All too often, a lack of Iron is posited as the reason for an individual’s consistent feelings of fatigue. Yet, in many cases, an important group of vitamins is overlooked when it comes to fatigue, listlessness, and apathy - B vitamins. If you want to find out more about Vitamin B deficiencies and how you can combat them, keep reading…

“My Iron is normal, so why am I still exhausted?”

If you’ve ever suffered from prolonged bouts of fatigue during your adult life, then you’ve possibly uttered this statement at some point. 

You know the drill, you’ve been feeling far too tired, for far longer than you have before. 

So, you visit your GP.

And, you’ve guessed it, they head straight towards the most obvious lab test - checking your Iron levels. 

Iron deficiency is, admittedly, common, and can be a clear driver of fatigue. 

However, it’s not the sole driver of fatigue. 

In far more cases than is commonly acknowledged, Vitamin B deficiencies can play as big - if not a bigger - role in chronic fatigue than Iron.

That’s because B vitamins act as essential coenzymes - helping to turn nutrients into ATP - your body’s energy substrate. B vitamins also play an important role in red blood cell production and nervous system function. As a result, a shortfall in B vitamins can result in classic tiredness, and related symptoms such as neurological and cognitive fatigue¹. 

In short - the classic diagnostic formula has been ‘tiredness = Iron deficiency’. However, this is NOT always the case - with Vitamin B deficiencies being an oft overlooked culprit.

What is Vitamin B deficiency? 

Iron may not necessarily be the cause of your chronic, long-lasting fatigue. Instead, it may well be a Vitamin B deficiency. 

But, what exactly does a Vitamin B deficiency entail? What does it look like?

You’ll find the answers in this next section. 

The B vitamins: what they do in your body

As we’ve previously written in our guide to multivitamins, the term Vitamin B actually refers to  a family of eight distinct - but related - water soluble vitamins: 

• Vitamin B1 (Thiamin). 

• Vitamin B2 (Riboflavin). 

• Vitamin B3 (Niacin). 

• Vitamin B5 (Pantothenic Acid).

• Vitamin B6.

• Vitamin B7 (Biotin). 

• Vitamin B9 (Folate). 

• Vitamin B12 (Cobalamin).

Whilst these are all distinct vitamins in their own right, they share common characteristics and physiological roles. Collectively, they act as coenzymes and essential metabolic helpers in hundreds of cellular processes². 

However, having said that, there are three core roles that the B vitamins play that can explain why an individual feels fatigued when they are deficient in said vitamins: 

1. Energy metabolism: this is perhaps the main reason why a Vitamin B deficiency could be the true cause of your fatigue. Many B vitamins act as cofactors in energy-producing pathways, helping your body to convert carbohydrates into usable energy. Take, for example, B1, which is a cofactor in your citric acid cycle, which is central to aerobic energy production. Likewise, B2 and B3 help generate the electron carriers that drive the ‘energy engine’ of your cells³.
   

2. Nervous system support: because certain B vitamins are integral to nerve function and neurotransmitter synthesis (especially B1, B6, and B12) a deficiency in these vitamins can worsen feelings of tiredness and worsen mental stamina⁴.
   

3. Cellular synthesis and repair: as we’ve already seen, B vitamins are important for cellular synthesis and repair. In particular, B9 and B12 are central to one-carbon metabolism, the biochemical network that enables DNA synthesis, methylation and amino acid homeostasis. Interruptions to this biochemical network (via Vitamin B deficiency), can result in the body having to rely on less-efficient pathways, which in turn results in persistent fatigue, weakness, and reduced exercise tolerance⁵. 

As you can see, the B vitamins are deeply entwined into the fundamental physiological processes that keep your body functional and healthy. A shortage of these critical vitamins can result in symptoms that are redolent of an Iron deficiency - which brings us onto our next point…

The Vitamin B9/12 and Iron deficiency mimicry problem

One of the key challenges facing medical professionals when diagnosing the underlying causes of chronic fatigue is the way in which Vitamin B9/B12 deficiencies mimic Iron deficiencies. 

