In a 28‑year‑old woman who experiences intermittent numbness or tingling of the lower limbs when she sits with her legs crossed, which mineral deficiencies should be considered?

Mineral Deficiencies in Positional Lower Limb Paresthesias

In a 28-year-old woman experiencing transient lower limb numbness when sitting cross-legged, the most likely culprit is mechanical nerve compression rather than a mineral deficiency, but if deficiencies are present, iron (ferritin <50 ng/mL), vitamin B12, and magnesium should be evaluated first.

Understanding the Clinical Context

The symptom pattern described—sensory loss specifically triggered by sitting with crossed legs that presumably resolves with position change—is highly characteristic of positional nerve compression rather than a systemic metabolic or nutritional deficiency 1, 2. However, underlying mineral deficiencies can lower the threshold for nerve dysfunction and exacerbate symptoms.

Primary Mineral Deficiencies to Consider

Iron Deficiency (Most Important)

Iron deficiency with ferritin <50 ng/mL is the single most important mineral deficiency to evaluate, as reduced intracellular iron in the substantia nigra impairs dopamine transport and can cause peripheral nerve dysfunction 1, 3.

• Check serum ferritin ideally in the morning after avoiding iron supplements for 24 hours 4
• Values <50 ng/mL warrant iron supplementation 1, 3
• Iron deficiency can present with paresthesias and peripheral neuropathy even without anemia 1, 4

Critical pitfall: Ferritin can be falsely elevated by inflammation, so consider checking transferrin saturation (<20% indicates functional iron deficiency) 4

Vitamin B12 Deficiency (Second Priority)

Vitamin B12 deficiency causes demyelination in both central and peripheral nervous systems, producing paresthesias, numbness in the trunk and limbs, and abnormal sensory function 1, 3.

• Neurological symptoms can occur without macrocytic anemia in approximately one-third of cases 1
• B12 deficiency particularly affects sensory rather than motor function, causing loss of proprioceptive, vibratory, tactile and nociceptive sensation 1
• Check serum B12 levels; consider methylmalonic acid (MMA) if B12 is borderline, as elevated MMA indicates tissue deficiency 1

Magnesium Deficiency (Third Priority)

Hypomagnesemia (<1.8 mg/dL or <0.74 mmol/L) can cause neuromuscular hyperexcitability and paresthesias 5, 6, 7.

• Most patients are asymptomatic until magnesium falls below 1.2 mg/dL 5
• Clinical manifestations include paresthesias and neuromuscular irritability 8, 9
• Serum magnesium can be normal despite intracellular depletion, so consider 24-hour urinary magnesium or magnesium load test if clinical suspicion is high 6, 7

Additional Deficiencies in Specific Contexts

Thiamine (Vitamin B1)

Thiamine deficiency causes peripheral neuropathy and neuritis, especially in lower limbs (Dry Beriberi), but typically presents with persistent rather than positional symptoms 1.

• Consider if there is history of prolonged vomiting, rapid weight loss, poor dietary intake, or alcohol abuse 1
• Symptoms include persistent paresthesias, not intermittent positional symptoms 1

Copper and Zinc

Copper deficiency presents with myeloneuropathy and neuromuscular abnormalities, but this is rare outside of bariatric surgery, high-dose zinc supplementation, or malabsorptive conditions 1.

• Zinc deficiency rarely causes neurological symptoms; more commonly presents with skin changes, hair loss, and taste alterations 1

Diagnostic Algorithm

Step 1: Confirm the symptom pattern

• Does numbness occur only with leg crossing and resolve with position change? This suggests mechanical compression, not deficiency 1, 2
• Are symptoms present at rest, worse at night, with urge to move? Consider restless legs syndrome and check ferritin 1, 3, 4
• Are symptoms persistent regardless of position? Consider peripheral neuropathy from B12 or other causes 1

Step 2: Initial laboratory evaluation

• Serum ferritin (morning, fasting from iron supplements) 4
• Complete blood count (CBC) to assess for anemia 4
• Vitamin B12 level 1
• Serum magnesium 5, 6
• Consider HbA1c to exclude diabetic neuropathy 4

Step 3: Interpret results in clinical context

• Ferritin <50 ng/mL: supplement iron 1, 3, 4
• B12 <200 pg/mL or borderline with neurological symptoms: supplement B12 1
• Magnesium <1.5 mEq/L: supplement magnesium 8

Common Pitfalls to Avoid

1. Do not assume positional symptoms indicate deficiency: Transient numbness with leg crossing is typically mechanical nerve compression (common peroneal or tibial nerve) 1, 2

2. Do not rely on CBC alone to exclude B12 deficiency: One-third of B12-deficient patients lack macrocytic anemia 1

3. Do not ignore normal serum magnesium if clinical suspicion is high: Intracellular depletion can exist with normal serum levels 6, 7

4. Do not check ferritin without considering inflammation: Acute phase reactants falsely elevate ferritin; add transferrin saturation if inflammation is present 4

When Deficiency is Unlikely

If symptoms are strictly positional (occur only with crossed legs, resolve immediately with position change), no nocturnal worsening, and no other neurological symptoms, mineral deficiency is unlikely to be the primary cause 1, 2. In this scenario, reassurance about avoiding prolonged leg crossing and evaluation for anatomical nerve compression may be more appropriate than extensive metabolic workup.