What Are the Different Types of Calcium?

Over-the-counter (OTC) calcium supplements are generally available in four different forms. The most common ones are calcium carbonate and calcium citrate. The other two, calcium lactate and calcium gluconate, contain less calcium per dose and tend to be harder to find.

The difference between the calcium supplement types, at least in terms of composition, is the second element paired with calcium to form a calcium salt. The amount of calcium actually absorbed by the body, called elemental calcium, can vary from one calcium salt to the next. Some types have greater bioavailability (the proportion of a drug that enters the bloodstream) than others, while some require food to break down and absorb the nutrient.

Still, having more elemental calcium does not mean that a supplement is necessarily "better." In the end, the amount of calcium that can be absorbed in one sitting is limited. As the intake of calcium increases, the rate of absorption will decrease as the excess mineral is excreted from the body in urine, which affects recommended dosing.

Knowing more about the differences between calcium supplements can help you make a more informed choice as a consumer.

Calcium Carbonate

Calcium carbonate, also known as calcite, tends to be the best value because it contains the highest amount of elemental calcium per dose (around 40% by weight). As a result, you need to take fewer pills per day to meet your daily needs.

Calcium carbonate is generally taken two to three times daily to treat hypocalcemia (low calcium) or to prevent calcium deficiency. Calcium carbonate should be taken with meals as this stimulates the production of stomach acids needed to absorb the nutrient. Drink a full glass of water after taking the dietary supplement. On the downside, calcium carbonate may cause mild constipation and bloating.

Calcium carbonate is also used in antacids like Tums to relieve heartburn, acid indigestion, and upset stomach.

Calcium Citrate

Calcium citrate is the calcium salt of citric acid. It absorbs about as well as calcium carbonate but does not need high stomach acid levels to break it down. As such, it may be a better choice for people with low stomach acidity, such as the elderly or those with chronic gastritis, inflammatory bowel disease (IBD), or celiac disease.

Calcium citrate can be taken either with food or on an empty stomach, and it's a better option for people who take acid-reducing heartburn medications. Because calcium citrate contains only 21% calcium, you may need to take a higher milligram (mg) dosage per day to meet your daily needs.

Calcium citrate should also be taken in two to three divided doses. It tends to be far less constipating than calcium carbonate.

Calcium Lactate

The calcium lactate is a less popular option given the lower amount of elemental calcium per dose (around 13% by weight). Because of this, you need to take more pills per day, making it a less convenient (and generally costlier) option than either calcium carbonate or calcium citrate.

On the plus side, calcium lactate can be taken on an empty stomach. Calcium lactate can also be found in certain over-the-counter antacids.

Calcium Gluconate 

Calcium gluconate is less commonly used as a nutritional supplement due to the low availability of elemental calcium (around 9% by weight).

Calcium gluconate is more commonly prescribed for medical emergencies, where it is delivered intravenously (into a vein) to treat hypocalcemia, hyperkalemia (high potassium), or an overdose of Epsom salt. Even so, severe cases of hypocalcemia may be more responsive to calcium chloride than calcium gluconate.

Constipation and stomach upset are common side effects of calcium gluconate.

Other Factors That Affect Absorption

Other factors can either enhance or impair calcium absorption. These include age (with infants absorbing up to six times more than adults) and the consumption of foods high in oxalic acid (including spinach, collard greens, sweet potatoes, rhubarb, and beans), which impedes absorption.

Magnesium and vitamin D are also essential to the absorption of calcium. Vitamin D helps the intestines absorb calcium but must first be activated by magnesium. Doing so increases absorption by 2-to-4 fold.

Given that 42% of Americans are vitamin D deficient, many calcium supplements today are fortified with vitamin D.

In general, calcium may have drug-drug interactions with other medications. It's a good idea to space out calcium from other medications by one to two hours. Calcium may decrease the effectiveness of the other medications.

Making a Choice

Generally speaking, calcium carbonate and calcium citrate are the preferred choices as they deliver higher elemental calcium, and are relatively low-cost. That's not to suggest that other types can't be used.

According to a 2014 review of studies in BoneKEy Reports, the differences in calcium absorption rates are relatively small and "of no practical significance" when selecting the appropriate supplement.

Unless a supplement is prescribed to treat clinical hypocalcemia, the choice should ultimately be based on which type is most tolerable, convenient, and affordable to you. More than anything else, palatability and acceptability ensure greater adherence to supplementation over the long run.

A Word From Verywell

Calcium supplements are helpful to fill the gap between how much calcium you get from your diet and the daily requirement needed per day.

If you are unsure whether you need calcium supplements or not, speak with your healthcare provider. (If you are eating a healthy, balanced diet, you may not.) If you think you are deficient, ask for a calcium blood test to check your levels.