Forms and chemistry: frequently asked questions
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What is the difference between vitamin D2 and vitamin D3 on a label?
Vitamin D2 (ergocalciferol) and D3 (cholecalciferol) are chemically different compounds with the same broad role in the body.
D3 is the form the body makes from sunlight and the form most UK supplements use, while D2 is usually made from irradiated fungi and is common in some vegan-labelled and older pharmaceutical products. Milligram for milligram, D2 and D3 are converted to IU using the same 40 IU per microgram factor, but they are not chemically interchangeable, so we compare them in separate tables rather than pooling their prices together. A label states which one a product contains; if neither is named, the product is treated as unverified rather than assumed to be D3.
Compare vitamin D3 and vitamin D2 products separately.
What is the difference between vitamin K2 MK-7 and MK-4?
MK-7 and MK-4 are both forms of vitamin K2 (menaquinone), differing in their chemical side-chain length, which affects how long each stays active once absorbed.
MK-7 is usually derived from fermented sources (such as natto) and MK-4 is more often made synthetically. Because the two forms behave differently once absorbed, labels state doses for each on different scales, and we compare MK-7 and MK-4 products in separate tables rather than counting a microgram of one as equal to a microgram of the other. A label names which form a product contains; if it only says “vitamin K2” with no form named, we cannot place it in either table with confidence.
Compare MK-7 products; an MK-4 table appears once a UK retailer lists one.
What is the difference between methylcobalamin and cyanocobalamin?
Both are forms of vitamin B12 (cobalamin): methylcobalamin is the form the body uses directly in its active state, while cyanocobalamin is a more stable, synthetic form that the body converts into an active form after absorption.
Cyanocobalamin is the more common and cheaper form in mass-market supplements because it is chemically stable and well studied; methylcobalamin is often marketed as the “natural” or “active” form and is common in vegan-labelled products, since it avoids the small cyanide-linked molecule cyanocobalamin carries (present only in a trace, non-harmful amount). Both state their B12 content in micrograms on the label, and we compare them by that stated amount within B12’s price tables.
Compare vitamin B12 products by form.
What is the difference between folic acid and methylfolate?
Folic acid is the synthetic, stable form of folate most commonly used in supplements and fortified foods; methylfolate (5-MTHF) is closer to the form folate takes naturally in food.
Methylfolate does not need the same conversion step folic acid does after absorption, and is often marketed towards people who metabolise folic acid less efficiently due to a common genetic variation, though that is a claim about the body’s processing, not the substance itself. Both state their folate content in micrograms, usually as “DFE” (Dietary Folate Equivalent) on more recent labels, which puts the two forms on a comparable scale.
What forms of magnesium, zinc, iron and calcium are sold in the UK, and how much of the mineral is in each?
Magnesium is commonly sold as citrate (about 16% elemental) or bisglycinate (about 11.7%). Zinc is commonly sold as citrate (about 32.1%) or bisglycinate (about 30.6%). Iron is commonly sold as bisglycinate (“gentle iron”) or fumarate (about 32.9%). Calcium is commonly sold as carbonate (about 40%) or citrate (about 21.1%).
These percentages are the share of each compound’s weight that is the elemental mineral, from the factors on our methodology page; a directly stated elemental amount on a label always wins over one estimated this way. This is a hand-written snapshot, not a page that regenerates automatically — the current numbers for any product are always in its own price table.
See the full factor table and each mineral’s price tables.
Which magnesium form has the most elemental magnesium by weight?
Of the common forms, magnesium oxide has by far the highest share of elemental magnesium by weight, at around 60.3%.
This is because oxygen is a very light atom compared with the organic acids and amino acids other forms are bound to, so a smaller compound weight of oxide delivers a larger elemental amount than citrate (about 16%), bisglycinate (about 11.7%) or L-threonate (about 8.3%) at the same compound weight — which is why oxide is often the cheapest way to buy a large stated number of milligrams. A higher elemental share is a fact about the chemistry, not a judgement of which form is best to take.
