Thiamine (Vitamin B1): Brain Fog, Energy, and the Truth

There are vitamins that are talked about a lot, and there are vitamins without which you simply cannot function. Thiamine, also known as vitamin B1, belongs to the second group. It is one of the most basic substances that every cell in the body, especially brain cells, needs to convert sugar into energy. Without enough thiamine, the brain loses its fuel, and the result is exactly the complaints that fill clinics today: brain fog, unrelenting fatigue, and difficulty concentrating.

This is precisely what makes thiamine a confusing supplement. On one hand, a true deficiency causes neurological symptoms that improve dramatically with supplementation. On the other hand, it is not a miracle cure that will clear brain fog for everyone. The gap between these two scenarios is the whole story, and it's why we rated thiamine yellow and not green. In this article, we'll explain what thiamine actually does in the brain, who is at real risk for deficiency, and what the research says about energy, fatigue, and cognitive function.

What is Thiamine (Vitamin B1)?

Thiamine is a water-soluble B vitamin that the body cannot store for long periods. It must be obtained from food or a supplement regularly. Here's what's important to understand about it:

• It is fuel for sugar metabolism. Thiamine is an essential cofactor for several key enzymes that break down glucose to produce cellular energy (ATP).
• The brain is especially hungry. The brain consumes about 20% of the body's energy and relies almost entirely on glucose, making it particularly sensitive to thiamine deficiency.
• Body stores are small. The body holds a supply of only about two to three weeks, so deficiency can develop relatively quickly in certain situations.
• It is water-soluble and very safe. Excess thiamine is excreted in urine, so toxicity at standard doses is virtually non-existent.

Food sources rich in thiamine include pork, legumes, whole grains, sunflower seeds, and brewer's yeast. A processed Western diet, high in sugar and low in whole grains, is actually a favorable environment for developing subclinical deficiency, without anyone noticing.

The Connection to the Brain: When the Fuel Runs Out

To understand why thiamine deficiency specifically harms the brain, you need to know three enzymes: transketolase, pyruvate dehydrogenase (PDHC), and alpha-ketoglutarate dehydrogenase (KGDHC). All three depend on thiamine to function, and all three are at the heart of the process by which brain cells produce energy from glucose.

When thiamine levels drop, these enzymes slow down. The result is a decrease in cellular energy production precisely in the organ most dependent on energy. Subjectively, this manifests as fatigue, mental slowness, and difficulty concentrating – what people today describe as brain fog. In severe cases, prolonged deficiency causes actual neurological damage.

This connection is not merely theoretical. In brain studies of Alzheimer's patients, the decrease in activity of the KGDHC and PDHC enzymes was found to be strongly correlated (approximately 0.77) with the severity of dementia, a correlation much higher than that of amyloid plaques or tau tangles. This doesn't mean thiamine cures Alzheimer's, but it illustrates how closely brain sugar metabolism, the process thiamine enables, is linked to cognitive function.

Current Evidence

Study 1: Thiamine Deficiency is Common in Diabetes, Thornalley 2007

One of the most important and less well-known findings in the field was published in the journal Diabetologia in 2007 by Paul Thornalley's group. The researchers measured thiamine levels in 26 patients with type 1 diabetes, 48 patients with type 2 diabetes, and 20 healthy individuals.

The result surprised many: Plasma thiamine concentration was 75% to 76% lower in diabetic patients compared to healthy controls (approximately 15 to 16 nmol/L in patients versus about 64 in healthy individuals). The reason was not poor diet but increased thiamine loss through the kidneys, by a factor of 16 to 24. That is, people living with diabetes lose thiamine at an accelerated rate, putting them at inherent risk for deficiency even if they eat well. This is one scenario where thiamine supplementation is particularly logical.

Study 2: Benfotiamine and Alzheimer's, Gibson 2020

A randomized, double-blind, placebo-controlled trial (Phase IIa) published in the Journal of Alzheimer's Disease in 2020, led by Gerald Gibson. 70 participants with mild cognitive impairment or early Alzheimer's received either benfotiamine (a fat-soluble form of thiamine at a high dose) or a placebo for 12 months.

The results were encouraging but cautious: The benfotiamine group showed a trend towards slowing cognitive decline, more pronounced in carriers of the ApoE4 gene, and the supplement was found to be safe. The researchers themselves emphasized that this is a small preliminary study requiring confirmation in larger trials (the follow-up BenfoTeam study is currently underway). This is a good example of intriguing but not conclusive evidence.

Study 3: High-Dose Thiamine and Fatigue, Costantini 2013

Italian physician Antonio Costantini published a series of case reports and small open-label studies in which high-dose thiamine (hundreds to thousands of mg per day) significantly alleviated chronic fatigue in patients with inflammatory bowel diseases, multiple sclerosis, and fibromyalgia. In the open-label study on inflammatory bowel diseases, fatigue resolved almost completely in all participants.

