Calcium: Its importance, food, absorption and side effects

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It is important to eat calcium-rich foods as part of your balanced diet. This metal makes up 40% of the bones and any lack of it in the diet can lead to osteoporosis. You also need a sufficient amount of vitamin D to absorb more calcium from food.

Calcium, in addition to being part of our skeleton, has important properties and is involved in many biochemical processes in the body. For example, it helps muscle muscles perform the task of contraction. When a nerve stimulates a muscle, the body releases calcium. When the body removes calcium from the muscle, the muscle relaxes. The role of calcium in muscle function includes maintaining the action of the heart muscle. Calcium relaxes the smooth muscle that surrounds blood vessels and this helps regulate blood pressure. Metal plays a key role in blood clotting, a complex process that has many steps involved in various substances. It is also a co-factor for many enzymes that cannot function effectively without it.

The amount of calcium that should be taken from the diet depends on age and various other conditions:

0-6 months: 200 mg
7-12 months: 260 mg
1-3 years: 700 mg
4-8 years: 1,000 mg
9-18 years: 1,300 mg
19-50 years: 1,000 mg
51-70 years: 1,000 mg for men and 1,200 mg for women
71 years and older: 1,200 mg

During pregnancy and lactation, women need 1,300 mg at the age of 14-18 and 1,000 mg at other ages. Your doctor may recommend extra calcium if you are menopausal, do not have a period due to anorexia nervosa or excessive exercise, are lactose intolerant, have a milk allergy, or follow a strict vegetarian diet.

The above amounts of calcium are the recommended intake from the diet and not those absorbed by the body. When you take 1 gram of calcium in your diet, the net absorption is about 300 mg (30%). This is the amount of calcium that the body actually needs, but absorption can vary considerably between foods.

Foods rich in calcium and absorption

Calcium, in its natural form, does not exist as a free element except in compounds with other ingredients (salts). These salts are mainly calcium citrate, calcium carbonate, calcium phosphate, calcium gluconate and milk calcium.

Dairy products are a good source of calcium. One serving is equivalent to a glass of milk (250 ml), a cup of yogurt (200 g) or a slice of cheese (40 g). Hard cheeses, such as parmesan, have a higher concentration of calcium than softer varieties. Small fish (sardines, atherina) eaten with their bones are also rich in calcium.

However, the bioavailability of calcium (absorption by the body) must be taken into account. There are several factors that can affect bioavailability.

  • Vegetable and oxalic acid. Vegetable acid is found in cereal bran and binds calcium and other minerals, making them insoluble and therefore non-absorbable by the gut. If you eat a lot of bread and other whole grains, you will need to get more calcium from your diet. Oxalic acid is a more potent inhibitor of calcium absorption than vegetable acid. It is found in cocoa, spinach, soy. In the case of calcium, it prevents intestinal absorption because it forms calcium oxalate. Less than 5% of the calcium in spinach is absorbed by the body. Also, a study showed that consuming spinach and milk at the same time reduces the absorption of calcium from milk. More than 50% of calcium is absorbed from broccoli, which has little oxalate.
  • Vitamin D deficiency. Vitamin D is critical for calcium absorption. It is produced in the body due to exposure to ultraviolet radiation but is also found in fatty fish and a small amount in eggs.
  • Caffeine. Caffeine in coffee and tea acts as a mild diuretic and calcium is excreted before the body can use it. Consumption of these drinks in small amounts, one or two cups a day, is relatively harmless as 240 ml of coffee leads to about 4 mg of calcium in the urine [5].
  • Salt. Sodium intake increases the excretion of calcium in the urine. An increase of 2.3 g. Sodium in the diet leads to 40 mg of calcium in the urine. This is a large amount. For a period of 20 years, assuming that the body has 1 kg of calcium, this is equivalent to 15% loss of bone mass.
  • Protein: Increased protein intake helps in the absorption of calcium but at the same time causes increased urinary excretion. A few years ago, it was thought that consuming a lot of protein contributed to osteoporosis because it caused calcium to be excreted in the urine. Recent research has shown that proteins stimulate the release of acid in the stomach, which in turn enhances calcium absorption [6]. Proteins are probably neutral in their role in osteoporosis. They may even play a positive role as part of the bone is made up of protein (minerals make up 60% and water 10%).
  • Alcohol. Alcohol consumption can affect the state of calcium by reducing its absorption and inhibiting liver enzymes that help convert vitamin D to its active form [3]. However, how much alcohol is needed to affect the state of calcium is unknown.
  • Magnesium. This metal plays a role in building strong bones. It is needed for the secretion of parathyroid hormone which increases the absorption of calcium.
  • Smoking. Studies show a reduced bone mass among smokers. The reason is not well understood, but smoking may interfere with the absorption of calcium from the intestine.

