Short answer: You're still deficient in B12 because most all-in-one supplements contain symbolic doses—often 5–10 mcg—designed to check a box on the label rather than deliver therapeutic support. Adult needs range from 100–1,000 mcg daily depending on absorption factors, age, and gut health1. Rebuilt's Pure Greens is a greens powder, not a B12 supplement — it contains no B12 at all, so if your bloodwork is low, a dedicated B12 product is what you need. If you're tired, foggy, or just got bloodwork showing low B12 despite taking a multivitamin, underdosing is almost always the culprit—and it's time to demand transparency from your supplement stack.
What Is the B12 Underdosing Problem in All-in-One Supplements?
The underdosing problem is simple: supplement manufacturers pack dozens of ingredients into a single capsule or scoop, leaving minimal room for effective doses of any individual nutrient. B12 ends up as window dressing—present on the label but absent in impact. A typical all-in-one multivitamin contains 6 mcg of cyanocobalamin, the synthetic form that requires conversion in your liver. Meanwhile, the National Institutes of Health notes that absorption declines sharply with age, digestive conditions, and certain medications, meaning you may absorb as little as 1–2 mcg from that 6 mcg dose1.
Compare that to clinical practice: physicians routinely prescribe 1,000 mcg sublingual or intramuscular B12 to correct deficiency. The gap between "meeting the RDA on paper" and "restoring optimal levels in your blood" is the underdosing problem in a nutshell. You take your supplement daily, trust the label, and wonder why fatigue, brain fog, and pins-and-needles persist. The answer lies in dose, form, and absorption—three variables most all-in-ones ignore in favor of ingredient count and marketing.
Why Does Form Matter? Cyanocobalamin vs. Methylcobalamin
Not all B12 is created equal. Cyanocobalamin is the cheap, stable, synthetic form found in most multivitamins. Your body must convert it to methylcobalamin or adenosylcobalamin—the active coenzymes—before it can support DNA synthesis, red blood cell formation, and myelin sheath integrity1. For some people, especially those with MTHFR gene variants or compromised methylation pathways, that conversion is sluggish or incomplete. You're taking B12, but your cells never see it.
Methylcobalamin, by contrast, is already active. It bypasses the conversion step and enters metabolic pathways immediately. Studies show methylcobalamin is retained longer in tissues and may be more effective for neurological support1. That is the form to look for when you buy a dedicated B12 product. Pure Greens contains no B12 — it is thirteen ingredients with every amount printed on the label, not a marketing grid.
How Much B12 Do You Actually Need?
The RDA for B12 is 2.4 mcg per day for adults—a baseline to prevent overt deficiency diseases like pernicious anemia. But "not diseased" is not the same as "optimal." Functional medicine practitioners and endurance athletes routinely use 100–1,000 mcg daily to support energy production, cognitive function, and recovery. Absorption is the bottleneck: even a 1,000 mcg oral dose may yield only 10–20 mcg absorbed, depending on intrinsic factor availability and gut health1.
If you're over 50, take metformin or proton-pump inhibitors, follow a vegan diet, or deal with digestive issues like bloating or irregular bowel movements, your absorption drops further. Stomach bloating and gut imbalances directly impair nutrient uptake, creating a vicious cycle: poor gut health → low B12 → fatigue and brain fog → worse digestion. Breaking that cycle takes two things: an adequately dosed B12 product, and enough vegetables behind it.
7 Concrete Tips to Fix Your B12 Deficiency (Even When Taking Supplements)
1. Check the Label for Dose and Form
Look for methylcobalamin or adenosylcobalamin, not cyanocobalamin. Check the dose: anything under 50 mcg in a daily multivitamin is symbolic. If the label lists 40+ ingredients, divide the total capsule weight by ingredient count—you'll see how little space each nutrient gets. Rebuilt's Pure Greens contains exactly 13 ingredients in an 11-gram serving with every amount printed on the label — B12 is not one of them.
2. Test Your Levels, Then Dose Accordingly
Ask your doctor for a serum B12 or methylmalonic acid (MMA) test. Serum B12 below 400 pg/mL often correlates with fatigue and cognitive symptoms, even if labs call it "normal." Use your results to guide dosing: mild deficiency may respond to 100–250 mcg daily, while severe cases may require 1,000 mcg or intramuscular injections initially. Rebuilt doesn't diagnose or treat deficiency, but we design our greens blend to support normal energy levels and cellular function for those already addressing nutritional gaps2,3.
