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Calcium Supplements: Side Effects, Bioavailability, & Dosage

Calcium is an essential mineral that supports many aspects of human health. Supplements can help meet the recommended daily allowance of 1000mg for calcium when food-based sources aren’t practical. This article is written to help find the best calcium supplement to meet an array of personal needs.

When buying calcium supplements, there are several important things to be aware of before clicking the add to cart button. Dosage, form, and meeting the recommended daily allowance are the most important but there are some others worth keeping in mind as well. This is an article mostly about calcium supplements but there are some important general nutritional considerations to make as well.


  • Calcium has to be combined with other compounds before it can be absorbed by your body.
  • Some of the “other compounds” are better absorbed than others.
  • Some of these “other compounds” may offer benefits in addition to calcium
  • Calcium supplements are typically 20% calcium and 80% “other compounds”
  • Any calcium beyond a single 500mg dose is likely to pass through your system without getting absorbed.
  • Vitamin D is required for calcium to be absorbed
  • Other compounds like magnesium may also help calcium absorption
  • Some foods can cause calcium to be poorly absorbed


First know this: not all calcium supplements are created equal. Factors such as age, sex, and existing health conditions like osteoporosis can influence how much calcium you should take. I can’t help you there—that’s for your doctor to answer.

Where I do seek to help is in the influence of other factors that affect the absorption of calcium supplements. After all, why pay for something your body isn’t going to use? As with many vitamins and minerals, there isn’t a lot of research comparing the bioavailability of different types of calcium.

There’s enough research to make some educated guesses, and that’s all I’ll try to do here. Take such recommendations with a grain of [calcium] salt.

Nutritional Guidelines for Calcium

foods high in calcium

First and foremost; let’s discuss how much calcium we’re supposed to get each day. These levels are based on the Dietary Reference Intakes for Calcium. These vary based on age, gender, and certain other life conditions. I’ve included an abbreviated table of recommended values here:

Life Stage EAR RDA
1–3 y 500 mg 700 mg
4–8 y 800 mg 1,000 mg
9–13 y 1,100 mg 1,300 mg
14–18 y 1,100 mg 1,300 mg
19–30 y 800 mg 1,000 mg
31–50 y 800 mg 1,000 mg
51–70 y 800 mg 1,000 mg
> 70 y 1,000 mg 1,200 mg
9–13 y 1,100 mg 1,300 mg
14–18 y 1,100 mg 1,300 mg
19–30 y 800 mg 1,000 mg
31–50 y 800 mg 1,000 mg
51–70 y 1,000 mg 1,200 mg
> 70 y 1,000 mg 1,200 mg

AI = Adequate Intake; EAR = Estimated Average Requirement; RDA = Recommended Dietary Allowance.

Source: Dietary Reference Intakes for Calcium & Vitamin D

Calcium Foods

I’m of the opinion that one should strive to meet nutritional from food first, and supplementation second. To get a better idea of which foods are highest in calcium, refer to the infographic below:

Calcium Foods Infographic

Tofu, sesame seeds, sardines, yogurt (milk also), and leafy greens like spinach are all high in calcium. The figures above have their total calcium sorted very casually by typical serving sizes.

For example; the chart above notes that both 1 cup of turnip greens and 1 ounce of cheese both contain roughly 200mg of calcium. This approach isn’t very efficient for displaying relative densities of calcium but pretty practical in my opinion. That is until you consider anti-nutrients.

Anti-nutrients & Calcium Absorption

bundle of organic spinach
Certain foods, even those rich in calcium like spinach, contain compounds that block the absorption of calcium

Calcium supplements are sold as chemical combinations of elemental calcium and other stabilizing agents. Our bodies don’t absorb calcium well by itself, and these other compounds help the process along. It’s like getting your kids to eat broccoli by covering it in cheese: there’s a chance they’ll accidentally eat some by itself—but the cheese maximizes your chances.

Minerals like calcium can also bond with compounds that make it more difficultactually impossible in many cases—to be absorbed. These compounds are called anti-nutrients and can be found alongside calcium in many calcium-rich foods.

Predicting the interaction between phytonutrients and mineral absorption is a tough study. Factors such as pH levels, processing methods, and especially person-to-person variables are all at play here (R). The important takeaway is that you’re not likely to absorb the full amount of calcium in foods. Plan your supplementation accordingly. The trouble is; not all calcium supplements are absorbed equally well.

