The vitamin deficiencies nobody warns you about on GLP-1 therapy.

The quiet problem with eating less

Micronutrient deficiencies develop slowly. Unlike nausea — which is immediate and obvious — a B12 deficiency that develops over six months of reduced intake shows up as fatigue, brain fog, and numbness that's easy to attribute to almost anything else.

This makes micronutrient gaps particularly insidious on GLP-1 therapy. People are often feeling better in obvious ways (weight loss, reduced joint pain, improved metabolic markers) while quietly developing deficiencies that will surface 6–18 months later.

Vitamin B12 — the highest priority

B12 is found almost exclusively in animal products: meat, fish, dairy, eggs. It is not found in plant foods in meaningful amounts. On a reduced-appetite diet, B12 intake is among the first to fall — particularly for people who were already eating limited animal products.

B12 deficiency is slow to develop (the body stores it for 2–5 years) but severe in consequence: neurological damage, anemia, and cognitive impairment that is not always reversible. Fatigue and brain fog are the early warning signs.

The form matters. Methylcobalamin (the active form) requires no liver conversion and is more bioavailable than cyanocobalamin (the synthetic form used in most cheap supplements). The dose in Anchor Complete is 500mcg methylcobalamin — above the RDA to account for absorption variability.

Vitamin D3

Vitamin D is synthesized through sun exposure but is also present in fatty fish, egg yolks, and fortified foods. Deficiency is common in the general population — by some estimates affecting 40% of Americans — and GLP-1 therapy doesn't create the deficiency but can deepen one that already exists.

D3 (cholecalciferol) is the form produced by the skin and is more effective at raising blood levels than D2. At 2,000 IU, the dose in Complete covers daily maintenance for most people; those with documented deficiency often need higher doses under medical supervision.

Iron and zinc

Iron comes from red meat and leafy greens (heme and non-heme iron, respectively). Zinc comes from meat, shellfish, and seeds. Both are common targets of deficiency in calorie-restricted populations. The symptoms — fatigue, weakened immune response, hair loss — overlap significantly with each other and with GLP-1-related side effects, making attribution difficult without blood work.

Iron supplementation should only be done with confirmed deficiency via blood test — excess iron is harmful. Zinc at moderate doses (8–11mg daily) is generally safe.

What blood tests to ask for

If you're on GLP-1 therapy and experiencing persistent fatigue, cognitive fog, or hair changes, ask your prescriber for a basic micronutrient panel:

• Serum B12 (and ideally MMA or homocysteine as functional markers)
• 25-hydroxyvitamin D
• Ferritin and serum iron (full iron panel)
• Zinc (whole blood or RBC zinc is more accurate than serum)

These are standard labs and rarely require a specialist referral.