Calcium is a mineral most often associated with healthy bones, although it also plays an important role in blood clotting, muscle contraction, and regulating normal heart rhythm. Today we’re going to talk about calcium and maintaining healthy bones, and what you should eat to get enough calcium. We’ll leave the other health functions of calcium for another time.
Calcium and osteoporosis
At birth, the human body contains about 26 to 30 grams of calcium. This amount increases rapidly after birth, reaching about 1,200 g in females and 1,400 g in males in adulthood. So, we have more than a kilogram of calcium in our body.
About 99% of the body’s calcium is stored in the bones, and the remaining 1% is found in the blood, muscles, and other tissues. To carry out vital daily functions (such as heart rhythm), the body works all the time to keep the amount of calcium in the blood constant. Contrary to what you may think, our skeleton and bones are not static. Bone works all the time: it absorbs calcium and releases it. If calcium levels in the blood fall too low, parathyroid hormone (PTH) will signal the bones to release calcium into the bloodstream. This hormone can also activate vitamin D to improve the absorption of calcium in the gut. At the same time, PTH signals the kidneys to release less calcium into the urine. When the body has enough calcium, another hormone called calcitonin does the opposite: it lowers calcium levels in the blood by stopping the release of calcium from the bones and signalling the kidneys to release more of it through urine.
The body gets the calcium it needs in two ways. One is by eating foods or supplements that contain calcium, and the other is by using calcium in the body. If you don’t get enough calcium through food, your bones are forced to release calcium into the blood. Ideally, the calcium “borrowed” from the bones is replaced at a later date. But this doesn’t always happen, especially when you’re older.
In healthy individuals who receive adequate calcium through food and who are physically active, bone production exceeds bone breakdown till approximately age 30. After that, the breakdown is usually greater than the production. This is sometimes referred to as “negative calcium balance,” which can lead to bone loss.
Getting enough calcium through diet at all ages can help slow the rate of bone loss. Therefore, it is important to eat foods rich in calcium literally every day.
How much calcium do you need?
The recommended dietary allowance of calcium (RDA) for adults is between 950 and 1200 milligrams per day.
A gradual, progressive calcium deficiency can occur in people who do not get enough calcium from their diet in the long term or who lose the ability to absorb calcium. The first early stage of bone loss is called osteopenia and, if left untreated, osteoporosis follows. Postmenopausal women are at greater risk of osteoporosis. During menopause, the amount of oestrogen in the body decreases, a hormone that helps increase calcium absorption and retain the mineral in the bones. Sometimes doctors prescribe hormone replacement therapy with oestrogen and progesterone to prevent osteoporosis. Women of all ages with amenorrhea also have an increased risk of osteoporosis. Amenorrhea is a condition where menstruation stops or is disrupted prematurely and is often seen in younger women with eating disorders or athletes who exercise at a very high level.
Too much calcium
Too much calcium in the blood is called hypercalcemia. The upper limit for calcium is 2500 mg per day from food and supplements combined. People over the age of 50 should not take more than 2000 mg calcium per day, especially from supplements, as this can increase the risk of certain conditions such as kidney stones, prostate cancer, and constipation. Too much calcium from supplements can also block the absorption of other minerals such as iron and zinc.
Symptoms of hypercalcemia include weakness, fatigue, nausea or vomiting, shortness of breath, chest pain and irregular heartbeat.
The best food sources of calcium
It was long thought that cow’s milk was the best source of calcium. It may be, but a recent meta-study showed that there doesn’t seem to be a link between drinking more milk and having a lower risk of osteoporosis. In addition, regular cow’s milk consumption is bad for all kinds of other reasons such as lactose intolerance, premature aging, increased insulin resistance… That is why I have selected fifteen other foods for you that contain a lot of calcium. The amount of calcium is indicated per 100 g of product:
- Parmesan: 950mg calcium;
- Cheddar: 707 mg calcium;
- Mozzarella: 693mg calcium;
- Chia seed: 630mg calcium;
- Tahini: 425 mg calcium;
- Sesame seeds: 4oo mg calcium;
- Feta: 371mg calcium;
- Tofu made with calcium salts: 300 mg calcium;
- Almonds: 273mg calcium;
- Beans: 200-235mg calcium;
- Kale: 250mg calcium;
- Ricotta: 225 mg calcium;
- Almond milk: 175 mg calcium;
- Soy milk: 155 mg calcium;
- Chinese cabbage (Pak-choy): 100 mg calcium.
Absorption of calcium
Calcium is a large mineral and not so easily broken down in the gut. The amount of calcium listed on the label of a food or supplement is the actual amount of calcium in the product, but not necessarily the amount the body will absorb. It has to do with a limited bioavailability. The term “bioavailability” refers to how efficiently the nutrient (in this case calcium) is absorbed.
Dairy products, for example, have a bioavailability of about 30%, meaning that only a third of the calcium is absorbed. Plant-based foods such as leafy vegetables generally contain less calcium, but have a higher bioavailability than dairy, about 50%.
A disadvantage of some plant foods is that they naturally contain so-called ‘antinutrients’. Examples of antinutrients are oxalates and phytates that bind to calcium and reduce its absorbability. Spinach contains the most calcium of all leafy greens, but it is also high in oxalates, making its bioavailability very low, only about 5%. However, it does not mean that you have to avoid spinach, which contains other valuable nutrients. Just do not to rely on spinach as a major source of calcium. A varied diet remains the key to health.
Also keep in mind that the exact amount of calcium absorbed into the body will vary from person to person based on their age, gender and health, as well as what other foods are eaten in the same meal. In general, eating a variety of calcium-rich foods can help offset any minor losses (due to “antinutrients” or “wrong” combinations).