3. Support Absorption with Gut-Friendly Nutrients
B12 absorption depends on intrinsic factor, stomach acid, and a healthy gut lining. Pure Greens is a daily vegetable base — spinach, broccoli, wheatgrass, barley grass and purple kale, spirulina, vitamin C and zinc, plus prebiotic chicory fibre with turmeric and ginger — that sits alongside your B12 product rather than replacing it. If you're dealing with bloating or irregular digestion, addressing gut health is step one—B12 supplementation is step two.
4. Avoid Conflicting Medications and Lifestyle Factors
Proton-pump inhibitors, H2 blockers, and metformin all interfere with B12 absorption. Alcohol and chronic stress deplete B12 stores. If you take any of these medications or skip breakfast and notice energy crashes by lunch, your cortisol surge may be draining B12 alongside other nutrients. Fasting without morning nutrition can amplify deficiencies—supporting your body with a greens blend first thing helps stabilize energy and nutrient status throughout the day.
5. Pair B12 with Folate and B6 (the Methylation Triad)
B12, folate, and B6 work together in methylation cycles that regulate mood, energy, and DNA repair. Deficiency in one often signals deficiency in another. Many all-in-ones dose folate as folic acid, which—like cyanocobalamin—requires conversion. Look for methylfolate (5-MTHF) and pyridoxal-5-phosphate (active B6). Rebuilt formulates with this triad in mind, ensuring your greens blend complements rather than competes with your body's biochemistry.
6. Consider Sublingual or Liquid Forms for Better Uptake
Sublingual B12 bypasses the gut entirely, dissolving under your tongue and entering the bloodstream directly. This is especially useful for anyone with digestive conditions, post-bariatric surgery, or low stomach acid. Pure Greens is a powder mixed into water or smoothies and contains no B12, so it is not an alternative to a sublingual product here.
7. Track Symptoms, Not Just Lab Numbers
Fatigue, brain fog, mood swings, tingling hands and feet, and pale skin are all hallmark B12 deficiency symptoms. Even if your serum B12 is "normal," persistent symptoms warrant investigation. Keep a daily log of energy, focus, and digestion for two weeks after starting a properly dosed B12 regimen. Most people notice improvements within 7–14 days—if you don't, recheck absorption, form, and dose. Boosting energy naturally requires more than one nutrient; it demands a system where every ingredient works synergistically, which is why Rebuilt keeps the formula tight and transparent.
Support Normal Energy and Vitality with Pure Greens
Rebuilt's Pure Greens contains 13 carefully selected ingredients — spinach, broccoli, wheatgrass, barley grass and purple kale, plus spirulina, vitamin C and zinc, and prebiotic chicory fibre with turmeric and ginger. Vitamin C contributes to normal energy-yielding metabolism and to the reduction of tiredness and fatigue; zinc contributes to normal cognitive function. It contains no B12. Every amount is printed on the label in an 11-gram serving — no proprietary blends, no filler, no lock-in subscriptions.
Shop Pure Greens and see exactly what you are getting.
Why All-in-One Supplements Can't Deliver Therapeutic Doses
Simple math: a standard capsule holds about 500 mg of powder. If a multivitamin contains 40 ingredients, each averages 12.5 mg per capsule—barely enough for minerals like magnesium or zinc, let alone vitamins that require microgram precision. Manufacturers work around this by using the cheapest, most compact forms (cyanocobalamin, folic acid, magnesium oxide) and dosing to the RDA, not to therapeutic need. The result is a supplement that looks comprehensive on Amazon but delivers little in your bloodstream.
Rebuilt takes the opposite approach: 13 ingredients, 11 grams per serving, every amount printed on the label. You won't find 40+ ingredients fighting for space. You will find vegetables, spirulina, vitamin C and zinc — and no B12 at all, because we would rather leave a nutrient out than list it in a dose that does nothing. Transparency isn't a marketing gimmick—it's the foundation of trust.
| Feature | Typical All-in-One | Rebuilt Pure Greens |
|---|---|---|
| Ingredient Count | 40–50+ | Exactly 13 |
| B12 | Cyanocobalamin, RDA-level | Not included — take a dedicated B12 product |
| Proprietary Blends | Common (hidden doses) | None—every ingredient fully disclosed |
| Serving Size | 1–2 capsules (~1 g total) | 11 g powder (ample room for effective doses) |
| Subscription Lock-In | Often required for discount | Optional, never mandatory |
What Are the Symptoms of B12 Deficiency and How Long Until You Feel Better?
