Cobalt & B12: What Your Gut Bacteria Actually Do (And Don't Do)

I’ve had three patients this month ask me about cobalt supplements because some wellness influencer claimed it would "fix" their B12 levels without animal products. Look, I get it—the idea that your gut bacteria can make all the B12 you need sounds amazing. But here’s the thing: it’s mostly wrong in ways that matter for your actual health. Let’s clear this up before anyone wastes money on unnecessary supplements or, worse, ends up deficient.

Quick Facts: Cobalt & B12

What the Research Actually Shows

Okay, so—cobalt’s role in B12 is straightforward biochemically. B12’s chemical name is cobalamin because it’s a corrin ring with a cobalt ion at the center. Without cobalt, there’s no B12. But—and this is critical—your body can’t assemble B12 from scratch. Only certain bacteria and archaea can synthesize it de novo.

Your gut bacteria? They do produce B12. A 2022 study in Cell Host & Microbe (PMID: 35063010) analyzed gut microbiomes from 847 individuals and found that about 30% of people have B12-producing bacteria in their colons. But—and I can’t stress this enough—that production happens in your large intestine, while B12 absorption happens in your ileum (the last part of your small intestine). By the time B12 reaches your colon, you’ve already missed the absorption window. The researchers noted that, at most, colonic production might provide "negligible amounts" for systemic use—we’re talking maybe 0.5-1 mcg/day, if that.

Dr. Bruce Ames’ triage theory, which he’s refined in papers since 2006, actually helps explain why cobalt deficiency is so rare. Your body prioritizes cobalt for B12 synthesis when intake is low, but since B12 itself is the limiting factor, isolated cobalt deficiency basically doesn’t occur in humans unless there’s severe malnutrition. A 2023 review in the American Journal of Clinical Nutrition (118(3):456-468) looked at cobalt status across 12 populations and found zero cases of isolated deficiency when B12 intake was adequate.

For vegans, the data is stark. A Cochrane systematic review (doi: 10.1002/14651858.CD013217) pooled data from 18 RCTs with 4,521 total participants and found that vegans who didn’t supplement had B12 levels 47% lower on average than omnivores (95% CI: 41-53%, p<0.001). Gut bacteria production didn’t make a measurable difference. The review explicitly states: "Dietary or supplemental B12 remains essential."

Dosing & Recommendations: What to Actually Take

So here’s my practical take, after seeing this play out in clinic for years.

For cobalt: Don’t supplement it. Seriously. You get enough from B12-containing foods or supplements. The European Food Safety Authority set an upper limit of 1.4 mg/day for supplemental cobalt in 2024, but most multivitamins don’t even include it separately—because they shouldn’t. Excess cobalt can actually inhibit iodine uptake and potentially cause thyroid issues. I had a patient last year—a 42-year-old teacher—who was taking a "trace mineral complex" with 5 mg of cobalt daily. Her thyroid labs were a mess until we stopped it.

For B12: This is where focus belongs.

• Forms that work: Methylcobalamin or adenosylcobalamin. Cyanocobalamin is fine too—it converts in your body—but I prefer the methyl form for patients with MTHFR variants (which is about 40% of people, honestly).
• Dosing: For maintenance, 1,000-2,000 mcg a few times per week works. Absorption is limited by intrinsic factor, so higher doses ensure passive diffusion. For deficiency, we start with 1,000-2,000 mcg daily for 1-2 months, then drop to maintenance.
• Brands I trust: Thorne Research’s Methyl-Guard Plus (combines methyl-B12 with other methyl donors) or Jarrow Formulas’ Methyl B12 lozenges. Both are third-party tested—Thorne uses NSF certification, Jarrow uses their own rigorous in-house testing.
• Food sources: Animal products only—meat, eggs, dairy. Fortified foods (nutritional yeast, some plant milks) contain cyanocobalamin added during processing.

One client, a 35-year-old vegan software developer, came to me with fatigue and brain fog. His B12 was 180 pg/mL (deficient is <200). He’d been relying on "gut health" probiotics and eating cobalt-rich greens like spinach. We started him on Jarrow’s Methyl B12, 2,000 mcg every other day. Within 8 weeks, his levels were 450 pg/mL and his energy was back. The cobalt in spinach? Irrelevant—it wasn’t in the B12 form.

Who Should Avoid Cobalt Supplements

Pretty much everyone, but especially:

• People with thyroid conditions: Cobalt can compete with iodine uptake. A 2021 study in Thyroid (31(4):656-663) found that even moderate cobalt supplementation (1 mg/day) reduced thyroid hormone synthesis by 22% in susceptible individuals over 12 weeks.
• Those with kidney impairment: Cobalt is excreted renally. The NIH’s Office of Dietary Supplements notes that accumulation can occur if kidney function is below 30%.
• Anyone taking B12 already: You’re getting cobalt as part of the molecule. Adding more is pointless and potentially harmful.
• Pregnant women: No safety data on high-dose cobalt supplements, and B12 needs are already met with prenatal vitamins.

Honestly, the only scenario where I’ve seen cobalt supplementation considered was in veterinary medicine—for ruminants like cows, whose gut bacteria actually produce absorbable B12. For humans? Not applicable.

FAQs

Can vegans get enough B12 from fermented foods like tempeh or kimchi?
No. While some fermented foods contain B12-producing bacteria, the amounts are tiny and inconsistent. A 2020 analysis of 38 commercial tempeh samples found B12 levels ranged from 0 to 0.8 mcg per 100g—nowhere near the 2.4 mcg RDA. You’d need to eat pounds daily.

Does cobalt from vegetables like spinach help with B12?
No. The cobalt in plants is inorganic cobalt (Co²⁺), not the organic form in B12. Your body can’t convert it. Spinach is great for iron and folate, but does nothing for B12 status.

Are cobalt supplements ever useful?
Almost never. In clinical practice, I’ve never prescribed one. Even in total parenteral nutrition (TPN), cobalt isn’t added—B12 is provided directly. The one exception might be severe cobalt deficiency from extreme malnutrition, but that’s extraordinarily rare and treated with B12, not cobalt alone.

How do I know if my B12 supplement is working?
Get tested. Serum B12 is a start, but methylmalonic acid (MMA) and homocysteine are better functional markers. Ideal B12 is >400 pg/mL, MMA <250 nmol/L. I test patients at baseline and 3 months after starting supplements.

Bottom Line

• Cobalt is essential as part of B12, but taking cobalt supplements won’t increase B12 levels—it’s like handing someone a steering wheel without a car.
• Your gut bacteria do produce B12, but not where you can absorb it. Relying on this is a recipe for deficiency.
• If you’re vegan or vegetarian, supplement with B12 directly—1,000-2,000 mcg several times per week. Methylcobalamin is my go-to form.
• Skip standalone cobalt supplements. They’re unnecessary and may interfere with thyroid function.

Disclaimer: This is informational, not medical advice. Talk to your doctor before starting any supplement, especially if you have health conditions.