What is calcium L-threonate, and what is it made of?

Calcium L-threonate is a salt in which calcium is joined to L-threonic acid, a small sugar acid with four carbon atoms. L-threonic acid is one of the substances that form when vitamin C (ascorbic acid) breaks down, in plants and in other settings. The calcium L-threonate used in a 2025 study led by Thai universities was 12.92% calcium by weight. In the European Union it is on the list of mineral substances that may be used to make food supplements. The Thai Reference Daily Intake (Thai RDI) for calcium is 1,000 mg a day; L-threonate itself has no Thai RDI.

In short:

• What it is: a calcium salt of L-threonic acid, formula C8H14CaO10
• Where threonic acid comes from: the breakdown of vitamin C; it is also found in human blood
• Calcium by weight: 12.92% in the material used in a 2025 Thai study
• In food: not a normal part of everyday foods; made as an ingredient
• Rules: on the EU list of calcium forms for food supplements since 2009
• Daily amount: Thai RDI for calcium 1,000 mg; no Thai RDI for L-threonate

This article explains what the salt is made of and where its parts come from. For calcium as a nutrient, see What is calcium, and how much do you need?

What is calcium L-threonate?

Calcium joined to L-threonic acid

A salt forms when a mineral such as calcium joins with an acid. In calcium L-threonate the acid is L-threonic acid. PubChem, the chemical database of the US National Library of Medicine, lists the formula as C8H14CaO10 and the molecular weight as 310.27, which corresponds to one calcium joined to two L-threonate parts. The "L" in the name refers to the shape of the molecule: threonic acid exists in two mirror-image forms, and the L form is the one in this salt.

Basic facts

Source: PubChem for the chemical data; Thai RDIs for the daily amount.

Where does L-threonic acid come from?

L-threonic acid is not found only in supplements. It also forms when vitamin C breaks down, for example in plants and in water, and it has been found in human blood.

In plants

A 2024 review by researchers at the University of Adelaide, Flinders University and the University of Edinburgh explains that in many plant species, oxalate and L-threonate are formed from the breakdown of vitamin C. The same review notes that in a few species, including the grapevine, vitamin C is also used to make tartaric acid, the acid found in grapes and wine.

A 2017 study from France and the University of Edinburgh followed labelled vitamin C in tomato plants. Oxalate and threonate accumulated in the leaves, and about 63% of the leaves' vitamin C was broken down each day. Breakdown was faster in the dark. The study measured leaves, not the fruit that we eat.

In the human body

A 1999 study from the University of Oxford analysed sugars in blood and in the fluid at the front of the eye, taken from people having eye surgery. The researchers found threonic and erythronic acids and concluded that they are normal components of these fluids. They suggested two possible origins: the breakdown of vitamin C, or proteins that have reacted with sugars. The study did not settle which of the two is the source.

In water

A 2004 study from Åbo Akademi University in Finland added vitamin C to household tap water that contained copper from the pipes. After 3 hours at room temperature, 93% of the added vitamin C had been oxidised, and it went on to break down into oxalic acid and threonic acid. Little vitamin C was oxidised in tap water without copper, and none in the bottled waters tested.

How much calcium does calcium L-threonate contain?

Calcium as a share of the weight

Every calcium salt is partly calcium and partly something else, so the share of calcium differs between forms. The calcium L-threonate used in a 2025 study led by researchers from King Mongkut's Institute of Technology Ladkrabang, Walailak University and other Thai universities had a purity of 99.8% and contained 12.92% calcium by weight. The US NIH Office of Dietary Supplements (ODS) gives figures for the two most common forms in supplements.

These numbers describe only how much of each salt's weight is calcium. The ODS also lists calcium sulfate, ascorbate, hydroxyapatite, gluconate, lactate and phosphate as forms used in supplements. For more about the forms, see What forms does calcium come in, and how do they differ?

How is calcium L-threonate used in food?

A Thai study of fish gelatin jelly

The 2025 Thai study made jelly from fish gelatin (from tilapia skin), sugar, water, citric acid and lychee flavour, adding 1,200 mg of calcium L-threonate to the recipe. Some jellies also contained furcellaran, a gum from red seaweed. The researchers measured how firm the jellies were and asked 50 untrained tasters to rate them on a 9-point scale.

Adding calcium L-threonate alone made the jelly slightly firmer, and the tasters rated it as highly as the plain jelly. The jelly with both furcellaran and calcium L-threonate was the firmest and contained 158.15 mg of calcium per 100 g. The jellies were made in a laboratory, so the results describe this recipe rather than products in shops.

How the EU lists it

In the European Union, Commission Regulation (EC) No 1170/2009 updated the lists of vitamin and mineral forms. It added calcium L-threonate to the list of mineral substances that may be used to make food supplements. The same regulation's list of forms that may be added to other foods does not include it.

How much calcium do you need a day?

The Thai RDI for calcium is 1,000 mg a day. Calcium is needed for the maintenance of normal bones and teeth. Calcium from food, drinks and every supplement counts toward the same total. Calcium salts are used in ordinary foods as well: tofu, for example, is set with salts, and the type of salt changes how much calcium it contains.

The USDA figures describe American foods. For calcium in Thai foods, see Which foods are high in calcium, and how much is in each?

Upper limits

The NIH ODS lists the US upper limit for calcium at 2,500 mg a day for adults aged 19 to 50 and 2,000 mg a day for adults over 50. An upper limit is the most that is unlikely to cause harm; it is not a target. How milk and supplements add up is covered in Can I take a calcium supplement together with milk?

What has EFSA assessed about L-threonate?

L-threonate has no Thai RDI. In 2024, EFSA's nutrition panel assessed magnesium L-threonate, another salt of the same acid, for use in food supplements by adults, excluding pregnant and breastfeeding women. Its opinion records that in 2008 EFSA's panel on food additives and nutrient sources assessed an intake of 2,700 mg of L-threonate a day and found no safety concern at that level, and the 2024 panel concurred for this use. This is the level EFSA assessed, not a recommended amount.

Who should talk to a doctor or pharmacist?

People who notice stomach discomfort

The NIH ODS says some people who take calcium supplements notice gas or bloating.

People who take regular medication

The ODS notes that calcium supplements can interact with some medicines, and some medicines affect calcium levels. Anyone who takes medication regularly should tell their doctor or pharmacist about every supplement they use.

Pregnant or breastfeeding women

If you are pregnant or breastfeeding, or take medication regularly, check with a doctor or pharmacist before taking supplements.

Dietary supplements are not intended to prevent, treat or cure any disease.