Without putting too fine a point on it, unless one is especially observant, it can be easy to confuse a Vitamin B deficiency with an Iron deficiency. 

Put it this way: Vitamin B9 and B12 are important for red blood cell synthesis. Any deficiencies in B9 or B12, can in turn result in reduced red blood cell populations. This, in turn, results in tissues receiving less oxygen, which triggers the classic symptoms associated with low Iron - fatigue, weakness, shortness of breath, and rapid heart rate⁶.

How, then, are you (or medical professionals for that matter) expected to tell the two deficiencies apart from each other? It boils down to the underlying mechanisms of each deficiency: 

• Iron deficiency: this particular deficiency results in smaller, paler red blood cells (known as microcytic anaemia) due to impaired haemoglobin synthesis.
  

• B9/B12 deficiency: this deficiency results in larger, immature red blood cells (macrocytic/megaloblastic anaemia), caused by impaired DNA synthesis⁷.

The key point here, however, is that both forms of deficiency reduce effective oxygen transport - making the individual feel persistently tired. Thus, the two deficiencies ‘mimicking’ each other and leading to incorrect diagnoses. 

The Vitamin B deficiency spectrum

Before we proceed, it’s important to note that Vitamin B deficiencies exist on a spectrum. There is not a generic ‘overarching’ deficiency condition. 

It’s this deficiency spectrum that means that some people may have ‘normal’ blood work, yet still feel persistent fatigue and tiredness. 

The deficiency spectrum can be broken down as follows: 

• Clinical deficiency: this refers to scenarios where the classic signs of a deficiency (e.g. anaemia, neuropathy) are obvious, with the lab values clearly out of normal range.
  

• Subclinical or functional: this is a state in which your blood vitamin levels may fall within the ‘normal’ band of lab values yet mean that you don’t have enough active B vitamins to sustain optimal biochemical function. 

It’s this latter point that is key here. You may ‘pass’ a Vitamin B lab test, yet still have a deficiency that is impacting your quality of life. 

To put this another way, standard serum tests for B12 are useful, but they may not detect early insufficiency at the cellular level. Specialised tests (e.g. methylmalonic acid for B12, homocysteine for B12/B9/B6 pathways) are often needed in these contexts⁸. 

For example, a homocysteine above 9 means you have a B9/B12 deficiency. As Dr. Dean St. Mart PhD has previously written on this subject: 

“High serum Cobalamin (Vitamin B12) has long been a misunderstood and underestimated anomaly. Vitamin B12 was discovered as the anti-pernicious anaemia factor that was found in the liver. No other micronutrient than Vitamin B12 is known to require a specific factor for its absorption.

Without this absorption factor, Vitamin B12 deficiency follows”.

Note: the Vitamin B12 absorption factor is called intrinsic factor (IF), as opposed to the extrinsic factor (which is Vitamin B12 itself).  

So, what does this all mean in simpler terms? Why is serum B12 alone misleading? And, what are the broader mechanisms at play? Let’s take a closer look: 

Why “normal” Vitamin B12 levels can still mean a deficiency

Vitamin B12 absorption is a multi-step, highly regulated process, and disruption at any point can lead to deficiency.

After ingestion, Vitamin B12 must first be released from food proteins by gastric acid and pancreatic enzymes, before binding to intrinsic factor (IF) secreted by gastric parietal cells. This vitamin B12-IF complex is then absorbed in the terminal ileum, where it binds to a specific receptor. 

Conditions such as achlorhydria (low stomach acid), intrinsic factor deficiency, or dysfunction of this ileal receptor can all predispose individuals to Vitamin B12 deficiency, even when dietary intake appears adequate.

Although Vitamin B12 is found predominantly in foods of animal origin, it is synthesised exclusively by bacteria, not plants or animals. Many animal-derived foods contain B12 because of bacterial synthesis within the food chain or bacterial symbiosis. This explains why restrictive diets, altered gut environments, or impaired digestion can all influence B12 status. 