Calcium supplements

The recommendation is to take the required dose of calcium through your diet but this is not always possible. Older people often do not consume enough calcium or are unable to absorb it properly. Other times there are diseases that prevent the absorption of calcium, such as e.g. celiac disease in which the absorption of fat-soluble vitamins and minerals is affected. Also, if you do not have good kidney function, you may be deficient in calcium through increased urine loss.

In such cases, taking supplements can increase your daily calcium intake by about 300 mg per day. 43% of adults in the United States take calcium supplements, compared with 70% of older women. However, studies are needed to examine whether the benefits of taking supplements outweigh any side effects.

As with food, calcium is found in dietary supplements in the form of salts. These compounds contain different percentages of “elemental” calcium, which is their essential content. In order for these salts to dissolve and for calcium to be absorbed, an acidic environment is usually required, which is provided by the gastric fluid of the stomach. Calcium is better absorbed when taken in amounts of up to 500-600 mg at a time with a meal. The various supplements contain different amounts of elemental calcium and the two main forms on the market are carbon and calcium citrate.

Calcium carbonate: This type is the most available and cheapest. It contains 40% elemental calcium which is a significant amount. If a pill has 500 mg of calcium carbonate, then it has 200 mg of elemental calcium. Calcium carbonate requires stomach acid to be absorbed, so these supplements should be taken with meals [7]. However, this form is more likely to cause side effects such as gas, bloating and constipation.

Calcium citrate: Contains 21% elemental calcium which means you will need to take more pills. Its advantage is that it does not depend on gastric acid, so it can be taken at any time, with or without food. It is useful in people who suffer from inflammatory bowel disease or whose stomach does not produce much acid. If you are taking medications that block stomach acids (H2 inhibitors or proton pump inhibitors), then calcium citrate is your choice.

Other forms of supplements contain less elemental calcium, for example lactate has 13% and gluconate has 9%.

It is best to talk to your doctor about when and how to take a supplement because it is important to take it properly. Calcium carbonate can reduce the absorption of iron (however, it does not appear to be significantly reduced by preparations containing iron and calcium together).

Side effects

As with other minerals or vitamins, it is important to get the right amount. Too much calcium can have side effects.

Sometimes blood calcium levels can be too high, leading to hypercalcemia which is characterized by stomach pain, nausea, irritability and other symptoms. This condition can be caused by many things, such as dehydration, thyroid disease and malignancy. However, the use of high-dose calcium or vitamin D supplements increases the risk, especially in postmenopausal women.

Calcium supplements are usually well tolerated but some people may experience bloating or constipation. If this happens, talk to your doctor. To avoid constipation, eat more fiber (from fruits, vegetables and whole grains), drink six to eight glasses of water a day, and be physically active.

Rarely, calcium supplements can cause kidney stones in people who are predisposed. However, the risk is considered very small and begins when the intake is over 3,000 mg per day [1].

High calcium intake – whether from dairy products or supplements – has been linked to an increased risk of prostate cancer and cardiovascular events in some, but not all, observation and intervention studies. There are currently no indications of such harmful effects when people consume 1,000-1,200 mg of calcium per day (combination of diet and supplements).

The use of calcium supplements at levels of 500-600 mg per day is considered safe but you should only take the supplements according to the instructions and after consulting your doctor.


Find at supplements with calcium!


  1. Calcium Intake and Health.
  2. The good, the bad, and the ugly of calcium supplementation: a review of calcium intake on human health.
  3. Calcium. Fact Sheet for Health Professionals.
  4.  Isotopic exchange of ingested calcium between labeled sources: evidence that ingested calcium does not form a common absorptive pool.
  5. Caffeine, urinary calcium, calcium metabolism, and bone.
  6. The impact of dietary protein on calcium absorption and kinetic measures of bone turnover in women.
  7. Calcium supplementation in clinical practice: a review of forms, doses, and indications.
  8. All About Calcium Supplements.

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