Unlike vitamin D or B12, thiamine rarely features in popular health conversations. However, thiamine plays a critical role in brain metabolism. Without adequate thiamine, neurons struggle to generate energy, and the brain — one of the body’s most energy-hungry organs — begins to malfunction
Cancer treatment today is often discussed in terms of cutting-edge drugs, survival curves, and remission rates. Yet, hidden beneath the headline victories of chemotherapy is a quieter, more insidious problem that directly affects how patients think, feel, and function. New evidence suggests that thiamine deficiency — a lack of vitamin B1 — may be an under-recognised but significant cause of neurological symptoms in patients with blood cancers undergoing chemotherapy. These symptoms are often mild, nonspecific, and easily dismissed, but they can seriously undermine both quality of life and the effectiveness of cancer care.
Thiamine is not a vitamin that typically commands public attention. Unlike vitamin D or B12, it rarely features in popular health conversations. However, it plays a critical role in brain metabolism. Without adequate thiamine, neurons struggle to generate energy, and the brain — one of the body’s most energy-hungry organs — begins to malfunction. In patients with hematological cancers such as leukemia, lymphoma, and multiple myeloma, the risk of thiamine depletion appears to be particularly high, especially once chemotherapy begins.
A recent retrospective analysis from a major academic hospital in Japan sheds light on this neglected issue. By examining patients with hematological cancers receiving chemotherapy, the study establishes a clear association between low thiamine levels and neurological symptoms ranging from cognitive impairment and poor attention to mood and emotional disturbances. Importantly, these were not dramatic, textbook cases of neurological collapse. They were subtle changes — confusion, emotional instability, difficulty concentrating — that are often attributed to “chemo brain,” stress, or ageing, rather than to a correctable nutritional deficiency.
Why thiamine matters more than we think
Thiamine is essential for glucose metabolism. It acts as a coenzyme in key biochemical pathways that generate energy within cells. When thiamine levels fall, the Krebs cycle — the engine of cellular energy production — begins to fail. The result is an energy crisis within neurons, accumulation of toxic metabolites, increased oxidative stress, and ultimately neuronal dysfunction.
In cancer patients, this vulnerability is amplified. Tumour cells have a voracious appetite for nutrients, including thiamine. Hematological cancers, in particular, involve rapidly dividing cells with high metabolic demands. These malignant cells effectively compete with normal tissues, including the brain, for available thiamine. Chemotherapy compounds the problem by accelerating metabolic stress, inducing gastrointestinal side effects that impair nutrient absorption, and increasing the body’s overall nutritional requirements.
Despite this, thiamine levels are rarely monitored in routine oncology practice. In most hospitals, thiamine is measured only when a patient presents with severe neurological symptoms, such as those seen in Wernicke–Korsakoff syndrome — a life-threatening condition classically associated with alcoholism. Even then, laboratory results may take days to return, delaying diagnosis and treatment. The tragic irony is that thiamine deficiency is both easy to detect and inexpensive to treat, yet it continues to be overlooked.
Subtle symptoms, serious consequences
The study examined 42 patients with hematological cancers who had their thiamine levels measured during chemotherapy. Nearly all were otherwise fit, with good performance status and no significant alcohol use — eliminating many traditional risk factors for thiamine deficiency. Yet, a significant proportion developed neurological symptoms.
These symptoms were not dramatic seizures or loss of consciousness. Instead, they included cognitive impairment, attention deficits, and mood or emotional disturbances — symptoms that are easily dismissed as psychological reactions to cancer or side effects of chemotherapy. However, when researchers compared patients with and without these symptoms, the difference in thiamine levels was striking. Those with neurological symptoms had significantly lower thiamine levels than those without.
More importantly, statistical analysis revealed that thiamine deficiency was the only factor significantly associated with neurological symptoms. Age, sex, albumin levels, vitamin B12, folate, and electrolyte abnormalities — all common suspects in neurological decline — showed no meaningful association. Thiamine stood alone.
This finding challenges a common assumption in oncology: that mild cognitive and emotional changes during chemotherapy are inevitable and untreatable. On the contrary, the data suggest that at least some of these symptoms may be preventable — or reversible — with timely recognition of thiamine deficiency.
