Your Three Medications — Composition First

Core Answer: When to Start?

Detailed Analysis: Do These Drugs Affect Your Tests?

1. Stool Routine (microscopy, occult blood, culture, ova & parasites)

• Suppress pathogenic bacteria you're trying to detect
• Alter commensal flora patterns used to interpret dysbiosis
• Potentially suppress parasites or give false-negative bacterial overgrowth results

2. Fecal Calprotectin

• Distinguish IBD from IBS (high sensitivity/specificity)
• Gauge mucosal disease activity in IBD
• Predict relapse

3. Blood Tests — CRP, CBC, ESR

4. Whole Abdomen Ultrasound

Summary Recommendation

Note: This analysis is for informational purposes. Your gastroenterologist who has access to your clinical history and investigation results is best positioned to finalize the timing. Given that your stool tests are already complete, starting as prescribed is medically appropriate.

B12 Dosing for Your Exact Profile: A Rigorous Analysis

Your profile, restated clinically

• 29-year-old male, lifelong vegetarian (dietary deficiency, not malabsorption disease)
• Serum B12: 155 pg/mL — confirmed deficient (normal range typically 200-900 pg/mL; functionally significant below ~200 pg/mL)
• No anemia, no neurological symptoms
• SIBO/IBS-type symptoms (creates a wrinkle — more on this below)
• Subclinical hypothyroidism (TSH ~5.8, normal T3/T4) — relevant to B12 co-deficiency risk
• On rifaximin + acotiamide + rebamipide

The Absorption Mechanics — This Is the Foundation of the Entire Decision

• Requires IF from gastric parietal cells, plus a functional terminal ileum
• Highly efficient, but the IF carrier system saturates at approximately 1-2 mcg per dose
• Beyond that saturation point, this mechanism contributes negligibly regardless of how much more you take

• No IF required, no ileum required
• About 1-2% of any dose is absorbed this way
• Operates proportionally — so larger doses yield proportionally larger absolute amounts absorbed via this route

Q1. Is 500 mcg/day likely enough for meaningful correction, or is 1500 mcg more rational initially?

• At 500 mcg/day, you absorb approximately 10-11 mcg/day via combined IF + passive diffusion. Your daily metabolic requirement is ~2.4 mcg, and total body stores need to rebuild from a deficient state to ~2-2.5 mg (most in the liver). That means roughly 10-11 mcg net absorbed daily going toward repletion.
• At 1500 mcg/day, you absorb approximately 16-18 mcg/day. That is a 50-60% increase in absorbed dose — not trivial.
• The BC Clinical Practice Guidelines explicitly recommend 500-2000 mcg/day for adults with dietary deficiency (your category), and specifically note that 1000-2000 mcg/day is appropriate for impaired absorption states.
• Your SIBO introduces uncertainty about how well your IF-mediated pathway is functioning (bacteria in the proximal small bowel can compete for and consume B12 before IF-mediated uptake occurs). This shifts you closer to the "mild impaired absorption" tier, where the higher dose has a stronger rationale.
• The Kuzminski RCT (1998) — the landmark study — showed that 2000 mcg oral daily was comparable to monthly IM injections even in pernicious anemia. This is achievable only because enough passive diffusion occurs at that dose range.

Q2. What matters most for your profile?

Q3. Is there evidence that 1000-2000 mcg produces meaningfully faster/more reliable correction than 500 mcg in vegetarians or mild malabsorption?

