Vitamin B1: what it is, what it tells you and when it’s tested

Vitamin B1, or thiamine, helps your body turn food into energy, especially sugars. Your brain, nerves and heart need it most. Your body keeps only a small store, so it has to come from food every day.

When it’s tested

Doctors order it when they suspect a shortage: in people who drink heavily, after weight-loss surgery, with long-lasting vomiting, with malnutrition or a very restricted diet. They also order it for confusion, trouble walking or tingling with no clear cause.

In an emergency the doctor takes the blood sample but does not wait for the result: they give vitamin B1 straight away, because it is safe and a delay can do harm.

It is not a routine test for people who feel well: with a varied diet a shortage is rare. That is why Rosa does not test it. Instead it looks at vitamin B12, gamma GT and MCV, which say a lot about B vitamins and alcohol.

What it tells you, and what it does not

It shows how much thiamine is in your blood. If it is low, your body is probably short of it. If it is high, a supplement is usually behind it.

There is no perfect test for vitamin B1, and a normal value does not always rule out a problem when the symptoms are clear. That is why your doctor looks mainly at your symptoms and history.

That is why it is read together with other tests:

• Vitamin B12: A shortage also causes tingling and nerve problems
• Gamma GT: It often rises with alcohol, a common cause of shortage
• MCV: Large red cells point to alcohol or vitamin shortages
• Folate: Another B vitamin that is often short at the same time

Normal vitamin B1 levels

The lab decides what is normal. Some lab reports give 32 to 95 ng/mL, the same for men and women. Other labs print slightly different limits, for example 28 to 85 µg/L, which is the same unit.

Some labs use nmol/L, with higher numbers. The range printed next to your result is what counts.

It is measured on whole blood, and the tube must be kept out of the light. You are usually asked to fast, and not to take supplements before the blood draw.

What your result means

Find the row for your result, in ng/mL. If your report uses another unit, compare your value with the range printed there.

• Below 32: Low. Your body may be short of it. It happens mostly with heavy drinking, after weight-loss surgery, with lasting vomiting or with a very poor diet. Talk to your doctor soon. If you have confusion, eye problems or trouble walking, call 112 or go to the emergency department.
• 32 to 95: Normal. Your body has enough. Nothing to do.
• Above 95: High. It usually comes from a supplement or a recent injection. Extra vitamin B1 leaves the body in urine and no harm is known. Usually nothing to do. Tell your doctor if you take supplements.

If your vitamin B1 is high or low

• Low vitamin B1: Mostly seen with alcohol, after weight-loss surgery or with lasting vomiting. It should be corrected quickly.
• High vitamin B1: It usually comes from a supplement. No harm is known.

Who is at risk of a vitamin B1 shortage

With a varied diet it is rare. It is more common in these cases:

• Alcohol: People who drink heavily often eat little, absorb less vitamin B1 and use up more. It is the most common cause.
• Weight-loss surgery: After a bypass or a sleeve you eat and absorb less. Your doctor checks it in the months after surgery.
• Long-lasting vomiting: From an illness, a treatment or anything else: your body loses what you eat.
• Very poor diets: Malnutrition, long fasts, or a diet made almost only of refined grains.

Symptoms of a vitamin B1 shortage

A shortage can cause:

• Tiredness, poor appetite, irritability
• Tingling, burning or weakness in your legs and feet
• Shortness of breath and swollen legs, if it affects the heart
• Confusion, eyes that move oddly or see double, an unsteady walk

The last group of symptoms can be Wernicke’s encephalopathy, an emergency: call 112 or go to the emergency department.

Vitamin B1 and alcohol

People who drink heavily are the most at risk. Alcohol cuts how much vitamin B1 you absorb and how much your body can use, and people who drink a lot often eat less too.

A severe shortage affects the brain. At first it can look like simple confusion or being drunk, which is why it is sometimes missed.

If you drink a lot, talk to your doctor: they can check your vitamin B1 and help you drink less.

Where it is found: what to eat

Vitamin B1 is in many foods. For example:

• Whole grains: wholemeal bread, pasta and rice, oats
• Pork
• Legumes: beans, peas, lentils
• Nuts and seeds, such as sunflower seeds and walnuts
• Yeast and fortified cereals

It is lost when grains are refined and in cooking water. A varied diet is usually enough: ask your doctor before taking supplements.

What can skew the result

A few things change the value without changing your stores.

• Recent supplements: They can raise the value. Say so before the blood draw.
• A vitamin B1 injection: If you have just had one, the value no longer shows what it was before.
• A tube left in the light: Thiamine breaks down in light. The lab protects the sample.
• The lab and the unit: Different methods and units give different numbers. Always compare with the range on your report.

When to worry

Talk to your doctor if:

• Your vitamin B1 is below the range on your report.
• You drink heavily, have had weight-loss surgery or have been vomiting for days.
• You have tingling or weakness in your legs, or swollen legs and shortness of breath.
• You have sudden confusion, double vision, eyes that move oddly or you cannot stand steadily: call 112 or go to the emergency department.

Frequently asked questions

What does vitamin B1 do? It helps your body turn food into energy, especially sugars. Your brain, nerves and heart need it most.

What are normal vitamin B1 levels? Some lab reports give 32 to 95 ng/mL, others 28 to 85 µg/L. The range printed on your own report is what counts.

What are the symptoms of a vitamin B1 shortage? Tiredness, poor appetite, tingling and weakness in your legs. In severe cases, shortness of breath and swollen legs, or confusion and eye problems: in that case call 112 or go to the emergency department.

Which foods contain vitamin B1? Whole grains, pork, legumes, nuts and seeds, and yeast. Refined grains contain little.

Is high vitamin B1 harmful? No harm is known. Extra vitamin B1 leaves the body in urine. A high value usually comes from a supplement.

Does Rosa test vitamin B1? No. Rosa measures vitamin B12, gamma GT and MCV, which say a lot about B vitamins and alcohol. If your doctor wants vitamin B1, they order it separately.

Sources

• ASST Grande Ospedale Metropolitano Niguarda, laboratory test catalogue: vitamin B1, reference values. Catalogo esami Niguarda
• Centro Diagnostico Italiano, test sheet: vitamin B1 in whole blood, reference values. Scheda esame CDI
• Galvin and colleagues, EFNS guidelines for diagnosis, therapy and prevention of Wernicke encephalopathy. European Journal of Neurology, 2010. PMID 20642790
• Whitfield and colleagues, thiamine deficiency disorders: diagnosis, prevalence, and a roadmap for global control programs. Annals of the New York Academy of Sciences, 2018. PMID 30151974
• EFSA Panel on Dietetic Products, Nutrition and Allergies, dietary reference values for thiamin. EFSA Journal, 2016. PMID 42006014