What is creatine monohydrate versus creatine HCl, by weight?
Creatine monohydrate is creatine bound to one water molecule, about 88% pure creatine by weight (our own worked example: “4.4 g creatine from 5 g monohydrate”).
Creatine HCl is creatine bound to hydrochloric acid instead, a different molecule with its own, different weight of pure creatine per gram. Because the two are different compounds rather than the same one at different concentrations, we compare them in separate tables rather than converting one figure into the other. Products sold as creatine HCl are commonly labelled at a smaller stated dose than monohydrate (our tables use 1.5 g as the comparison dose for HCl against monohydrate’s 5 g), reflecting how manufacturers typically formulate and market HCl products.
Compare creatine monohydrate and creatine HCl separately.
What is the difference between fish oil and algae oil on the label?
Fish oil is extracted from oily fish (or occasionally krill); algae oil is extracted from cultivated marine algae — the original source of the omega-3 in fish, since fish accumulate it by eating algae.
Algae oil is a vegan and vegetarian source of the same EPA and DHA fatty acids fish oil provides, though it often carries a different balance of EPA to DHA than typical fish oil, and is usually a smaller, more specialised part of the market — which tends to be reflected in price per dose. We compare fish oil and algae oil in separate tables, both ranked on their combined EPA and DHA content rather than the total oil weight.
What does "triglyceride form" versus "ethyl ester" mean for fish oil?
These describe the chemical form the omega-3 fatty acids are bound in, which affects how concentrated the oil can be made.
Triglyceride form is closer to how omega-3 occurs naturally in fish, while ethyl ester form is produced during concentration (replacing part of the triglyceride’s structure with ethanol), often used to reach a higher EPA and DHA concentration per capsule. Re-esterified triglyceride (rTG) oil is ethyl ester oil converted back towards the triglyceride structure after concentration, combining a higher EPA and DHA content with a form closer to the natural one. All three state their EPA and DHA content on the panel, and we compare them within the same fish oil class by that combined figure.
Compare fish oil products by EPA + DHA content.
What is curcumin versus turmeric?
Turmeric is the whole spice root; curcumin (strictly, curcuminoids) is the group of compounds within turmeric most supplements are concentrated for.
Curcuminoids give turmeric its yellow colour and are typically standardised to a stated percentage, such as 95% curcuminoids, in an extract. A “turmeric” product may be the whole root powder, with a relatively low and variable curcuminoid content, while a “curcumin extract” product is concentrated specifically to deliver a stated curcuminoid amount in a smaller weight. We compare products by their curcuminoid content where it is stated, since that is the figure that actually varies the price per dose.
Compare curcumin and turmeric products.
What is KSM-66?
KSM-66 is a specific, branded ashwagandha root extract, standardised to contain at least 5% withanolides, the group of compounds ashwagandha is characterised by.
It is made using a proprietary, low-temperature extraction process from the root only, rather than the root and leaf together. It is one of several branded, standardised ashwagandha extracts on the market, alongside others such as Sensoril (root and leaf, standardised higher, to about 10% withanolides) and Shoden (standardised to about 35% withanolides, at a smaller stated dose). We compare branded extracts in their own tables, separate from unbranded extract and root powder, since their standardisation and typical dose differ.
Compare KSM-66 ashwagandha products.
What is the difference between ascorbic acid and a buffered vitamin C salt?
Ascorbic acid is plain vitamin C, which is naturally acidic; a buffered salt binds vitamin C to a mineral to reduce that acidity, which some people find gentler.
Buffered salts include sodium ascorbate, calcium ascorbate and the branded Ester-C, each adding a small amount of that mineral alongside the vitamin C. Both state their vitamin C content on the label, usually “as ascorbic acid” even on a buffered product, which puts every form on the same comparable scale. We compare standard vitamin C — ascorbic acid and buffered salts together, since both state the same kind of figure — separately from liposomal vitamin C, which uses a different delivery method and is priced differently.
