Calcium (Ca) is a unique mineral that is an essential constituent of all forms of life. Calcium is the fifth most abundant element in our bodies and the most abundant mineral. It is the main mineral in bone, being stored as hydroxyapatite. The skeleton contains approximately 99% of the body’s total calcium where it supports their structure and function. We need adequate dietary calcium intake to facilitate normal growth and maintain healthy bones and teeth.
Serum calcium is very tightly regulated and does not fluctuate with changes in dietary intakes; the body uses bone tissue as a reservoir for, and source of calcium, to maintain constant concentrations of calcium in blood, muscle, and intercellular fluids. Both intracellular and extracellular calcium concentrations are tightly controlled within narrow limits by the calciotrophic hormones. This is crucial to human survival as the interaction of calcium ions with proteins alter molecular activity. The ordered movement of ionic calcium plays an essential role in regulation of muscle contraction, nerve conductivity, ion transport, enzyme activation, blood clotting and the secretion of hormones and neurotransmitters. Though less than 1% of total body calcium is needed to support these critical metabolic functions, life without calcium is not possible and even small fluctuations in plasma calcium concentrations can have serious consequences.
Bone itself undergoes continuous remodelling, with constant resorption and deposition of calcium into new bone. The balance between bone resorption and deposition changes with age with bone formation exceeding resorption in periods of growth (children and adolescents). In early and middle adulthood both processes are relatively balanced. In aging adults, particularly among postmenopausal women, bone breakdown exceeds formation, resulting in bone loss that increases the risk of osteoporosis over time.
Milk, yogurt, and cheese are rich natural sources of calcium and are the major food contributors of this nutrient to people in westernized countries. Non-dairy sources include vegetables, such as Chinese cabbage, kale, and broccoli. Spinach provides calcium, but its bioavailability is poor. Most grains do not have high amounts of calcium unless they are fortified; however, they make a significant contribution of calcium to the diet given the frequency with they are consumed. Foods fortified with calcium include many fruit juices and drinks, tofu, and cereals. The two main forms of calcium in supplements are carbonate and citrate. Calcium carbonate is more commonly available and is both inexpensive and convenient.
Role in Health and Disease
Over the short term, inadequate intakes of dietary calcium produce no obvious symptoms as circulating blood levels of calcium are tightly regulated. Despite this, hypocalcaemia (low serum calcium) and hypercalcaemia (high serum calcium) are common medical emergencies. Low blood calcium may cause seizures, muscle spasms, lethargy, reduced appetite, heart arrhythmias, and numbness and tingling due to increased neuromuscular activity. Over the long term, inadequate calcium intake causes osteopenia which if untreated can lead to osteoporosis. Calcium deficiency can also cause rickets, though it is more commonly associated with vitamin D deficiency.
Hypercalcemia may induce thirst, mild confusion, irritability, loss of appetite, general fatigue. renal insufficiency, soft tissue calcification, hypercalciuria (high levels of calcium in the urine) and kidney stones. Although very high calcium intakes have the potential to cause hypercalcemia, it more commonly arises from primary hyperparathyroidism or malignancy. High calcium intake can cause constipation and may also interfere with the absorption of iron and zinc- though this effect is not well established. High intake of calcium from supplements, but not foods, has been associated with increased risk of kidney stones.
Diet influences calcium metabolism via the acid-base balance. The acid-base balance is kept under tight homeostatic control via the physiological buffering systems of the lungs, kidneys and plasma protein buffers. If the capacity of these systems is exceeded, bone mineral is mobilised to contribute bicarbonate and other anions to buffer the acid. Fruit vegetables are rich in alkaline potassium salts; increasing their consumption reduces dietary acid load, which may contribute to the conservation of bone mineral.
Urinary calcium excretion rises when the filtered load of calcium increases or tubular reabsorption decreases. Acidifying agents, dietary sodium, protein and caffeine raise excretion. Phosphorus, alkaline agents lower excretion. One teaspoon of salt (NaCl) raises urinary calcium by 40mg calcium/day. Purified sulphur containing amino acids (methionine and cysteine) cause significant calciuria, but this is mitigated by phosphate in whole foods. There is evidence that vegetarians with an alkaline urine excrete less urinary calcium than meat eaters, who have an acid urine. Magnesium deficiency can also cause hypocalcaemia, as it is a cofactor for the secretion or parathyroid hormone. If left untreated, calcium deficiency leads to death.
Requirements and Toxicity
Our dietary needs vary according to gender, ethnicity, age and magnitude of obligatory calcium loss. The recommended daily allowance (RDA) established by the US Food and Nutrition Board of the Institute of Medicine (IOM) (2010), are as follows: 0-6 months 200mg/day, 7-12 months 260mg/day 1-3 years 700mg/day, 4-8 years 1,000mg/day, 9-18 years- 1,300 mg/day , 19-70 years 1,000 mg/day, >70 years 1,200 mg/day.
The Tolerable Upper Intake Levels (ULs) for calcium established by the Food and Nutrition Board are 0–6 months 1,000 mg, 7–12 months 1,500 mg, 1–8 years 2,500 mg, 9–18 years 3,000 mg, 19–50 years 2,500 mg, 51+ years 2,000 mg per day. Getting too much calcium from foods is rare; excess intakes are more likely to be caused using supplements.
There is evidence that the body’s ability to absorb calcium declines in both men and women from the age of 70 and onwards. This is due to lower intestinal resistance and reduced renal synthesis of calcitriol, potentially contributing to the development of senile osteoporosis.
The best sources of calcium are those calcium containing foods which are also low in sodium, sulphur containing amino acids, and high in alkaline potassium salts and phosphate. These sources serve as a dietary source of calcium, without disturbing the acid-base balance, protecting the bodies deposits of calcium in the bones. Such foods are of plant origin, including green vegetables (turnip greens, kale, Chinese cabbage, broccoli, peas, cabbage, spinach), legumes, nuts (almonds, walnuts), chia seeds, soy milk, tofu, cereals and dried fruit.
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