Chemotherapy and the thiamine crash
One of the most revealing aspects of the study was its analysis of chemotherapy-naïve patients. By tracking thiamine levels before and after the initiation of chemotherapy, researchers observed a clear decline in thiamine levels following treatment. The lowest levels were recorded five to eight weeks after chemotherapy began.
This temporal relationship strongly suggests that chemotherapy itself accelerates thiamine depletion. The reasons are likely multifactorial: increased metabolic demand, enhanced thiamine consumption by tumour cells stressed by chemotherapy, reduced dietary intake due to nausea or anorexia, and impaired absorption from the gastrointestinal tract.
For clinicians, this raises a critical question. If chemotherapy predictably lowers thiamine levels, should monitoring and supplementation not be built into standard cancer care protocols — especially for high-risk groups such as patients with hematological malignancies?
The cost of ignoring the brain
Neurological symptoms are not just an inconvenience. Cognitive impairment can interfere with a patient’s ability to understand treatment plans, adhere to medication schedules, and recognise early signs of complications. Mood disturbances and depression reduce motivation, impair social support, and are associated with poorer cancer outcomes. In severe cases, untreated thiamine deficiency can progress to irreversible neurological damage or death.
Historically, the medical community has paid a heavy price for underestimating thiamine deficiency. Autopsy studies suggest that up to 80 percent of cases of Wernicke–Korsakoff syndrome are diagnosed only after death. While the patients in this study did not develop such extreme conditions, the lesson is clear: waiting for severe symptoms is a dangerous strategy.
What makes this issue particularly troubling is that thiamine deficiency does not always announce itself clearly. Patients may have “subclinical” deficiency — levels low enough to impair brain function, but not low enough to trigger alarm bells. Without routine screening, these patients fall through the cracks.
A case for preventive monitoring
The study’s authors are careful not to overstate their conclusions. They acknowledge limitations, including the small sample size, retrospective design, and lack of objective cognitive testing or brain imaging. Yet, the signal is strong enough to warrant attention.
Their central argument is pragmatic rather than radical: thiamine levels should be considered a reference marker in patients with hematological cancers undergoing chemotherapy. Not every patient with low thiamine will develop neurological symptoms, but monitoring provides clinicians with an early warning system. It allows for timely supplementation before mild symptoms escalate into serious neurological complications.
From a public health perspective, this approach makes sense. Thiamine testing is inexpensive compared to advanced imaging or neuropsychological evaluations. Thiamine supplementation is safe, cheap, and widely available. In an era where cancer care is becoming increasingly expensive, this is a rare example of a low-cost intervention with potentially high impact.
Rethinking “chemo brain”
The concept of “chemo brain” has gained traction in recent years, describing the cognitive fog experienced by many cancer patients during and after treatment. While chemotherapy undoubtedly affects the brain through multiple mechanisms, thiamine deficiency may be one piece of this complex puzzle.
By reframing some cognitive and emotional symptoms as potentially nutritional rather than purely psychological or toxic, clinicians can adopt a more nuanced approach to patient care. This does not diminish the seriousness of chemotherapy-related cognitive changes; instead, it expands the toolkit available to address them.
Looking ahead
The findings from this study highlight an urgent need for larger, prospective investigations. Future research should examine different chemotherapy regimens, longer follow-up periods, and broader neurological outcomes, including peripheral neuropathy. Objective cognitive assessments and brain imaging could further clarify the relationship between thiamine deficiency and neurological dysfunction.
However, waiting for perfect data should not delay reasonable clinical action. The evidence already suggests that thiamine deficiency is common, underdiagnosed, and clinically relevant in patients with hematological cancers receiving chemotherapy.
Cancer care is not only about killing tumour cells. It is about preserving the patient’s ability to think clearly, feel emotionally stable, and remain engaged in their own treatment. Thiamine, a humble vitamin discovered over a century ago, may play a far more important role in this mission than we have been willing to acknowledge. In the push toward ever more sophisticated cancer therapies, it may be time to remember a simple truth: sometimes, protecting the brain starts with recognising what it is quietly losing.
