Signs of vitamin B12 deficiency, and the groups who actually need to watch for it

Adults need about 2.4 micrograms of vitamin B12 a day, and most people who eat meat, fish, eggs or dairy get it without trying [s1]. Full-blown deficiency, with its blood and nerve consequences, is relatively uncommon [s3]. But it is not rare, and it does not fall evenly: it clusters in older people, in those who avoid animal foods, and around a short list of medicines and operations. The useful question is not whether B12 matters, but whether you are in one of those groups.

What the deficiency feels like

B12 is needed to make red blood cells and to maintain the nervous system, so a shortage strikes on two fronts [s4]. The blood side produces anaemia: fatigue, lack of energy, pale skin, breathlessness on exertion, and sometimes a swollen, red, sore tongue [s2]. You may have no symptoms, or only mild ones [s2].

The nerve side is what makes B12 different from a run-of-the-mill vitamin gap. A low level held for a long time can damage nerves, causing numbness and tingling in the hands and feet, loss of balance, difficulty concentrating and, in severe cases, confusion or changes in mental status [s2]. Those neurological problems can appear without obvious anaemia, and they may become permanent if treatment is not started promptly [s2]. That is the reason clinicians take even mild, unexplained B12 symptoms seriously.

Who is actually at risk

Deficiency comes from three routes: too little intake, poor absorption, or a metabolic problem [s3]. The at-risk groups map onto those routes.

Absorption is the big one, and it is mostly about age. Because 10 to 30 per cent of older people can no longer absorb the B12 naturally bound to food, the Institute of Medicine advises that people over 50 meet their requirement mainly through fortified foods or a supplement, where the vitamin is in a form that is easier to take up [s1]. Layered on top of ageing are the medical causes: pernicious anaemia, an autoimmune loss of the stomach protein needed to absorb B12; surgery that removes parts of the stomach or small intestine, including some weight-loss operations; and digestive diseases such as Crohn's and coeliac disease [s2].

Several everyday exposures also blunt absorption. Long-term use of metformin, long-term use of antacids and other heartburn medicines, heavy alcohol use, and repeated abuse of nitrous oxide (laughing gas) all appear on the list [s2]. These do not guarantee deficiency, but they raise the odds enough to warrant attention over years, not weeks.

Intake is the other clear route. B12 is not present in plant foods unless they are fortified, so a strict vegetarian or vegan diet is a recognised cause, as is poor nutrition in infancy or pregnancy [s2] [s4]. For people eating that way, fortified breakfast cereals, some nutritional yeasts, or a plain B12 supplement close the gap [s4].

How real deficiency is confirmed

The picture is muddier than a single number suggests. A review in Nature Reviews Disease Primers noted that subclinical B12 deficiency affects somewhere between 2.5 per cent and 26 per cent of the general population depending on the definition used — a range wide enough to show how unsettled the cut-offs are, and how much a "low" result depends on the threshold [s3]. Beyond the serum B12 level, clinicians can measure methylmalonic acid and homocysteine, which rise when B12 is functionally short, and test for the antibodies of pernicious anaemia [s2] [s3].

One reassuring piece of physiology: the body stores B12 in the liver, enough for a few years' worth, which is why dietary deficiency develops slowly and why a recent switch in diet is not an emergency [s4].

What to take from this

If you are over 50, eat little or no animal food, have had stomach or bowel surgery, or take metformin or long-term acid-suppressing drugs, B12 is worth a conversation with a clinician — especially if numbness, balance problems or unexplained fatigue have crept in [s1] [s2]. For a younger person eating a mixed diet with no absorption problem, routine worry is not warranted [s1]. This article is informational and not a substitute for medical assessment; confirmed deficiency, particularly from pernicious anaemia, needs proper treatment rather than a self-chosen supplement [s2].