Once absorbed, Vitamin B12 transport and cellular delivery depend on a family of carrier proteins called transcobalamins (TCBs). Around 80% of circulating Vitamin B12 is bound to transcobalamin I and III (also known as haptocorrins), which act primarily as storage and transport forms. However, transcobalamin II (TCB II) is the critical carrier responsible for delivering Vitamin B12 into tissues and cells, including the liver and DNA-synthesising cells.

The fraction of Vitamin B12 bound to TCB II is known as holotranscobalamin, often referred to as “active B12”. Clinically, this represents the biologically active portion of Vitamin B12 that can actually enter cells and support metabolic processes such as DNA synthesis and methylation. Measuring holotranscobalamin therefore provides a more accurate picture of functional B12 status than serum B12 alone.

Importantly, serum B12 levels can be misleading. In some cases, elevated or normal serum cobalamin reflects increased binding to haptocorrins rather than effective cellular delivery. This can occur when TCB II availability is reduced, when liver uptake is impaired, or when Vitamin B12 leaks back into the circulation from damaged hepatic tissue. In these situations, cellular B12 delivery is compromised despite apparently sufficient - or even high - serum levels.

This is why Vitamin B12 deficiencies exist on a spectrum - from overt deficiency with low serum levels, to functional deficiency where transport, uptake, or intracellular utilisation is impaired. That’s why some people experience classic B12 deficiency symptoms, including fatigue and neurological issues, despite “normal” blood test results.

Note: in recent years, supplements designed to boost NAD (nicotinamide adenine dinucleotide) have become popular for energy, longevity, and metabolic health. However, it’s important to note that if you’re going down this path, increased NAD activity can actually raise your body’s need for B vitamins. Thus, you should consider a good-quality Vitamin B Complex supplement.

What are the symptoms of a Vitamin B deficiency? 

If you’re reading this, the chances are you may be wondering if you are currently experiencing a Vitamin B deficiency. 

Our advice at this point - if you think you may have a Vitamin B deficiency - is to go and see a medical professional immediately. 

However, if you simply want to be aware of the symptoms of such a deficiency, then you’ll find them below. 

Fatigue

This is perhaps the easiest symptom of a Vitamin B deficiency to identify. Deficiencies in Vitamin B12, B9, and B6, result in persistent fatigue that, crucially, doesn’t improve with rest. 

This fatigue may be accompanied by generalised weakness, reduced exercise tolerance, shortness of breath (during periods of exertion), headaches, and even heart palpitations. 

Clinical reviews have demonstrated that fatigue and emotional weakness are amongst the typical presenting features in individuals with compromised B12 status⁹. 

Cognitive and mood changes

B vitamins - especially B6, B9, and B12 - play an important role in neurotransmitter synthesis and those methylation pathways that support brain chemistry.

As a result, a Vitamin B deficiency can result in you experiencing brain fog, poor concentration, low motivation, irritability, mood-swings, and even anxiety-style symptoms. In the worst cases, Vitamin B deficiencies can result in severe depression¹⁰.

Oral and GI issues

The third key symptom area that a Vitamin B deficiency can affect is your oral, and gastrointestinal health. 

Let’s take a look at each in turn: 

• Oral health: a Vitamin B deficiency can trigger conditions such as glossitis and mouth ulcers. Deficiencies also strongly correlate with a condition called angular cheilitis (cracks at the corner of your mouth). Furthermore, you may experience mucosal changes with a Vitamin B complex deficiency¹¹.
  

• Digestive health: gastrointestinal complaints related to Vitamin B deficiencies span everything from reduced gastric acid production through to chronic diarrhoea and altered gut motility¹². 

How to prevent and/or fix a Vitamin B deficiency

If you suspect you may be suffering from a Vitamin B deficiency, your first action should always be to seek immediate advice from a medical professional. However, if you want to ensure that you don’t develop a deficiency in the first place then there are dietary, and supplementation-based actions that you can take. 