B12 deficiency creeps in slowly. Early signs include fatigue that doesn't resolve with sleep, difficulty concentrating, mood swings, and a feeling of "running on empty" even after rest. As deficiency progresses, you may experience tingling or numbness in hands and feet (peripheral neuropathy), pale or jaundiced skin, glossitis (smooth, sore tongue), and balance problems. Severe deficiency can cause irreversible neurological damage, which is why early intervention matters1.
Once you start supplementing with a properly dosed, bioavailable form of B12, most people notice improvements within one to two weeks. Energy stabilizes, brain fog lifts, and mood evens out. Neurological symptoms may take longer—several weeks to months—depending on severity. The key is consistency: B12 is water-soluble, so your body excretes excess, but chronic low intake means chronic depletion. A dedicated B12 product is what maintains your levels here; a greens blend such as Pure Greens is a daily vegetable base alongside it, not a replacement.
How Rebuilt Thinks About Dosing
Rebuilt built Pure Greens around a simple principle: list fewer ingredients and print every amount. Here's what that means in practice:
- No B12 at all: We leave it out rather than print a token dose — if you are deficient, buy a dedicated B12 product.
- 13 ingredients, 11-gram serving: Vegetables, spirulina, vitamin C and zinc, plus prebiotic chicory fibre with turmeric and ginger.
- Vitamin C and zinc: Vitamin C contributes to normal energy-yielding metabolism and to the reduction of tiredness and fatigue; zinc contributes to normal cognitive function.
- Full transparency: No proprietary blends, no hidden doses—every ingredient listed with its exact amount.
- Danish-developed: Every ingredient and every amount is printed on the label.
- No subscription lock-in: Order once, subscribe if it works for you, cancel anytime—your choice, your pace.
When you mix Pure Greens into your morning routine, you're not hoping for results—you're engineering them. One scoop delivers the nutrients your all-in-one multivitamin promises but can't fit. You support normal energy levels, cognitive function, and digestive wellness in a single step, freeing you to focus on work, movement, and becoming the best version of yourself2,3.
Can You Get Enough B12 from Food Alone?
Theoretically, yes—if you eat animal products daily. B12 is found almost exclusively in meat, fish, eggs, and dairy. A 3-ounce serving of salmon provides about 4.8 mcg; a cup of milk, 1.2 mcg; one egg, 0.6 mcg1. Add those up across a day, and you can hit the RDA. But remember: the RDA prevents deficiency, it doesn't optimize function. And if you're vegan, vegetarian, over 50, or dealing with gut issues, food alone rarely suffices.
Even omnivores struggle. Modern farming practices, soil depletion, and food processing reduce nutrient density. Stress, alcohol, and medications further drain B12 stores. Vitamin D deficiency despite supplementation follows the same pattern: underdosing and poor absorption. The solution is the same: choose supplements designed to deliver, not just to check a box. Rebuilt makes that choice simple.
Where to Find a Properly Dosed B12 Supplement
Rebuilt does not sell a standalone B12 supplement, so if your levels are low, buy a dedicated B12 product: look for methylcobalamin or adenosylcobalamin, a dose stated in micrograms on the label, and no proprietary blend hiding it. What Rebuilt does make is Pure Greens — thirteen transparently listed ingredients including spirulina, vitamin C and zinc. Vitamin C contributes to normal energy-yielding metabolism and to the reduction of tiredness and fatigue. It is a greens powder, not a B12 supplement.
We're not here to sell you 40 ingredients you don't need—we're here to give you 13 you can read straight off the label. Your bloodwork tells the B12 story, and a greens powder is not a substitute for it. Trust the process, fuel the work, and rebuild yourself one scoop at a time.
Start Your Day with Pure Greens
Rebuilt's Pure Greens is a daily vegetable base, not a B12 supplement — it contains no B12. Thirteen ingredients, every amount printed on the label, Danish-developed. Vitamin C contributes to normal energy-yielding metabolism and to the reduction of tiredness and fatigue; zinc contributes to normal cognitive function. No filler, no fluff, no lock-in.
