin D2, also known as ergocalciferol, and vitamin D3, or cholecalciferol (9). Research shows that vitamin D3 can increase the serum levels of vitamin D in the body faster than vitamin D2 (10,11).

Similar to vitamin A, the original source comes into play. While Vitamin D3 is considered to be more bioavailable, it is typically derived from animal-based sources like fish or dairy. For some people who prefer plant-based sources, D2 is the better option.

Most adults should consume at least 15 mcg or 600 international units (IU) of vitamin D daily (9). However, if your doctor finds through lab testing that you are low in vitamin D, then you may need to take higher doses daily or weekly until your level is in a healthy range.

Besides the form of vitamin D, the vehicle in which vitamin D is in can also affect how it’s absorbed in the body. Research shows that the body can absorb vitamin D3 in a microencapsulated and oil-based form better than vitamin D3 in micellized form (12). In simpler terms, natural lecithin, or fat-based capsule allows for the body to better absorb vitamin D3 in the body versus a water-soluble vehicle.


Vitamin E

Vitamin E has strong antioxidant properties. It reduces the number of free radicals created when your body converts fat to energy. It also supports the immune system and plays a role in gene expression.

Vitamin E is a fat-soluble vitamin that comes in eight forms that include the alpha, beta, gamma, and delta forms of tocopherol and tocotrienol (13). Some research shows that alpha forms of tocopherol and tocotrienols are more bioavailable than delta- or gamma- forms (14). Although the alpha form is believed to be the most effective, many supplements often combine these forms and list them as “mixed tocopherols” to extract any benefit from the different forms working together is synergy.

Although both the natural and synthetic forms of vitamin E are nearly identical from a molecular standpoint, many believe that the natural form provides a greater benefit. You will commonly see vitamins listed on a nutritional label as starting with d-alpha or dl-alpha. The “d” indicates that it is the natural form, while “dl” signifies that it is the synthetic version.

Most adults should consume about 15 mg of vitamin E in the alpha-tocopherol form daily, while those who breastfeed should consume about 19 mg each day (13).


Other factors affecting bioavailability

Besides the form or vehicle of the vitamin you choose, there are other factors that also affect the way compounds absorb in the body. These factors include (15):

  • Whether the body is in a fed or fasted state
  • Type of food eaten with the supplement
  • Health status of one’s digestive tract
  • Acute or chronic illness
  • Nutrition status
  • Genetics
  • Age since older adults and newborns break down drugs more slowly than others

Of note is that the body may better store fat-soluble compounds in the body if consumed with a food source of fat. On the other hand, the body may absorb less vitamin C and other water-soluble compounds if a person is taking diuretics that cause water to excrete more from the body (15).


The Bottom Line

Shopping for the best vitamins involves more than just checking the label for the highest amount of a compound. The amount of a vitamin you take won’t mean much if your body only absorbs a small amount of what is present in a product. That’s why it’s vital to be aware of the forms of each common vitamin that your body absorbs best.

Not only that, but it’s crucial to look at the vehicle in which the vitamin you are taking is inside. And let’s not forget the impact that the state of your body has on the way compounds absorb in the body. If you have trouble absorbing foods, then you may need a special form of certain compounds to provide the most benefit.

No matter what form of vitamin you decide to take, always be sure to talk with your doctor or another healthcare provider before starting any new supplement routine. This will make sure you stay safe by only taking those compounds that will work well with your current health status and prescribed treatment routine.



  1. Srini Srinivasan (April 2001) “Bioavailability of Nutrients: A Practical Approach to In Vitro Demonstration of the Availability of Nutrients in Multivitamin-Mineral Combination Products,” The Journal of Nutrition, Volume 131, Issue 4, April 2001, Pages 1349S–1350S,
  2. National Institutes of Health Office of Dietary Supplements (updated February 14, 2020) “Vitamin A.”
  3. Oregon State University Linus Pauling Institute Micronutrient Information Center (last reviewed March 2015) “Vitamin A.”
  4. National Institutes of Health Office of Dietary Supplements (updated June 3, 2020) “Folate.”
  5. Bayes, J., Agrawal, N., and Schloss, J. (published 2019) “Folate Supplementation in Healthy Populations and Animal Models: A Systematic Review.” The Journal of Alternative and Complementary Medicine, 25:2.
  6. National Institutes of Health Office of Dietary Supplements (updated February 27, 2020) “Vitamin C.”
  7. Oregon State University Linus Pauling Institute Micronutrient Information Center (last reviewed August 3, 2016) “Supplemental Forms.”
  8. Carr, A. C., & Vissers, M. C. (2013). “Synthetic or food-derived vitamin C–are they equally bioavailable?” Nutrients5(11), 4284–4304.
  9. National Institutes of Health Office of Dietary Supplements (updated October 9, 2020) “Vitamin D.”
  10. Lehmann, U., et al. (2013) “Bioavailability of vitamin D(2) and D(3) in healthy volunteers, a randomized placebo-controlled trial.” J Clin Endocrinol Metab. 2013 Nov;98(11):4339-45. doi: 10.1210/jc.2012-4287. Epub 2013 Sep 3.
  11. Bina, N., et al. (2019) “Efficacy of Vitamin D3 versus Vitamin D2 in deficient and insufficient patients: An open-label, randomized controlled trial.”Ibnosina Journal of Medicine and Biomedical Sciences, 11:2, 57-61.
  12. Šimoliūnas, E., Rinkūnaitė, I., Bukelskienė, Ž., and Bukelskienė, V. (2019) “Bioavailability of Different Vitamin D Oral Supplements in Laboratory Animal Model.” Medicina. 55(6):265.
  13. National Institutes of Health Office of Dietary Supplements (updated July 31, 2020) “Vitamin E.”
  14. Yang, C.S., et al. (April 2020) “Vitamin E and cancer prevention: Studies with different forms of tocopherols and tocotrienols.” Mol Carcinog., 59(4):365-389. doi: 10.1002/mc.23160. Epub 2020 Feb 3.
  15. Pressman, P., Clemens, R. A., & Hayes, A. W. (2017). “Bioavailability of micronutrients obtained from supplements and food: A survey and case study of the polyphenols.” Toxicology Research and Application
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