Folic acid and homocysteine: what the blood test means and whether supplements help

Lab Tests Online UK describes homocysteine as a sulphur-containing amino acid measured in the blood. The test is used to help assess the risk of cardiovascular disease and to investigate conditions such as vitamin B12 or folate deficiency and disorders affecting blood clotting, though its main practical uses are narrower than that sounds. It is used to find out whether someone has the rare inherited condition homocystinuria, and to help decide whether a person is folate-deficient or vitamin B12-deficient, because homocysteine can be raised in both conditions.

Homocysteine is measured in all newborn babies in the UK as part of newborn screening, according to the same source. In adults it is requested only in particular circumstances, for example if someone has symptoms consistent with homocystinuria, such as dislocation of the lens of the eye or some blood clots, or occasionally when vitamin B12 levels are borderline and suspicion of deficiency is high.

Lab Tests Online UK says a high level may suggest vitamin deficiency or, very rarely, homocystinuria, but many other factors can cause it, for example kidney disease or a non-fasting specimen. Levels can increase with age, when a person smokes, and with certain drugs, such as carbamazepine, methotrexate and phenytoin. Levels are lower in women than men, and women's concentrations increase after menopause, possibly because of reduced oestrogen production.

You may be asked to fast for 10 to 12 hours beforehand. It also advises telling your doctor and pharmacist about any traditional or herbal drugs you are taking, since these may interfere with results.

Lab Tests Online UK says there is evidence of an association between homocysteine levels and cardiovascular disease such as heart attack and stroke. However, reduction of homocysteine levels with B vitamins does not reduce the risk, so there is no evidence that homocysteine itself is causing the problem. For that reason, measurement of homocysteine to assess cardiovascular risk, and treatment with folic acid for example, is not standard recommended care.

The British Heart Foundation takes a similar line on supplements in general. It says research shows that supplements do not help to stop or improve heart and circulatory diseases, that in some cases supplements can even be harmful, and that a healthy, varied, balanced diet provides the vitamins and minerals needed. It advises talking to your doctor before spending money on supplements, adding that if you need them they will prescribe or recommend them.

The NHS describes folic acid as a man-made version of vitamin B9, also known as folate. It is used to treat folate deficiency anaemia and to help a baby's brain, skull and spinal cord develop properly and reduce the risk of conditions like spina bifida. Other uses include stopping folate levels getting too low during dialysis and preventing side effects from methotrexate.

If you are pregnant the NHS says you should take a folic acid supplement for the first 12 weeks of pregnancy, and if you are planning a pregnancy you should start before you get pregnant, ideally 3 months before. Some women need a higher dose, so check with a pharmacist, midwife or doctor. For folate deficiency anaemia, the NHS says folic acid tablets are used to restore folate levels and usually need to be taken for 4 months, and good dietary sources of folate include green vegetables such as broccoli, Brussels sprouts and peas.

The NHS says vitamin B12 and folate deficiency can cause extreme tiredness, lack of energy, pins and needles, a sore and red tongue, mouth ulcers, muscle weakness, vision problems, psychological problems and problems with memory and judgement. Pernicious anaemia, in which the immune system attacks healthy cells in the stomach and prevents absorption of B12, is the most common cause of B12 deficiency in the UK. Certain medicines, including anticonvulsants and proton pump inhibitors, can affect how much of these vitamins the body absorbs.

This is why the NHS lists vitamin B12 deficiency anaemia or pernicious anaemia as situations in which folic acid may not be suitable, and why the cause of a low result is worth having confirmed by a clinician. The NHS says a deficiency can often be diagnosed from symptoms and a blood test, that it is important to diagnose and treat it as soon as possible, and that some problems caused by the condition can be irreversible if untreated.

The NHS says possible side effects of folic acid tablets include bloating, wind, feeling sick and loss of appetite, and that the liquid can also cause diarrhoea, vomiting and a skin rash. Rarely it can cause a serious allergic reaction (anaphylaxis), with symptoms that can include a swollen face, throat or tongue, a raised itchy rash and difficulty breathing, and it advises calling NHS 111 if you think you might be having serious side effects.

The NHS says folic acid may not be suitable if you have had an allergic reaction to it before, have vitamin B12 deficiency anaemia or pernicious anaemia, have a coronary stent or have cancer. It lists medicines that may not mix well with folic acid, including epilepsy medicines such as phenytoin, some antibiotics such as co-trimoxazole, sulfasalazine, lithium and aspirin, and advises telling your doctor about any medicines, herbal remedies, vitamins or supplements you take. It also says it is best to avoid or limit alcohol because it can affect how well folic acid works. Taking too many multivitamin tablets can be dangerous.

A raised homocysteine result is usually followed by checking for the possible causes the sources describe, such as low B12 or folate, kidney function, medicines and whether the sample was fasting. Do not start high-dose B vitamins on your own because of a single result. The NHS says folic acid may not be suitable if you have vitamin B12 deficiency anaemia, and that deficiency should be diagnosed and treated as soon as possible. Discuss the result and any supplements with your GP or pharmacist.

On the NHS, homocysteine is not a routine test for assessing heart risk, in line with the laboratory guidance above. Private blood test providers do offer homocysteine and vitamin panels, and results should be reviewed with a clinician who can interpret them alongside symptoms, kidney function, diet and medicines. Folic acid medicines can be prescribed, and some supplement types can be bought from shops and pharmacies.