Shop Pure Greens and see exactly what you are getting.
Frequently Asked Questions About B12 Deficiency and Underdosing
Why am I still deficient in B12 even though I take a multivitamin every day?
Most multivitamins contain 5–10 mcg of cyanocobalamin, a synthetic form that requires conversion and is poorly absorbed—especially if you're over 50, take certain medications, or have gut issues. The dose is symbolic, not therapeutic. Switching to a supplement with methylcobalamin in a higher dose (100–1,000 mcg) is what restores levels1.
What is the difference between cyanocobalamin and methylcobalamin?
Cyanocobalamin is the cheap, stable, synthetic form of B12 used in most supplements. Your liver must convert it to methylcobalamin or adenosylcobalamin before your cells can use it. Methylcobalamin is already active, bypasses conversion, and is retained longer in tissues—making it more effective for neurological support and energy production1. Pure Greens contains no B12, so look for methylcobalamin in the dedicated B12 product you buy.
How much B12 should I take daily if I'm deficient?
The RDA is 2.4 mcg, but therapeutic doses range from 100–1,000 mcg depending on severity, age, and absorption. Mild deficiency may respond to 100–250 mcg daily; severe cases may require 1,000 mcg or intramuscular injections initially. Always test your levels with a doctor and adjust dosing based on symptoms and bloodwork. Pure Greens plays no part in that — it contains no B12.
Can spirulina and chlorella help with B12 absorption?
While spirulina and chlorella contain trace amounts of B12 analogs (not all bioavailable), their primary benefit is supporting gut health, reducing oxidative stress, and promoting a balanced microbiome—all of which improve nutrient absorption overall2,3. Pure Greens contains spirulina but no B12, so treat it as a vegetable base rather than a B12 source.
How long does it take to correct a B12 deficiency?
Most people notice improvements in energy, mood, and focus within 7–14 days of starting a properly dosed B12 regimen. Neurological symptoms like tingling or numbness may take several weeks to months to resolve, depending on severity. Consistency is key: B12 is water-soluble, so your body doesn't store large reserves. Daily support through a greens blend ensures you maintain optimal levels long-term1,2,3.
Should I take B12 with other B vitamins?
Yes. B12, folate (B9), and B6 work together in methylation cycles that regulate energy, mood, and DNA repair. Deficiency in one often signals deficiency in another. Look for supplements that include methylfolate and pyridoxal-5-phosphate (active B6) alongside methylcobalamin. Pure Greens contains none of these B vitamins, so look for them in the dedicated B12 or B-complex product you choose.
Where can I buy a B12 supplement that actually works?
Look for methylcobalamin or adenosylcobalamin in doses above 50 mcg, full ingredient transparency, and the dose stated in micrograms. Rebuilt does not carry a B12 supplement — buy one from a brand that specialises in single-nutrient dosing. Pure Greens sits alongside it as a daily greens powder with vitamin C and zinc, not as a B12 source.
Claims Overview
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12 Fact Sheet for Health Professionals. Accessed August 2026. Available at: https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
- Kalafati M, et al. Ergogenic and antioxidant effects of spirulina supplementation in humans. Med Sci Sports Exerc. 2010;42(1):142-151. PMID: 19936191.
- Lu HK, et al. Preventive effects of Spirulina platensis on skeletal muscle damage under exercise-induced oxidative stress. Eur J Appl Physiol. 2006;98(2):220-226. PMID: 16944194.
References
- National Institutes of Health, Office of Dietary Supplements. Vitamin B12 Fact Sheet for Health Professionals. Accessed August 2026. Available at: https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
- Kalafati M, Jamurtas AZ, Nikolaidis MG, et al. Ergogenic and antioxidant effects of spirulina supplementation in humans. Med Sci Sports Exerc. 2010;42(1):142-151. PMID:19936191.
- Lu HK, Hsieh CC, Hsu JJ, Yang YK, Chou HN. Preventive effects of Spirulina platensis on skeletal muscle damage under exercise-induced oxidative stress. Eur J Appl Physiol. 2006;98(2):220-226. PMID:16944194.
These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.
