Calcium is a vital mineral, renowned for its crucial role in maintaining strong bones and teeth. However, its importance extends far beyond skeletal health, influencing blood clotting, muscle function, heart rhythm regulation, and nerve signal transmission. Approximately 99% of the body’s calcium is stored in bones, with the remaining 1% circulating in the blood, muscles, and other tissues.
To sustain these essential daily functions, the body diligently maintains a consistent level of calcium in the blood and tissues. When blood calcium levels dip too low, the parathyroid hormone (PTH) prompts the bones to release calcium into the bloodstream. PTH also activates vitamin D, enhancing calcium absorption in the intestines. Simultaneously, PTH signals the kidneys to reduce calcium excretion in urine. Conversely, when calcium levels are sufficient, calcitonin, another hormone, lowers blood calcium by inhibiting calcium release from bones and instructing the kidneys to eliminate more calcium through urine.
The body obtains its calcium requirements through two primary avenues: consuming calcium-rich foods or supplements and drawing from existing calcium stores within the body. Inadequate dietary calcium intake leads the body to extract calcium from bones. Ideally, this “borrowed” calcium is replenished later, but this doesn’t always occur and cannot always be achieved solely through increased calcium consumption.
Top view of a wooden board featuring various cheeses, nuts, broccoli, canned fish, and a jug of milk, showcasing calcium-rich foods
Recommended Daily Calcium Intake
The Recommended Dietary Allowance (RDA) for calcium varies based on age and gender.
- Women: 1,000 mg daily (ages 19-50); 1,200 mg daily (ages 51+)
- Pregnant and Lactating Women: 1,000 mg daily
- Men: 1,000 mg daily (ages 19-70); 1,200 mg daily (ages 71+)
Calcium and Its Impact on Health
Let’s delve into how calcium affects various aspects of health:
Blood Pressure
Numerous studies have explored the relationship between total calcium intake (from both food and supplements) and blood pressure. Some suggest a potential link between calcium and lower high blood pressure. However, inconsistencies in study designs, such as small participant pools and variations among study populations, preclude a definitive recommendation to increase calcium intake above the RDA for managing high blood pressure. Larger, long-term trials are necessary to determine whether increased calcium intake or calcium supplements can effectively lower high blood pressure.
Cardiovascular Health
Certain studies have raised concerns about calcium supplements and cardiovascular health. These studies suggest that calcium supplements might elevate the risk of cardiovascular events in both men and women. The theory is that high-dose supplements can cause hypercalcemia (excessive calcium in the blood), potentially leading to blood clots or arterial hardening, thereby increasing the risk of cardiovascular disease. While the connection remains unclear, clinical guidelines from the National Osteoporosis Foundation and the American Society for Preventive Cardiology indicate that calcium from food or supplements has no association (positive or negative) with cardiovascular disease in generally healthy adults. These guidelines advise against exceeding the Upper Limit for calcium, which is 2,000-2,500 mg daily from food and supplements.
Bone Health
Calcium is indispensable for maintaining bone health. Bone is dynamic tissue, constantly undergoing remodeling. Throughout life, bones are continuously broken down and rebuilt. Osteoblasts are bone cells responsible for building bone, while osteoclasts break down bone when calcium is needed. In healthy individuals with adequate calcium intake and physical activity, bone production surpasses bone destruction until around age 30. After this point, destruction typically exceeds production, resulting in “negative calcium balance” and potential bone loss. Women often experience more significant bone loss later in life due to menopause, which reduces hormone levels that aid in bone building and preservation. Adequate dietary calcium intake at all ages can help slow bone loss, but it cannot entirely prevent it. Calcium absorption tends to decrease with age; therefore, consuming very high amounts of calcium may not always resolve the issue. Studies on calcium intake and bone density in postmenopausal women have yielded mixed results, potentially due to factors such as focusing solely on supplement intake without considering dietary calcium or failing to account for hormone replacement therapy or vitamin D supplementation.
Colorectal Cancer
Epidemiological studies suggest that high calcium intake (from food and/or supplements) may offer protection against colorectal cancer. However, randomized controlled trials involving calcium supplements, with or without vitamin D, have produced inconsistent results, possibly due to their relatively short durations. Colorectal cancer development can take 7-10 years or longer, making it challenging for shorter trials to capture any changes in the colon.
Kidney Stones
Previously, experts recommended that individuals with kidney stones limit their calcium intake, as calcium is a major component of calcium-oxalate stones. However, current understanding suggests the opposite: insufficient calcium-rich foods can increase the risk of stone formation. Research from large trials like the Women’s Health Initiative and the Nurses’ Health Study has found that high calcium intake from foods reduces the risk of kidney stones in women. This effect does not extend to supplements, as calcium in pill form has been shown to increase the risk.
