B12 Deficiency: Signs, Symptoms, and Labs — A Simple Guide to a Sneaky Energy Problem

Let me tell you something I see constantly — women walking around exhausted, foggy, irritable, struggling to lose weight, dealing with numb hands, headaches, mood swings… and they assume it’s stress, age, burnout, motherhood, thyroid, “getting older,” or all of the above.

Then their labs come back with a “normal” B12 of 350 and their doctor says, “Looks fine!”

But here’s the thing…
Fine isn’t always optimal.
And B12 is one of the most overlooked causes of fatigue I see.

If your brain feels like you’re thinking through mud, if your energy crashes, if you forget what you walked into the room to do — B12 might be worth checking.

Let’s dive into what B12 actually does, how deficiency shows up, what labs matter, and how to catch issues early.

What B12 Actually Does (and Why It Matters)

Vitamin B12 (cobalamin) is a major player in energy, mood, nerve health, and metabolism.

You need B12 for:

• Energy production

• Red blood cell formation

• DNA & neurotransmitter synthesis

• Nervous system function

• Brain health & memory

• Methylation + detox pathways

When it’s low, the whole system glitches.
Think of B12 as the spark plug — your car may “run” without it, but not well, not fast, and definitely not efficiently.

Low B12 = low energy.
Low B12 = anxiety + depression symptoms.
Low B12 = numbness, burning feet, tingling hands.
Low B12 = brain fog and memory issues.

It’s wild how many symptoms connect back to one nutrient.

Signs & Symptoms of Low B12

Symptoms can be subtle — or loud.

Common signs:

• Fatigue & low stamina

• Brain fog or slow thinking

• Memory issues or word-finding difficulty

• Tingling, numbness, “pins & needles”

• Headaches or migraines

• Pale skin

• Mood changes — anxiety or depression

• Dizziness / feeling off balance

• Shortness of breath with exertion

• Muscle weakness

• Low motivation

• Cold hands/feet

• Poor exercise tolerance

And here’s a big one:

You can have symptoms even if your B12 is in the lab’s “normal” range.

This is why knowing optimal vs normal matters.

Understanding Your B12 Labs

Standard Serum B12 Ranges

(These vary by lab, but generally):

• Normal: ~200–900 pg/mL

• Functional optimal: 450–900 pg/mL

• Symptoms often appear below 450, even if flagged as normal.

Why the difference?

Because ranges are set to detect severe deficiency, not optimal function.
And I don’t know about you — but I want optimal, not “bare minimum survival mode.”

Other Labs That Matter With B12

Sometimes serum B12 looks okay, but the body isn’t using it well.
These labs help clarify the picture:

What Causes B12 Deficiency?

Not just diet — let’s clear that up.

Common reasons B12 drops:

• Low stomach acid (VERY common with age + stress)

• Pernicious anemia / autoimmune issues

• Vegan/vegetarian diets (B12 is mostly animal-based)

• Metformin use (yep — blood sugar meds can reduce absorption)

• PPIs or antacids (reduce stomach acid needed for absorption)

• Bariatric surgery or gut malabsorption

• Chronic inflammation or H. pylori

• Poor intrinsic factor production

Quick breakdown like you requested:

• Vegans/Vegetarians: Harder to get B12 from diet — testing yearly is smart.

• Metformin: Research shows it can lower B12 levels over time.

• Bariatric/gut issues: Reduced absorption — supplementation often required.

• Pernicious Anemia: Autoimmune condition affecting intrinsic factor.

• Aging/Low Stomach Acid: Absorption drops naturally; deficiency is common.

Functional Clues B12 Might Be Low (Even with “Normal” Labs)

If these line up — test.

• You need caffeine to feel human

• Afternoon crashes hit hard

• Anxiety but no clear cause

• Forgetting words mid-sentence

• Feeling “slower” mentally

• Numbness/tingling hands or feet

• Exercise recovery feels harder

• Shortness of breath walking stairs

It’s not always burnout — it can be biology.