We’ve detailed these below. 

Dietary changes

A “food-first” approach leverages whole foods rich in B vitamins to boost levels gradually and sustainably. The B-vitamin family is widely distributed across food groups, however, some vitamins are more concentrated in certain sources. These sources include: 

• Animal products: foodstuffs such as eggs, meat, fish, and dairy products contain high levels of B12. In fact, B12 is almost exclusively found in animal-derived foods. This is an especially important point as humans can’t endogenously produce B12 - meaning dietary sources are essential¹³. It’s for this reason that Vegans typically experience B12 deficiencies.
  

• Fortified foods: many breakfast cereals, plant milks, and yeast extracts are fortified with B12 and folic acid to help close dietary gaps and give the average consumer a ‘Vitamin B boost’¹⁴. However, it’s worth noting that folic acid is a synthetic form of Vitamin B9 that must be converted in the body into its active form (5-Methyltetrahydrofolate). Some individuals have reduced capacity to perform this conversion due to genetic variations affecting folate metabolism (such as MTHFR gene variants), meaning fortified food may by less effective for meeting their functional folate requirements.
  

• Legumes, leafy greens, whole grains: these types of ‘dark greens’, pulses, and grains are excellent sources of certain B vitamins (especially B9, B1, B2, and B3). Folate in particular is handy as the body uses it to support DNA synthesis and cell division¹⁵.

Dietary changes - such as increasing the volume of foodstuffs outlined above - can go a long way in ensuring that you don’t experience a Vitamin B deficiency. 

However, for some people, it is necessary to consider taking a dedicated Vitamin B supplement. There are a number of reasons why you may have to take this route. 

Vitamin B supplementation

As we’ve mentioned above, sometimes dietary changes aren’t enough to address a shortfall in Vitamin B intake. Thus, some people will choose to supplement Vitamin B. 

Some of the reasons why people supplement Vitamin B, include: 

• Dietary restrictions: if you’re a vegan, then unfortunately, you are a prime candidate for a Vitamin B deficiency. Numerous studies support this¹⁶, indicating that vegans should strongly consider supplementing their diets with additional sources of Vitamin B.
  

• Malabsorption: some people may eat plenty of Vitamin B-rich foodstuffs, but suffer from conditions (such as pernicious anaemia, coeliac disease, inflammatory bowel disease) that disrupt the absorption of B vitamins¹⁷.
  

• Medication: certain medications (and the chronic use of these), can impair Vitamin B absorption, meaning supplementation may be necessary despite an ostensibly adequate diet¹⁸.
  

• High demand or certain life stages: at certain points of your life (e.g. if you are pregnant or elderly) - or if you are under significant physiological stress - you may need additional Vitamin B intake beyond your diet¹⁹.

It’s very important to note here that if your Vitamin B deficiency is sufficiently advanced (e.g. that you are suffering from neurological symptoms or significant anaemia), then dietary supplementation won’t be enough of a remedial action. Instead, you should seek professional advice and support. 

How to choose the right Vitamin B support for your body

When addressing fatigue - especially in cases where Iron isn’t the primary issue - the right B vitamin strategy matters just as much as whether you supplement in the first place. 

As we’ve seen, B vitamins are involved in cellular energy metabolism, neurotransmitter synthesis, nerve support, and red blood cell formation - meaning that a deficiency can impact these systems. 

Rather than selecting cheap, generic Vitamin B supplements, you should choose a well-formulated, science backed, premium Vitamin B supplement. Why? Because, you need to consider: 

• Bioavailability: you must choose supplements such as those produced by Supplement Needs that are highly bioavailable - meaning the body can use them immediately without requiring extensive metabolic conversion first.
  

• Full B-vitamin coverage: ensure you select a supplement that covers all the B vitamins, not just an individual B vitamin alone.
  

• Transparent dosing and quality controls: any Vitamin B supplements you choose should have clear labelling and be transparent about the formulas that they have used. 