It is important to view these findings critically. These are open-label studies and small case series, without a randomized placebo group, so they are far from proof. Costantini's hypothesis, that fatigue in certain diseases stems from subclinical thiamine deficiency or a disruption in its cellular transport, is interesting and warrants more serious research, but it remains a hypothesis.

What About Benfotiamine?

If you've come across the term benfotiamine, it's worth knowing briefly. Benfotiamine is a fat-soluble derivative of thiamine that is absorbed differently and raises thiamine levels in certain tissues more effectively than regular thiamine. For this reason, it was the form chosen in Gibson's brain studies, and it is also extensively studied in the context of diabetic nerve complications (neuropathy).

For most people whose goal is simply to ensure adequate B1 supply, regular thiamine (thiamine hydrochloride or mononitrate) is perfectly sufficient and cheaper. Benfotiamine is relevant mainly in specific contexts like diabetic neuropathy or when aiming to raise tissue levels with high doses, preferably under medical supervision.

Should You Start Taking Thiamine?

This is why we rated thiamine yellow, not green. The logic is simple and important: Thiamine fixes a real problem when it exists, but it is not a universal nootropic.

• In a true deficiency, the effect is reliable and fast. Someone suffering from thiamine deficiency will typically experience significant relief from brain fog and fatigue after supplementation. In severe cases (Wernicke's encephalopathy), this is a medical emergency treated with intravenous thiamine.
• In a healthy, well-nourished person, the benefit is low. If there is no deficiency, an extra dose of B1 is unlikely to clear brain fog or add energy. The body will simply excrete the excess.
• Who is truly at risk for deficiency: Diabetics (increased renal loss), the elderly, heavy alcohol users, people after bariatric surgery, and those with a poor, processed diet.
• Brain fog is a symptom, not a diagnosis. It can stem from poor sleep, stress, underactive thyroid, anemia, or other deficiencies. Thiamine will only help if its deficiency is the cause.

In terms of safety, thiamine is considered one of the safest supplements. It is water-soluble, and excess is excreted in urine (sometimes turning it a light yellow). There is no defined toxicity threshold at standard doses, and no tolerable upper intake level has been set because no toxicity has been found. One practical caveat: If you are already taking a B-complex or multivitamin, it likely already contains thiamine, and there is no need to double up without reason.

What to Take Away from the Research?

1. If you are in a risk group (diabetes, older age, high alcohol consumption, post-bariatric surgery), it's worth checking with your doctor if thiamine supplementation is right for you. This is where the likelihood of real benefit is highest.
2. If you are healthy and looking for a solution to brain fog, start with the basics. A blood test (including thyroid function, B12, iron), sleep, stress management, and a balanced diet will provide a more real answer than a random dose of B1.
3. Practical dosage: 50 to 100 mg per day is a common, safe dose, usually more than enough to correct a dietary deficiency. In certain medical conditions, much higher doses are used, but this should be under a doctor's supervision.
4. Don't double up without reason. Check the label of your multivitamin or B-complex before adding a separate thiamine supplement.
5. Diet before supplement. Whole grains, legumes, seeds, and meat provide thiamine naturally. A supplement is a complement to a poor diet or a medical condition, not a substitute for good eating.

Those interested in trying can purchase thiamine (Vitamin B1) on iHerb in various dosages, including benfotiamine forms. To check which supplements are truly suitable for your state of brain fog and fatigue based on your age, gender, and goals, try our personal supplement selector which provides a ranked list based on evidence quality.

The Broader Perspective

Thiamine is an excellent reminder of a principle that recurs again and again in the supplement field: Correcting a deficiency is not the same as enhancing a normal baseline. When the body is deficient in an essential vitamin, supplementing it can be life-changing. When the body is full and functioning, the same supplementation will hardly be noticed. Most of the confusion around brain supplements stems from blurring these two states.

The practical lesson: Before looking for the pill that will clear the brain fog, it's worth asking what is causing the fog in the first place. For some people, especially in risk groups, the answer is indeed a nutritional deficiency that thiamine can fix. For others, the cause lies in sleep, stress, or the thyroid, and no dose of B1 will solve it. Brain health, as always, is built first from the foundations, and the right supplement is the one that matches your real problem.

References:
Gibson GE. et al., Benfotiamine and Cognitive Decline in Alzheimer's Disease: Results of a Randomized Placebo-Controlled Phase IIa Clinical Trial, Journal of Alzheimer's Disease, 2020;78(3):989-1010 (DOI: 10.3233/JAD-200896)
Thornalley PJ. et al., High prevalence of low plasma thiamine concentration in diabetes linked to a marker of vascular disease, Diabetologia, 2007;50(10):2164-2170 (DOI: 10.1007/s00125-007-0771-4)
Costantini A., Pala MI., Thiamine and Fatigue in Inflammatory Bowel Diseases: An Open-label Pilot Study, Journal of Alternative and Complementary Medicine, 2013;19(8):704-708