Iron vs Vitamin B12 Deficiency in Perimenopause: Which Is Making You Tired?
Iron and B12 deficiency both cause fatigue and brain fog during perimenopause. Learn the differences, who is at risk, and how to get the right blood tests.
Why deficiencies are easy to miss in perimenopause
Fatigue, brain fog, low mood, and poor concentration are reported by a large proportion of women during perimenopause, and they are often attributed entirely to hormonal change. However, iron deficiency and vitamin B12 deficiency produce an almost identical set of symptoms, and both become more common in midlife for reasons that go beyond hormones. Heavy or irregular periods, which are a hallmark of perimenopause, significantly increase iron losses. B12 absorption declines with age and is affected by medications that many midlife women use, including proton pump inhibitors and metformin. Without appropriate blood testing, these nutritional deficiencies can go undetected for years.
How iron deficiency presents
Iron deficiency, and the anaemia that develops when it becomes severe, causes persistent tiredness, breathlessness on exertion, difficulty concentrating, pale skin, cold hands and feet, and sometimes a craving for non-food substances such as ice or clay, known as pica. Hair shedding is common and can be significant. Restless legs syndrome, a condition in which unpleasant sensations in the legs disrupt sleep and drive an urge to move them, is closely associated with low iron stores. In perimenopausal women with heavy periods, iron stores can become depleted even if haemoglobin is still technically within the normal range, meaning a ferritin test rather than a simple full blood count is needed to detect early deficiency.
How B12 deficiency presents
Vitamin B12 deficiency produces fatigue, brain fog, and low mood, much like iron deficiency and perimenopause. However, it also has some distinguishing features. Tingling or numbness in the hands and feet is a common early sign of B12 deficiency that does not occur in iron deficiency or perimenopause. A smooth or sore tongue, sometimes called glossitis, is another clue. B12 deficiency can cause a form of anaemia called megaloblastic anaemia, where red blood cells are abnormally large, and it can also damage the nervous system if left untreated for a long time. Memory difficulties and mood disturbances in B12 deficiency can be severe and are sometimes mistaken for early dementia in older women.
Who is at greatest risk in perimenopause
Women who have heavy or prolonged periods during perimenopause are at increased risk of iron deficiency due to cumulative blood loss. Vegetarians and vegans are at higher risk of B12 deficiency because B12 is found almost exclusively in animal products. Women who have been taking proton pump inhibitors or H2 blockers for acid reflux for more than a year are at risk of both B12 and iron deficiency, as stomach acid is needed for absorption of both. Those who have had bariatric surgery, have Crohn's disease or coeliac disease, or have pernicious anaemia, an autoimmune condition that prevents B12 absorption, face higher risk. Both deficiencies are more common than many women and their doctors realise.
Getting the right tests
If you suspect either deficiency, the most useful starting point is a full blood count, iron studies including serum ferritin, and a serum B12 level. Ferritin below 30 micrograms per litre is associated with symptoms of iron deficiency even when haemoglobin is normal, and many experts recommend a ferritin level above 50 or even 70 to support hair, energy, and cognitive function optimally. A serum B12 below 200 nanomoles per litre is generally considered deficient, though some clinicians treat on the basis of symptoms at levels up to 300. Methylmalonic acid and homocysteine are more sensitive markers of functional B12 deficiency if serum B12 is borderline. Requesting these tests specifically is often necessary as they are not always included in routine blood panels.
Treating iron deficiency
Iron deficiency is treated with oral iron supplements, typically ferrous sulfate or ferrous fumarate, taken on an empty stomach or with vitamin C to maximise absorption. Side effects including constipation and stomach discomfort are common. Some women tolerate newer forms such as ferric maltol or liposomal iron better. Taking iron on alternate days rather than daily may improve absorption and reduce side effects. Addressing the underlying cause is equally important: if heavy periods are responsible, managing those through hormonal or other means alongside supplementing iron produces better results than supplementation alone. Intravenous iron is available for women who cannot tolerate oral forms or who need faster restoration.
Treating B12 deficiency
Treatment for B12 deficiency depends on its cause. If absorption is the problem, as in pernicious anaemia or post-bariatric surgery, intramuscular B12 injections are the most reliable treatment because oral supplements cannot be absorbed reliably. For dietary deficiency in vegans or vegetarians, high-dose oral B12 or sublingual B12 supplements are effective. The response to treatment can be dramatic: many women notice improved energy and clarity within weeks. Because both iron and B12 deficiency can coexist with perimenopause and with each other, correcting nutritional deficiencies is often an essential step before assessing how much of the fatigue and cognitive difficulty is truly hormonal in origin.
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