Types of Calcium Supplements

large pile of supplement capsules and tablets

I’m only going to mention several popular forms of calcium here to keep things simple. When considering these types, there are three things to keep in mind:

  1. Overall percentage (ratio) of elemental calcium
  2. Bioavailability compared to other calciums
  3. Brand/Product Quality

Total Percentage of Elemental Calcium

Mineral supplements aren’t well absorbed without being chemically-bound to other more readily-absorbed compounds. Below are some forms of calcium found in supplements as well as the compound to which they’re bound:

Calcium Compound Bound Compound % Calcium
Calcium Carbonate Carbon 41%
DiCalcium Malate Malic Acid 29%
Calcium Citrate Citric Acid 21%

There are certainly other types of calcium supplements out there but these are the most common in my experience. At first glance, it seems that Calcium Carbonate (CaCO3) is the best choice. After all, it has the most elemental calcium? It turns out this simply isn’t the case.

Regardless of the type of calcium supplement you ultimately choose, it’s always important to select from a trusted supplement brand. Check out my article listing the supplement brands I trust for more insight there—or my article on how to find a quality supplement brand before purchase.

Bioavailability of Calcium

urinary testing results

A 1988 study of 21 human subjects found that roughly 15% more calcium citrate was absorbed when compared to calcium carbonate (R). A more current study is currently underway but the results are yet to be shared (R).

Other research has been conducted to show how different forms of calcium are, even taking into account whether or not doses were taken with meals (PDF).

Most studies are small, test conditions vary wildly, and each form isn’t compared in each study. The bottom line? It’s mostly about making a best-guess given a sub-optimal amount of data.

Personally, I choose calcium citrate or dicalcium malate as they are fairly dense and shown slightly higher absorption rates compared to other forms. Would I take calcium carbonate if no other forms were available? Absolutely.

Maximum Dosages

Relative bioavailability isn’t the only factor affecting how much of your calcium supplements get utilized. When you take a calcium supplement, your body is only able to digest it so quickly.

This means that, at some point, you could take enough calcium that your digestive tract couldn’t absorb it before it passed. Turns out the optimal single dose of calcium is ~500mg (R).

That means taking any more than 500mg at one time isn’t likely to increase the amount your body actually absorbs. In other words: that’s when you start wasting money.

Synergistic Compounds

In addition to absorption rate and form, it’s important to recognize some other factors that contribute to calcium supplementation (and absorption from food also.) Firstly, just know that the body absorbs calcium in several distinct ways which can shift based on how much calcium your body has access to on a daily basis (R).

If you have a high calcium diet your body’s absorption process will be a little less dramatic. If you have a lower-calcium or calcium-deficient diet things get a little more ravenous.

In the late 1990s, researchers altered the DNA of some mice so they had fewer Vitamin D receptors. Compared to regular mice, these test mice developed infertility, alopecia, had severely impaired bone formation, and calcium deficiency.

As barbaric as it may sound to some, this study helped showcase the integral role that Vitamin D plays in calcium absorption (R). There have been plenty of other studies to verify the general idea that Vitamin D is required for calcium absorption as well.

The officially-recommended amount of daily Vitamin D intake is 600mg for most adults and 800IU daily for people over 70. The “official” maximum daily dose is listed as 4000UI (R). Many doctors that I’ve discussed the topic with have expressed their belief that 2000-4000IU daily is closer to what should be taken daily, and that much larger doses can be tolerated for up to 6 months.

I’ve personally taken 10,000IU daily for 6 months without negative side effects. That was to get low-normal levels (read as: actually really low) up to par. For me, it worked like a charm.

Age-Related Concerns

Calcium absorption decreases with age and women near, during, and after menopause are at particularly high risk due to certain hormonal changes (R). This helps explain, at least partially, an increased risk of bone-related diseases such as osteoporosis as we age.

Research has shown that certain types of hormone-replacement therapies can help improve existing bone damage and slow the progression of future deterioration. This has been observed as increased utilization of calcium (R). Two common hormone therapies related to calcium absorption are estrogen and calcitriol.

There is a very heated debate over the safety of hormone replacement therapy. One side argues that it increases the risk of cancer significantly. The other side argues that Bioidentical Hormone Therapy (BHT) does away with this risk. The official word from the FDA is that BHT therapy is totally woo-woo and is “not supported by credible scientific evidence.”

There are compelling cases for the safety of BHT and why non-bioidentical hormones aren’t safe (R)(R). Personally, the argument of using human hormones rather than horse hormones makes sense. If you’re considering BHT, regular hormone therapy, or are dealing with age-related calcium issues my best advice is to talk to your doctor.


Calcium is, without doubt, one of the essential minerals that help our bodies get through the day. Diets rich in leafy greens, cheeses, certain fish, and ::cough:: tofu are all great options for ensuring you meet the RDA.

Calcium supplements can be life-savers in cases where dietary restriction is necessary. I’ve had to avoid many high-calcium foods, particularly dairy, and have relied on minimal calcium supplementation to ensure I don’t start breaking bones.

There’s a lot of factors to consider if you’re looking to buy a calcium supplement. The type and dose are factors but—in my opinion only—taking any calcium supplement is better than taking none if you’re at risk for a deficiency. Just don’t take more than 500mg per serving or you’re wasting your money!