When shopping for a multivitamin, it may seem like they all contain a mix of nutrients that simply get absorbed once you swallow the pill. Sad to say, it’s not that easy. Actually, if you don’t know what to look for, you might be wasting money on ingredients your body can't even use. Each nutrient comes in a variety of different forms which determines how efficiently your body will absorb it. So what should you be looking for to get the most out of your supplements? Feel confident, not confused, with these science-backed guidelines.
What is bioavailability?
Bioavailability is the relative absorption of a nutrient from food or a supplement. The bioavailability of a nutrient varies based on the form, foods and vitamins it is consumed with, interaction with medications and for some it can also be genetic. With regard to vitamins, we are concerned with how much of the nutrient is typically absorbed, used, and stored for future use.
Certain vitamins and minerals have multiple forms that are interchangeably used. Below we discuss the most common and best alternatives for optimal absorption.
This water-soluble vitamin plays an important role in red blood cell production, brain health, DNA production, metabolism, and energy.
There are two main forms of vitamin B12 found in supplements: methylcobalamin, which is naturally occurring, and cyanocobalamin, a synthetic version. A major difference between the two forms is how they are absorbed into the body. Methylcobalamin is absorbed as is, since it is in the form found in the human body. Cyanocobalamin is first converted into methylcobalamin and then absorbed. Because cyanocobalamin is the more stable form, it is absorbed at a slightly higher rate than methylcobalamin, but research shows three times as much cyanocobalamin is excreted through urine than methylcobalamin. Therefore methylcobalamin may be better retained by our body. It is important to note that both forms of B12 have been shown to improve B12 deficiencies especially when paired with a balanced diet (1,2).
Methylcobalamin is the naturally occurring form of vitamin B12 and is typically preferred over the synthetic version of cyanocobalamin.
There are two forms of vitamin D: vitamin D2 and vitamin D3. Most commonly, plant foods contain vitamin D2 and animal products contain small amounts of vitamin D3. The best source of vitamin D however is from the sun. Research shows that exposing the arms, legs, and face to the sun for 5-30 minutes daily or at least 2x/week between the hours of 10 AM and 4 PM is adequate to encourage appropriate vitamin D synthesis (3). It is important to note, this varies dramatically based on lifestyle, age, skin color, location, and seasonality. Since most of us spend the majority of our time dressed and indoors, we tend to be deficient in this nutrient. In fact, current research shows that 46.1% of adults are deficient in vitamin D with the highest rates among Blacks and Hispanics (4).
Research shows that although both vitamins D2 and D3 increase serum D levels, vitamin D3 is absorbed more efficiently and has a longer half-life, therefore has a larger impact on serum levels (5,6). This is why most supplements contain vitamin D3. It is important to note, in nutritional supplements, vitamin D3 is commonly sourced from sheep's wool i.e. not vegan, but considered vegetarian. If the source is vegan, it will typically be stated as vitamin D3, cholecalciferol from algae or lichens.
Vitamin D3 is the more bioavailable form of vitamin D. However, if you want to ensure that it is vegan, make sure the supplement claims the source is algae or lichen.
These essential fatty acids help to decrease inflammation, promote brain and heart health and support mood. Widely available from both plants, in the form of ALA (flaxseeds, walnuts, chia), and animals, such as EPA and DHA (salmon, sardines, grass-fed beef), many people believe a balanced diet delivers an adequate daily dose of omega-3. However current research shows this isn't the case and that 68% of adults and 95% of children are not meeting the dietary guidelines for omega-3 fatty acids (7).
In terms of bioavailability, ALA isn't an efficient source of omega-3 since it first needs to be converted to EPA and then DHA, but this conversion rate tends to be inconsistent and low. Research shows only 1–10% of ALA is converted into EPA and 0.5–5% into DHA (8,9). While flaxseeds, chia seeds, and other plant-based foods are a healthy part of any diet, they don't provide the biggest bang for your buck of omega-3s (10).
Many supplements contain omega-3 DHA exclusively. However, while EPA can be converted to DHA within the body, that conversion process is a one-way street—DHA cannot be retro-converted into EPA (11).
The most bioavailable forms of omega-3 fatty acids come from a combination of EPA and DHA, because our bodies do not need to go through a conversion process to use them. In supplements, fish oil and cod liver oil are popular sources of EPA and DHA. Also, algal oil is a great plant-based source of bioavailable EPA and DHA and should be considered over ALA forms to meet requirements.
Opt for omega-3 fatty acids supplements that have both DHA and EPA over ALA or DHA alone.
Zinc is a mineral widely found in our food supply and has been getting lots of buzz due to the role it plays in immunity, healing, skin health and repair. There are many forms of zinc available for supplementation including: zinc gluconate, sulfate, acetate, picolinate, orotate and citrate. The percentage of elemental zinc varies by form and will be clearly shown on the nutrition label. Recent research shows average absorption of zinc from zinc citrate is about 61.3% similar to the absorption from zinc gluconate at 60.9%. Both of these perform better when compared to other forms such as zinc oxide at 49.9% (12).
Look for zinc citrate or zinc gluconate as these forms have higher absorption rates than zinc oxide.
Many individuals may be low in iron for a variety of reasons including everything from diet, age, activity level and genetics. We absorb iron best from foods we eat from both plant and animal sources. Plants provide non-heme iron and animal sources, heme iron. Heme Iron is more bioavailable as our bodies absorb about 15 to 35% and non-heme iron is less bioavailable often due to cofactors like dietary enhancers (vitamin C) or inhibitors (oxalates) (13).
