Vitamin B12 for women: deficiency, testing and who's at risk
Low vitamin B12 is a common, treatable cause of fatigue and brain fog. Who's at risk, the symptoms, how it's tested, and how to correct it.
Reviewed by Emma Carter, Senior Health & Nutrition Editor
Vitamin B12 keeps your blood and nerve cells healthy and helps make DNA. When it runs low, the symptoms are easy to mistake for stress or a busy life, which is why it is one of the most under-recognized causes of fatigue and brain fog in women. The good news is that it is simple to test and very treatable once found.
Why B12 matters
Your body uses B12 to make healthy red blood cells, to keep your nervous system working, and to build DNA. Unlike many nutrients, it comes almost entirely from animal foods, so your diet plays a large part in whether you run low. Your liver also stores a few years' worth, which is why a deficiency tends to build slowly and quietly, and why it can catch people out long after their diet changed.
How much you need, and where to get it
Adults need about 2.4 micrograms a day, rising to 2.6 in pregnancy and 2.8 while breastfeeding. The richest sources are animal foods.
| Source | Notes |
|---|---|
| Meat, fish and shellfish | Among the richest natural sources |
| Eggs and dairy | Reliable options for vegetarians |
| Fortified foods | Some plant milks, breakfast cereals and nutritional yeast |
| Unfortified plant foods | Essentially none |
That last row is the catch for anyone eating a plant-based diet, and the reason a supplement matters there.
Symptoms of low B12
Signs build slowly and are easy to overlook:
- Fatigue, weakness and breathlessness
- Pins and needles in the hands or feet, or balance problems
- A sore, smooth or swollen tongue
- Memory lapses, poor concentration and low mood
Left untreated, B12 deficiency can damage the nerves, and some of that damage may not fully reverse, so it is worth catching early rather than waiting.
Who is at risk
Some women are far more likely to run low. Check whether any of these apply to you:
- Vegans and vegetarians, since plant foods contain almost no B12 unless fortified
- Older adults, because absorption declines with age
- People with gut conditions such as celiac or Crohn's, or who have had gut surgery
- Those on metformin or long-term acid-reducing medicines, since metformin, proton pump inhibitors and H2 blockers are all recognized causes (CMAJ, 2022)
- Pregnant and breastfeeding women, whose needs rise
Age is the one that creeps up quietly: B12 deficiency runs at about 6% in people under 60 and rises to roughly 20% in those over 60 (Fujita Medical Journal, 2022).
How it is tested, and the folate trap
A blood test measures your B12 level, usually alongside folate because the two work closely together. If your level is borderline, your doctor may check markers like MMA or homocysteine, which reflect B12 activity more precisely and are recommended exactly when the B12 result is equivocal (CMAJ, 2022), or test for pernicious anemia, an autoimmune cause where the body cannot absorb B12.
Testing both B12 and folate matters for an important reason. Both deficiencies cause a similar anemia, and taking folic acid can correct that anemia while a hidden B12 deficiency keeps quietly damaging the nerves. That is why B12 should be checked and corrected in its own right, not assumed to be fixed by a folate supplement.
How it is treated
Treatment depends on the cause. Established deficiency is corrected with B12 injections or high-dose oral supplements, and any underlying issue is addressed. Injections are often used for pernicious anemia or when absorption is the problem; oral doses can work well for dietary shortfalls. Your doctor decides which fits your situation.
Preventing it on a plant-based diet
If you eat little or no animal food, do not rely on unfortified plants. A regular B12 supplement, or reliably fortified foods, keeps your level topped up. Both common forms, cyanocobalamin and methylcobalamin, work. This is one of the few supplements with a clear, evidence-based case on a vegan or largely plant-based diet, and it sits alongside the other micronutrients worth keeping an eye on.
When to see a doctor
Ask about a B12 test if you have ongoing fatigue, tingling, balance problems or memory issues, especially if you follow a plant-based diet or take metformin or acid-reducing medicines. It is a simple test for a common, treatable problem, and catching it early protects your nerves.
Frequently asked questions
What are the symptoms of low vitamin B12?
Fatigue, weakness, breathlessness, a sore or smooth tongue, pins and needles in the hands or feet, balance problems, and memory or concentration difficulties. Because these are vague, low B12 is often missed until it is tested.
How much vitamin B12 do women need?
Adults need about 2.4 micrograms a day, rising to 2.6 in pregnancy and 2.8 while breastfeeding. It is found almost only in animal foods, so if you eat little or none, a supplement or fortified foods are the reliable way to meet it.
Who is at risk of B12 deficiency?
Vegans and vegetarians, since B12 is found almost only in animal foods, older adults whose absorption declines, people with gut conditions like celiac or Crohn's or who have had gut surgery, and anyone on metformin or long-term acid-reducing medicines.
How is B12 deficiency tested and treated?
A blood test measures your level, usually alongside folate. Borderline results can be clarified with markers like MMA or homocysteine. Established deficiency is corrected with B12 injections or high-dose oral supplements, plus treating any underlying cause.
Can I get enough B12 on a vegan diet?
Not from unfortified plant foods, which contain virtually none. Vegans and most vegetarians need a regular B12 supplement or reliably fortified foods such as some plant milks, cereals or nutritional yeast. It is one supplement genuinely worth taking on a plant-based diet.
References
- Vitamin B12 deficiency · Canadian Medical Association Journal, 2022
- Vitamin B12: Fact Sheet for Consumers · NIH Office of Dietary Supplements
- Involvement of folate and vitamin B12 deficiency in patients with normocytic anemia · Fujita Medical Journal, 2022