Calcium is a vital mineral, renowned for its crucial role in maintaining strong bones and healthy teeth. However, its benefits extend far beyond skeletal health. It plays a critical part in blood clotting, muscle contraction, regulating heart rhythms, and nerve function. With 99% of the body’s calcium residing in bones and the remaining 1% distributed throughout blood, muscles, and tissues, it’s evident that maintaining adequate calcium levels is essential for overall well-being.
top view wooden board with various cheese, nuts, broccoli, canned fish, and jug of milk
The body meticulously regulates calcium levels in the blood and tissues to ensure optimal functioning. When blood calcium levels dip too low, the parathyroid hormone (PTH) triggers the bones to release calcium into the bloodstream. PTH also activates vitamin D, enhancing calcium absorption in the intestines and prompting the kidneys to conserve calcium by reducing its excretion in urine. Conversely, when calcium levels are sufficient, calcitonin steps in to lower blood calcium by inhibiting calcium release from bones and encouraging the kidneys to eliminate more calcium through urine.
The body obtains calcium through two primary mechanisms: consuming calcium-rich foods or supplements and drawing upon the calcium reserves stored in bones. If dietary intake is insufficient, the body will resort to extracting calcium from bones. While ideally, this borrowed calcium is replenished later, it doesn’t always occur, even with increased calcium consumption.
Recommended Daily Calcium Intake
The Recommended Dietary Allowance (RDA) for calcium varies based on age and sex. Women aged 19-50 require 1,000 mg daily, increasing to 1,200 mg for women aged 51 and older. Pregnant and lactating women need 1,000 mg. Men aged 19-70 require 1,000 mg, while those 71 and older need 1,200 mg.
Calcium and Health: Examining the Evidence
The impact of calcium on various health conditions has been extensively researched.
Blood Pressure
Several studies suggest a possible link between total calcium intake (from both food and supplements) and lower blood pressure. However, inconsistencies in study designs, such as small participant groups and variations among participants, hinder definitive recommendations for increasing calcium intake to treat high blood pressure beyond the RDA.
Cardiovascular Disease
Some studies have raised concerns about calcium supplements potentially impacting heart health, suggesting an increased risk of cardiovascular events with their use. This may be due to high-dose supplements causing hypercalcemia (toxic levels of calcium in the blood), which could lead to blood clotting or hardening of the arteries. However, a consensus guideline from the National Osteoporosis Foundation and the American Society for Preventive Cardiology states that calcium from food or supplements has no discernible relationship (positive or negative) to cardiovascular disease in generally healthy adults. The guideline advises against exceeding the Upper Limit for calcium, which is 2,000-2,500 mg daily from food and supplements.
Bone Health
Calcium is undeniably crucial for bone health. Bones are dynamic tissues constantly undergoing remodeling, where osteoblasts build bone and osteoclasts break it down. In healthy individuals with sufficient calcium intake and physical activity, bone production surpasses bone destruction until around age 30. After this, destruction typically exceeds production, leading to bone loss. Adequate dietary calcium intake throughout life may help slow bone loss, but it cannot completely prevent it. Absorption of calcium becomes less efficient with age, so very high intakes do not always resolve the issue.
Colorectal Cancer
Epidemiological studies suggest a protective role of high calcium intakes against colorectal cancer. Calcium supplements, with or without vitamin D, have yielded mixed results in randomized controlled trials, potentially due to the relatively short duration of these trials compared to the long development time of colorectal cancer (7-10 years or longer). The World Cancer Research Fund and the American Institute for Cancer Research reported strong evidence that calcium supplements (over 200 mg daily) and high-calcium dairy foods likely decrease the risk of colorectal cancer, possibly by binding to toxic substances in the colon and preventing tumor cell growth.
Kidney Stones
Contrary to past recommendations, not eating enough calcium-rich foods can increase the risk of kidney stone formation. Research suggests that a high intake of calcium foods decreases the risk of kidney stones in women, but calcium supplements may increase the risk. Calcium-rich foods, primarily dairy, may reduce stone formation by lowering the absorption of oxalates, which contribute to calcium-oxalate stones.
What Foods Are Rich in Calcium?
Calcium is abundant in many foods, extending beyond the usual suspects like milk and dairy products. Fruits, leafy greens, beans, nuts, and certain starchy vegetables are excellent sources.
Understanding Calcium Bioavailability
It’s important to understand calcium bioavailability – the proportion of calcium in a food that the body can actually absorb and utilize. While the Nutrition Facts label indicates the total calcium content, it doesn’t reflect the amount absorbed. Dairy foods, for example, have a bioavailability of around 30%. Plant-based sources like bok choy, while containing less overall calcium, exhibit a higher bioavailability of 50%. Calcium-fortified orange juice and calcium-set tofu have a bioavailability similar to milk.
Some plant foods contain anti-nutrients like oxalates and phytates that can bind to calcium and reduce its bioavailability. Spinach, though rich in calcium, has a low bioavailability due to its high oxalate content. The key takeaway is not to avoid these foods but to recognize that they may not be reliable sources of calcium. It is best to schedule your meals so that you do not eat “calcium-binding” foods like spinach at the same meal as calcium-rich foods or with calcium supplements.
When interpreting food labels to achieve your daily calcium requirements, remember that the RDAs are established considering calcium bioavailability in food. Keep in mind that individual calcium absorption rates can vary depending on metabolism and the composition of other foods consumed in the same meal. A diverse diet rich in calcium-rich foods generally helps compensate for any minor absorption losses.
Signs of Calcium Deficiency and Toxicity
Deficiency
Calcium deficiency, known as hypocalcemia, can arise from conditions like kidney failure, digestive tract surgeries, or medications that hinder calcium absorption.
Symptoms of hypocalcemia:
- Muscle cramps or weakness
- Numbness or tingling in fingers
- Abnormal heart rate
- Poor appetite
Long-term inadequate calcium intake or impaired absorption can lead to osteopenia (early-stage bone loss) and eventually osteoporosis.
Individuals at risk include:
- Postmenopausal women: Menopause reduces estrogen levels, impacting calcium absorption and bone retention.
- Amenorrhea: Disrupted menstrual cycles, common in women with anorexia nervosa or athletes training at high intensity.
- Milk allergy or lactose intolerance: Inability to digest lactose or milk proteins.
Toxicity
Excessive calcium in the blood, termed hypercalcemia, has an Upper Limit (UL) of 2,500 mg daily from food and supplements. Individuals over 50 should limit intake to 2,000 mg daily, especially from supplements, to minimize the risk of kidney stones, prostate cancer, and constipation. High doses over extended periods can potentially lead to calcium accumulation in blood vessels, resulting in heart problems. Furthermore, calcium can interfere with the absorption of other minerals like iron and zinc.
Symptoms of hypercalcemia:
- Weakness, fatigue
- Nausea, vomiting
- Shortness of breath
- Chest pain
- Heart palpitations, irregular heart rate
Factors Affecting Calcium Needs
Certain nutrients and medications can influence your calcium requirements by either decreasing calcium absorption or increasing calcium excretion in urine. These include corticosteroids, excess sodium in the diet, phosphoric acid (found in dark cola sodas), excessive alcohol consumption, and oxalates.
By understanding the importance of calcium, recognizing calcium-rich food sources, and being aware of factors that influence calcium absorption and utilization, individuals can make informed dietary choices to support their bone health and overall well-being.