In supplements, iron generally comes in the following forms: fumarate, gluconate, sulfate and bisglycinate. The most common form you’ll find in store is iron sulfate, but many people complain about issues with this form. In terms of bioavailability, research has shown that iron bisglycinate has an absorption rate that is four times higher than that of iron sulfate (14). Additionally, many find iron bisglycinate to have fewer (if any) gastrointestinal tract side effects (15).
Iron bisglycinate is more bioavailable and has fewer gastrointestinal side effects than iron sulfate, which is the most common form in supplements.
Magnesium is a mineral in the body that is involved in over 300 enzyme systems that regulate a variety of reactions in the body from blood pressure, to bone health, to glucose control, nerve and muscle function and overall energy production. Magnesium is widely available in plant foods, however many refined food products lose magnesium in processing. As an abundant mineral in a healthy diet about 30% to 40% of the dietary magnesium consumed is absorbed by the body, however recent research shows almost half of all Americans aren’t getting enough of this mineral (16). This is where supplementation comes in.
There are two different types of magnesium. The first are inorganic forms, which do not dissolve in water — magnesium carbonate, magnesium chloride, magnesium gluconate, and magnesium oxide. The second are organic forms, those that do dissolve in water and are the most bioavailable — magnesium L-threonate, magnesium malate, magnesium citrate and magnesium glycinate. The inorganic forms (e.g. magnesium oxide) provide a high loading of elemental magnesium, but exhibit a very limited bioavailability as a result of their poor solubility (17). For this reason, it’s typically best to opt for one of the organic magnesium forms, which have been proven to have a higher absorption rate (18).
Organic forms, e.g. magnesium citrate and magnesium glycinate, have a higher absorption rate than inorganic forms like magnesium carbonate and magnesium oxide.
Most commonly associated with bone density and oral health, calcium is also involved in muscle contraction, heart health, blood clotting and nerve function. The bioavailability of calcium depends on the type of calcium and the foods it is obtained from and with which has a direct effect on its absorption. Calcium is present in both animal and plant products, however it’s less bioavailable from plants due to the presence of nutrients that block its absorption such as oxalates and iron.
The two most common forms of calcium are calcium citrate and carbonate. It is important to note the amount of calcium absorbed by the body is considered elemental calcium and this is clearly marked on nutrition labels. For example, calcium carbonate is 40% calcium by weight, whereas calcium citrate is 21% calcium. While more potent than calcium citrate, calcium carbonate can cause mild constipation. Calcium citrate supplements are absorbed more easily than calcium carbonate and don’t have the same GI side effects.
While lower in elemental calcium, calcium citrate has a higher absorption rate than calcium carbonate and can be taken with or without food.
Now that you are familiar with the most bioavailable forms of all for these nutrients you can select the best supplement to meet your personal nutrition goals and needs. Remember everyone is different so read labels and select what is best for you and your health goals.
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(2) Takasaki Y, Moriuchi Y, Tsushima H, et al. Rinsho Ketsueki. 2002;43(3):165-169.
(4) Forrest KY, Stuhldreher WL. Prevalence and correlates of vitamin D deficiency in US adults. Nutr Res. 2011;31(1):48-54. doi:10.1016/j.nutres.2010.12.001
(5) Tripkovic L, Lambert H, Hart K, et al. Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: a systematic review and meta-analysis. Am J Clin Nutr. 2012;95(6):1357-1364. doi:10.3945/ajcn.111.031070
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(8) Gerster H. Can adults adequately convert alpha-linolenic acid (18:3n-3) to eicosapentaenoic acid (20:5n-3) and docosahexaenoic acid (22:6n-3)?. Int J Vitam Nutr Res. 1998;68(3):159-173.
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(10) Burdge GC. Metabolism of alpha-linolenic acid in humans. Prostaglandins Leukot Essent Fatty Acids. 2006;75(3):161-168. doi:10.1016/j.plefa.2006.05.013
(11) Brenna JT. DHA retroconversion revisited: dietary DHA spares endogenous EPA. Am J Clin Nutr. 2019;110(4):789-790. doi:10.1093/ajcn/nqz125
(12) Wegmüller R, Tay F, Zeder C, Brnic M, Hurrell RF. Zinc absorption by young adults from supplemental zinc citrate is comparable with that from zinc gluconate and higher than from zinc oxide. J Nutr. 2014;144(2):132-136. doi:10.3945/jn.113.181487
(13) Ems T, St Lucia K, Huecker MR. Biochemistry, Iron Absorption. In: StatPearls. Treasure Island (FL): StatPearls Publishing; April 21, 2022.
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(15) Bumrungpert A, Pavadhgul P, Piromsawasdi T, Mozafari MR. Efficacy and Safety of Ferrous Bisglycinate and Folinic Acid in the Control of Iron Deficiency in Pregnant Women: A Randomized, Controlled Trial. Nutrients. 2022;14(3):452. Published 2022 Jan 20. doi:10.3390/nu14030452
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(17) Coudray C, Rambeau M, Feillet-Coudray C, et al. Study of magnesium bioavailability from ten organic and inorganic Mg salts in Mg-depleted rats using a stable isotope approach. Magnes Res. 2005;18(4):215-223.
(18) Fiorentini D, Cappadone C, Farruggia G, Prata C. Magnesium: Biochemistry, Nutrition, Detection, and Social Impact of Diseases Linked to Its Deficiency. Nutrients. 2021;13(4):1136. Published 2021 Mar 30. doi:10.3390/nu13041136
Amy Shapiro is a registered dietitian and founder of Real Nutrition