PE13.4 | Calcium Magnesium Nutrition — Summary & Reflection

KEY TAKEAWAYS

Calcium Magnesium Nutrition — Key Points:

• Calcium body stores: 99% in bone/teeth as hydroxyapatite; 0.1% ECF as ionised (biologically active), albumin-bound, and complexed fractions. Serum total calcium normal range: 8.5–10.5 mg/dL.

• Calcium RDA (ICMR/NIN 2020): Infancy 200–260 mg/day (AI); 1–3 yr 700 mg; 4–8 yr 1000 mg; 9–18 yr 1300 mg/day (highest, for pubertal skeletal accretion). Key Indian sources: dairy (milk, curd, paneer), ragi (~350 mg/100 g), sesame seeds, dried fish, green leafy vegetables. Bioavailability reduced by phytates, oxalates, vitamin D deficiency.

• Magnesium body stores: 60% bone, 38% intracellular, only 2% in serum. Normal serum Mg: 1.7–2.2 mg/dL. Rich Indian sources: green leafy vegetables, nuts/seeds, whole grains, legumes. RDA peaks at 410 mg/day (14–18 yr males).

• Calcium homeostasis: CaSR on parathyroid → PTH → (bone resorption + renal Ca reabsorption + 1α-hydroxylase activation → 1,25-OH2-D → intestinal Ca absorption). Calcitonin opposes. High phosphate (cow's milk feeds) → falls ionised Ca → late-onset neonatal hypocalcaemia.

• Magnesium–calcium link: Mg2+ required for PTH secretion (exocytosis) AND for PTH receptor Gs-protein signalling. Hypomagnesaemia → functional hypoparathyroidism → refractory hypocalcaemia. Replace Mg first.

• Neonatal hypocalcaemia: Early-onset (0–72 h) — prematurity + IDM; late-onset (after day 4–7) — cow's milk high-phosphate feeds, Mg deficiency. Treatment: IV calcium gluconate 1–2 mL/kg of 10% solution (max 20 mL) slowly with cardiac monitoring.

• Hypocalcaemic tetany: Trousseau sign (carpal spasm with BP cuff inflation — most sensitive), Chvostek sign (facial twitch — less specific), laryngospasm, seizures. Emergency: slow IV calcium gluconate.

• Nutritional rickets: Frontal bossing, rachitic rosary, Harrison's sulcus, widened wrists/ankles, genu varum. X-ray: cupped/frayed metaphyses, widened growth plate. Biochemistry: low Ca, low PO4, high ALP, high PTH, low 25-OH-D. Treatment: vitamin D 2000–4000 IU/day × 3 months + oral calcium. Distinguish from XLH rickets (low PO4 with normal Ca and 25-OH-D — treat with phosphate + calcitriol).

• Hypomagnesaemia management: IV MgSO4 25–50 mg/kg over 30–60 min (max 2 g/dose) for symptomatic cases; oral Mg for mild/maintenance. Always check Mg in refractory hypocalcaemia.

REFLECT

Take a moment to consolidate and apply what you have learned in this SDL:

1. Clinical integration: You are seeing a 3-year-old child with tetany and a serum calcium of 7.0 mg/dL. Your resident colleague is about to give IV calcium gluconate. What additional serum test must you insist on before concluding the workup is complete, and why? How would a positive result change your management?

1. Dietary counselling practice: A mother brings her 15-month-old for a well-child visit. The family is vegetarian (no egg, no fish) and the child has not been introduced to dairy yet. Using the ICMR/NIN 2020 RDA as your benchmark and the knowledge of Indian food sources, name three specific foods you would recommend and explain to the mother how to incorporate them daily to meet the child's calcium RDA of 700 mg/day.

1. Mechanism synthesis: A medical student says: "If a child has hypomagnesaemia, just give calcium — that will fix the low calcium level." Using the PTH secretion and PTH receptor mechanisms, explain in two to three sentences why this approach is incorrect and what must be done instead.

1. Connecting to systems: The early-onset neonatal hypocalcaemia vignette involved an infant of a diabetic mother (IDM). Trace the mechanism: maternal hyperglycaemia → foetal pathophysiology → neonatal Mg/Ca disturbance → clinical presentation. How does this illustrate the importance of calcium and magnesium as a linked pair rather than independent minerals?