Calcium

• Essential in bone and tooth mineralization (1, 2). Calcium metabolism is regulated by the parathyroid hormone, vitamin D and calcitonin (synthesized by specialized thyroid gland cells).
• Stimulates glycogen (a glucose polymer) breakdown in the liver when energy is needed, e.g. during intense physical activity.
• Regulates activity of several key enzymes in carbohydrate metabolism (related to energy production). Ca²⁺ together with ADP and ATP are allosteric regulators of several citric acid cycle enzymes (pyruvate dehydrogenase (3), isocitrate dehydrogenase and a-ketoglutarate dehydrogenase). It activates the enzymes catalyzing the following reactions:
  1) Pyruvate + CoASH + NAD⁺ ----------> Acetyl-CoA + CO₂ + NADH
  2) Isocitrate + NAD⁺ --------------> a-ketoglutarate + CO₂ + NADH
  3) a-ketoglutarate + NAD⁺ --------> succinyl-CoA + NADH + CO₂
• Triggers muscle contraction - calcium ions are released from intracellular storage sites and bind to troponin C, a component of the contractile apparatus causing the fiber to contract (4). Regulates heart rhythm.
• Mediates many hormonal signals such as those triggered by epinephrine (for smooth muscle contraction) (5) and acetylcholine (a neurotransmitter involved in the transmission of nerve impulse).
• Involved in blood clotting by modulating the activity of the proteins that take part in the process (6). For instance, the sluggish prothrombin activation factor (a protein) becomes very active in the presence of Ca²⁺, activated proaccelerin and a membrane phospholipid. Prothrombin and its activation factor require Ca²⁺ in order to bind to phospholipid membranes at the site of injury. These two proteins are anchored to the membrane via calcium bridges.
• Calcium can act as an intracellular second messenger when extracellular signals cause a transient rise in the cytosolic calcium concentration. As a result processes such as muscle contraction (7) and glycogen breakdown are triggered.
• Involved in gastrointestinal physiology (8).
• Interactions: Calcium interacts with Zn²⁺, Mg²⁺, vitamin D and with vitamin K. Calcium absorption in the gut can be inhibited by aluminium (in cookware, aluminium foil, anticid products), high levels of sodium, magnesium, and dietary fiber. High levels of sodium, alcohol, phosphates (in soft drinks), sugar and high protein intake increase calcium excretion.
• Best food sources: Dairy products, tofu, kale, green leafy vegetables, canned salmon, cooked soybeans, pecans, wheat germ, miso paste, romaine lettuce, dried apricots, roasted peanuts, blackcurrants, cabbage, whole wheat bread, eggs, globe artichoke, pumpkin seeds, celery, cashews, barley, carrots, sweet potatoes, brown rice, and fish.