• Strong evidence (Kuzminski 1998, Cochrane 2018, BC Guidelines): Very high oral doses (1000-2000 mcg) can normalize serum B12 and correct metabolic markers (MMA, homocysteine) in deficiency states — even pernicious anemia — comparable to IM.
• Specifically for vegetarians: BC Guidelines state that vegetarians are expected to have normal IF-mediated absorption (no parietal cell damage), so the 500-2000 mcg range is appropriate. They do not have malabsorption — they have inadequate intake — meaning their IF pathway is intact and working.
• The Sil et al. 2018 RCT (Korean Journal of Pain, PMID 30013732) — a small but directly relevant comparison in peripheral neuropathy patients: 500 mcg ×3/week (total 1500 mcg/week) produced significantly higher serum cobalamin at end of treatment (1892 ± 234 IU) than 1500 mcg ×1/week (1438 ± 460 IU). The important finding: frequency and total weekly dose both matter more than single-dose size. Daily 500 mcg (3500 mcg/week) would likely outperform once-weekly 1500 mcg.
• For vegetarians specifically, correction at 500 mcg/day is achievable. Faster correction at 1000-1500 mcg/day is plausible and mechanistically supported, but RCT data comparing these doses specifically in vegetarians without malabsorption is absent from the literature.

• Strong evidence: high-dose oral B12 (1000-2000 mcg) corrects deficiency even with malabsorption
• Clinical practice reasoning: 1500 mcg > 500 mcg for faster correction in someone with SIBO uncertainty
• Mechanistic plausibility: more absorbed per day at 1500 than 500 (real but modest ~50-60% more)
• Supplement marketing myth: "500 mcg does almost nothing" or "you need 5000 mcg" — both false

Q4. Could 1500 mcg worsen your GI symptoms (IBS/SIBO)?

• Vitamin B12 at oral doses up to 2000 mcg has no documented GI side effects in clinical trials. There is no plausible mechanism by which it would cause bloating, altered motility, or worsen SIBO.
• Unlike some supplements (magnesium, iron, zinc), B12 is not osmotically active, not fermented by gut bacteria, and does not alter bowel motility at these doses.
• Rebamipide (your Velgut) and acotiamide (Actapro) both work on mucosal and motility targets that are completely unrelated to B12 pharmacology. No interaction.
• Rifaximin's modest microbiota modulation does not interact with B12 absorption or metabolism.

Q5. Any meaningful risk or downside to 1500 mcg/day for 2-3 months?

• B12 is water-soluble and any excess above what is absorbed or retained is renally excreted. There is no established upper tolerable limit (UL) because no adverse effects from high oral doses have been documented.
• Some epidemiological studies have raised a signal about very high serum B12 (>900 pg/mL) being associated with certain cancers — but this reflects underlying disease causing high B12 release (liver disease, myeloproliferative disorders), not supplementation causing harm. Supplementation at 1500 mcg in a deficient individual will not push serum B12 to those pathological levels.
• At your current level of 155 pg/mL, you have significant depletion. The therapeutic goal is to normalize and then maintain — not to overshoot into harm.

Q6. What would I choose if this were my situation?

• Vegetarians with dietary B12 deficiency (intact IF, no ileal disease) respond well to oral high-dose replacement. 1500 mcg is within the well-documented efficacious range.
• At 155 pg/mL, you have meaningful depletion. You are not borderline; this requires genuine repletion, not just prevention.

• SIBO-type symptoms create uncertainty about whether IF-mediated absorption is fully efficient (bacteria may compete). The additional passive diffusion from 1500 vs 500 mcg provides a buffer against this.
• Subclinical hypothyroidism (TSH 5.8) is a recognized co-traveler of B12 deficiency — both are common in vegetarians, and inadequate B12 can impair thyroid metabolism at the cellular level. Faster repletion is prudent.
• Your GI workup is active. If SIBO is confirmed and treated, your absorption will improve — meaning even 500 mcg may become adequate later. But during the uncertainty, the higher dose is the rational choice.
• The Sil 2018 data suggests that frequent dosing matters — daily 1500 mcg (or even daily 500 mcg) is far superior to once-weekly 1500 mcg.

• ~16-18 mcg absorbed/day at 1500 mcg vs ~10-11 mcg/day at 500 mcg. That is real additional repletion over 90 days: roughly 1440-1620 mcg vs 900-990 mcg total absorbed. Meaningful when total body stores need to go from depleted to ~2000-2500 mcg.

Practical Summary