Compare standard vitamin C products.
What is the difference between calcium carbonate and calcium citrate in elemental calcium?
Calcium carbonate is about 40% elemental calcium by weight, the highest of the common forms; calcium citrate is about 21.1%.
A carbonate tablet needs roughly half the compound weight of a citrate tablet to provide the same amount of calcium. Carbonate is generally the cheaper form to manufacture given how much elemental calcium it packs in per gram, while citrate is a larger, more soluble molecule some people find easier to take alongside food. Both state their elemental calcium content on the label, or it is estimated from the compound weight using these factors, and we compare the two forms in separate tables.
Compare calcium carbonate and calcium citrate separately.
What is the difference between L-theanine and green tea extract?
L-theanine is a single amino acid, found naturally in tea leaves and sold either isolated or under a branded name such as Suntheanine.
Green tea extract is a wider concentration of the plant’s compounds, including L-theanine but also catechins such as EGCG and some caffeine, unless decaffeinated. A product labelled simply “L-theanine” states a specific milligram amount of the isolated amino acid, which is the figure we compare products on; a green tea extract product’s theanine content is usually much smaller and less consistently stated, since the extract is not concentrated for theanine specifically. We list green tea extract as an accepted addition alongside L-theanine rather than a directly comparable form.
Compare L-theanine products.
What do "D-alpha" and "DL-alpha" tocopherol mean?
“D-alpha” tocopherol is the natural form of vitamin E; “DL-alpha” is the synthetic form, a roughly equal mix of the D form and its mirror-image L form.
Only the D form behaves the same way in the body as natural vitamin E, which is why the two convert to IU at different rates: natural D-alpha is about 1.49 IU per milligram, while synthetic DL-alpha is lower per milligram. A label’s IU figure therefore means a different weight of vitamin E depending on which form is named. Where a label states only “vitamin E” or “tocopherol” without naming D or DL, the product is treated as unverified.
See vitamin E products and how we convert IU.
What is beta-carotene versus retinol on a vitamin A label?
Retinol is preformed vitamin A, found in animal-source ingredients and most direct vitamin A supplements; beta-carotene is a plant pigment the body converts into retinol as needed.
Beta-carotene (a carotenoid) is found in vegan-labelled vitamin A products and many multivitamins. The two are not interchangeable milligram for milligram: retinol converts to IU at 1 IU to about 0.3 micrograms, while beta-carotene’s conversion is different and less exact, because the body only converts part of it, at a variable rate depending on the person. Labels state which source a product uses — “retinol”, “retinyl palmitate/acetate” (both preformed) or “beta-carotene” — and we read the figure according to the source actually stated.
Compare vitamin A products.
What is the difference between whey concentrate and whey isolate on the label?
Whey concentrate is a less processed form of whey protein, typically around 70–80% protein by weight, retaining more of the fat, carbohydrate and lactose naturally present in whey.
Whey isolate is filtered further to remove most of that fat, carbohydrate and lactose, typically reaching 90% or more protein by weight. Because isolate is more processed, it is usually the more expensive form per kilogram to manufacture, which is reflected in price per gram of protein even though both state their protein content per serving on the label. Whey hydrolysate is a third, further-processed form, where the protein itself has been partially broken down; we compare each within its own class.
Compare whey concentrate and whey isolate prices per gram of protein.
What does "electrolytes" cover on a label?
“Electrolytes” refers to a group of minerals that carry an electrical charge in the body and are lost through sweat — typically sodium, potassium, magnesium, calcium and chloride.
These are listed together on a label because a product is formulated to replace several of them in one serving, rather than as separate single-ingredient doses. Because an electrolyte product is designed around the serving as a whole rather than a single active ingredient, we price it per serving rather than per milligram of any one mineral, so it sits in its own comparison type distinct from single-mineral products such as plain magnesium or plain calcium. The exact minerals and amounts included vary by product, so the panel is worth checking directly.
Compare electrolyte products per serving.