How do I know if I have a vitamin deficiency?

Vitamin deficiencies can have a significant impact on your physical and mental well-being. As well as potentially affecting your energy levels, mood, skin, and immune health, they can also lead to chronic illness further down the line. Identifying vitamin deficiencies can be key to managing your health and well-being, but how do you know if you’re deficient? You might assume a simple blood test will give you the answer, but it’s not always that simple.

In the UK, the NHS only tests for a small number of vitamins, usually vitamin D, B12, and folate and, even then, only if you have clear symptoms that justify the test. The usual ‘full blood test’ from your GP is unlikely to include any vitamins unless specifically requested.

When you are able to access blood tests for vitamins, you may find that even this doesn’t give you all the information you need. A result within the ‘normal’ range doesn’t necessarily mean your levels are optimal for you. Some people experience symptoms even when their blood levels are technically ‘normal’ because they are at the lower end of the range, which may not be enough to support their health. This is known as a subclinical deficiency, which means your levels aren’t low enough to be diagnosed as deficient, but they’re still not good enough for optimal health and could still be causing some symptoms.

Why blood tests may not provide the full picture

Blood tests don’t give us information about how well your body is utilising nutrients. You might have a decent level of vitamins in your blood, but that doesn’t mean your cells are absorbing and using them effectively. Some vitamins, like B12 and magnesium, are better assessed at a cellular level, and this is more difficult to test for.

Also, recent supplementation can temporarily raise blood levels without reflecting your body’s actual long-term status. This means that if you take a daily supplement before a test, your results might look fine, even if your body is still lacking the nutrients it needs. This is especially the case with B12, where supplementation in the weeks before can potentially impact blood test results. Urinary levels of the metabolite methylmalonic acid can give us more solid information on how well your body is absorbing and using B12.

Common vitamin deficiencies and their symptoms

Since standard blood tests don’t always give the full picture, paying attention to your health and noting any symptoms is important. Some symptoms are commonly associated with low levels of specific vitamins.

Vitamin D deficiency

Signs of low levels of vitamin D include fatigue, poor sleep, mood issues (especially during the winter months), a tendency to catch colds and viruses, and muscle and joint pain. People who have less sun exposure are more prone to deficiency. Between October and March, we are unable to get sufficient vitamin D from sunlight in the UK, meaning that we rely on stored vitamin D. If levels are already marginal, they will dip over the winter months.

Vitamin B12 deficiency

Vitamin B12 deficiency can cause tiredness, a swollen or burning tongue, brain fog, vertigo, tingling or numbness in the hands and feet, and memory and cognitive issues. Some of these symptoms can also be seen with B9 deficiency. There can be problems absorbing B12 in the stomach if pernicious anaemia is present or if you need to take medication that might affect absorption. Vegans and vegetarians are at risk of deficiency due to lower dietary intake.

Vitamin C deficiency

Signs of vitamin C deficiency can be varied, but skin issues can be a sign, especially easy bruising, poor collagen formation, or bleeding gums. You might find that cuts and wounds take longer to heal. It can also affect our immune health.

Vitamin A is also connected with skin and immune health, so it would overlap with some of these symptoms, but this vitamin is also important for vision and eye health. Signs of a vitamin A deficiency might include dry eyes or poor eyesight.

Factors that affect nutrient absorption

Nutrient needs are individual and depend on genetics, health conditions, diet, and lifestyle. Some people have genetic variations that affect how well they absorb or process certain nutrients. For example, MTHFR mutations can make it harder to convert folic acid into its active form, meaning you may need more folate from your diet. Others may have genetic differences that increase their need for vitamin D or vitamin A.

Your digestive health also influences how well you absorb nutrients. Conditions like IBS, IBD, or coeliac disease can reduce nutrient absorption, making deficiencies more likely despite dietary intake. High stress levels, poor sleep, and alcohol consumption can also deplete essential nutrients, making it even more important to assess individual needs rather than relying on general recommendations.

If you suspect a deficiency, an experienced nutritionist can assess your symptoms, diet, health history, and lifestyle to identify potential deficiencies. This will form part of their initial assessment before producing an individualised programme for you. They will be able to guide you on the best way to correct deficiencies, and if you do need to supplement, they can guide you on the correct dose and forms to avoid imbalances. A personalised, symptom-based approach can be the best way to identify and correct any potential deficiencies.