Perhaps most importantly, don’t let price be the governing factor over your choice. As we’ve written previously, cheap supplements really are a false economy!

Supplement Needs Advanced Vitamin B Complex

References: 

1. Hanna M, Jaqua E, Nguyen V, Clay J. B Vitamins: Functions and Uses in Medicine [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC9662251/ (Accessed on 18th January 2026).

2. ibid.

3. Yoshii K, Hosomi K, Sawane K, Kunisawa J. Metabolism of Dietary and Microbial Vitamin B Family in the Regulation of Host Immunity [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC6478888/ (Accessed on 18th January 2026).

4. Tardy A, Pouteau E, Marquez D, Yilmaz C, Scholey A. Vitamins and Minerals for Energy, Fatigue and Cognition: A Narrative Review of the Biochemical and Clinical Evidence [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7019700/ (Accessed on 18th January 2026).

5. Lyon P, Strippoli V, Fang B, Cimmino L. B Vitamins and One-Carbon Metabolism: Implications in Human Health and Disease [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7551072/ (Accessed on 18th January 2026).

6. Cleveland Clinic. Vitamin Deficiency Anemia [online]. Available at: https://my.clevelandclinic.org/health/diseases/17732-vitamin-deficiency-anemia (Accessed on 18th January 2026).

7. Hariz A, Bhattacharya P. Megaloblastic Anemia [online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK537254/ (Accessed on 20th January 2026).

8. Lyon P, Strippoli V, Fang B, Cimmino L. B Vitamins and One-Carbon Metabolism: Implications in Human Health and Disease [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC7551072/ (Accessed on 20th January 2026).

9. National Library of Medicine. Vitamin B12 Deficiency [online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK441923/ (Accessed on 20th January 2026).

10. Han A, Almeida L, Anand N, et. al. Exploring neuropsychiatric manifestations of vitamin B complex deficiencies [online]. Available at: https://www.frontiersin.org/journals/psychiatry/articles/10.3389/fpsyt.2025.1569826/full (Accessed on 24th January 2026).

11. Boukssim S, Chbicheb S. Oral manifestations of vitamin B12 deficiency associated with pernicious anemia: A case report [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC11254220/ (Accessed on 24th January 2026).

12. Wikipedia. Vitamin B6 [online]. Available at: https://en.wikipedia.org/wiki/Vitamin_B6 (Accessed on 24th January 2026).

13. Wikipedia. B vitamins [online]. Available at: https://en.wikipedia.org/wiki/B_vitamins (Accessed on 24th January 2026).

14.  NHS. Vitamin B12 management in adults [online]. Available at: https://www.nottsapc.nhs.uk/media/3xufkasz/vitamin-b12-treatment-guideline.pdf (Accessed on 24th January 2026).

15. Wikipedia. B vitamins [online]. Available at: https://en.wikipedia.org/wiki/B_vitamins (Accessed on 24th January 2026)..

16. Niklewicz A, Smith A, Smith A, et. al. The importance of vitamin B12 for individuals choosing plant-based diets [online]. Available at: https://pmc.ncbi.nlm.nih.gov/articles/PMC10030528/ (Accessed on 24th January 2026).

17. Ankar A, Kumar A. Vitamin B12 Deficiency [online]. Available at: https://www.ncbi.nlm.nih.gov/books/NBK441923/ (Accessed on 24th January 2026).

18. NHS. Dietary advice for vitamin B12 & folate deficiency [online]. Available at: https://www.uhcw.nhs.uk/download/clientfiles/files/Patient%20Information%20Leaflets/Medicine/Haematology/Dietary%20advice%20for%20Vitamin%20B12%20and%20folate%20deficiency.pdf (Accessed on 24th January 2026).

19. NHS. B vitamins and folic acid [online]. Available at: https://www.nhs.uk/conditions/vitamins-and-minerals/vitamin-b/ (Accessed on 24th January 2026).