Top Food Sources of Calcium
Calcium is widely available in various foods, extending beyond milk and dairy products. Fruits, leafy greens, beans, nuts, and certain starchy vegetables are also excellent sources.
Here’s a list of foods rich in calcium:
- Dairy products: Milk, cheese, yogurt
- Leafy green vegetables: Kale, collard greens, spinach (though bioavailability may be lower)
- Fortified foods: Plant-based milks (almond, soy, oat), tofu, orange juice
- Canned fish with bones: Sardines, salmon
- Nuts and seeds: Almonds, sesame seeds
- Beans and lentils: White beans, kidney beans, lentils
Understanding Calcium Bioavailability
Bioavailability refers to the proportion of calcium in food that the body can absorb and utilize. Calcium, being a large mineral, isn’t easily broken down in the gut. The amount of calcium listed on a food label represents the total calcium content, not necessarily the amount absorbed.
Dairy foods typically have a bioavailability of around 30%. Plant-based foods like leafy greens may have lower overall calcium content but higher bioavailability. For instance, bok choy contains approximately 160 mg of calcium per cup cooked, with a bioavailability of 50%, meaning about 80 mg is absorbed.
Some plant foods contain “anti-nutrients” like oxalates and phytates, which can bind to calcium and reduce its bioavailability. Spinach, despite having a high calcium content (260 mg per cup cooked), also has high oxalate levels, resulting in a low bioavailability of only about 5% (13 mg).
The key takeaway is not to avoid spinach, as it offers other valuable nutrients, but rather not to rely on it as a primary calcium source. You can also strategically time your meals, avoiding the consumption of calcium-binding foods like spinach alongside calcium-rich foods or supplements.
When reviewing food labels to meet your daily calcium requirements, continue to aim for the RDAs established for your age group and gender. These RDAs consider calcium bioavailability in food. Remember that the exact amount of calcium absorbed varies among individuals based on metabolism and other foods consumed during the same meal. Generally, a diverse diet rich in calcium-rich foods can help compensate for any minor losses.
Calcium Deficiency and Toxicity
Deficiency
Calcium blood levels are carefully regulated. If the diet lacks sufficient calcium, bones will release calcium into the bloodstream, often without noticeable symptoms. A more severe calcium deficiency, known as hypocalcemia, can arise from conditions such as kidney failure, digestive tract surgeries (e.g., gastric bypass), or medications like diuretics that interfere with absorption.
Symptoms of hypocalcemia include:
- Muscle cramps or weakness
- Numbness or tingling in fingers
- Abnormal heart rate
- Poor appetite
A gradual, progressive calcium deficiency can occur in individuals who consistently lack adequate dietary calcium or experience impaired calcium absorption. The early stage of bone loss is termed osteopenia, which, if untreated, can progress to osteoporosis. Individuals at risk include:
- Postmenopausal women: Menopause reduces estrogen levels, a hormone that enhances calcium absorption and retention in bones. Hormone replacement therapy (HRT) with estrogen and progesterone may be prescribed to prevent osteoporosis.
- Amenorrhea: A condition characterized by the early cessation or disruption of menstrual periods, often observed in younger women with anorexia nervosa or athletes engaged in intense physical training.
- Milk allergy or lactose intolerance: Occurs when the body cannot digest lactose, the sugar in milk, or the proteins in milk, casein or whey. Lactose intolerance can be genetic or acquired.
Toxicity
Excessive calcium in the blood is called hypercalcemia. The Upper Limit (UL) for calcium is 2,500 mg daily from food and supplements. Individuals over 50 should limit their intake to 2,000 mg daily, especially from supplements, as higher doses can increase the risk of kidney stones, prostate cancer, and constipation. Some research suggests that, in certain individuals, high doses of calcium over extended periods can lead to calcium accumulation in blood vessels, potentially causing heart problems. Calcium, being a large mineral, can also impede the absorption of other minerals like iron and zinc.
Symptoms of hypercalcemia include:
- Weakness, fatigue
- Nausea, vomiting
- Shortness of breath
- Chest pain
- Heart palpitations, irregular heart rate
Factors Affecting Calcium Needs
Certain nutrients and medications can elevate your calcium requirements by either reducing calcium absorption in the gut or increasing calcium excretion in urine. These include:
- Corticosteroids (e.g., prednisone)
- Excess sodium in the diet
- Phosphoric acid (found in dark cola sodas)
- Excess alcohol
- Oxalates
Additional Resources
- Vitamins and Minerals
References
Last reviewed March 2023
