Brain fog: causes of poor concentration and the role of diet

Forgetting words, trouble concentrating, slower thinking and mental fatigue are symptoms often described as brain fog. For some people they appear after a sleepless night or during a period of increased stress. For others they last for weeks and get in the way of work, study and everyday responsibilities.

Brain fog is not a disease in its own right, nor a diagnosis that points to a specific cause. Similar difficulties can accompany nutritional deficiencies, sleep disorders, chronic illness or the period after an infection. That is why the right approach depends on the symptoms, medical history and test results.

What does brain fog look like?

Patients most often describe problems with sustaining attention, recalling information and carrying out tasks that require planning. They may read the same passage of text several times, forget why they walked into a room, or need more time to complete routine activities. Difficulty finding words and a lower tolerance for mental effort sometimes come along too.

A 2025 scientific review shows that the term covers various combinations of problems with attention, memory, fatigue and psychological wellbeing [1]. The subjective sense of reduced mental performance does not always match the results of short cognitive tests. So it also matters to assess how the symptoms affect day-to-day functioning.

The most common causes of brain fog

Too little sleep and poor sleep quality

Sleep allows the processes involved in attention and memory to work properly. A 2024 meta-analysis of 44 studies found that even a single night with sleep opportunity restricted to 2–6 hours increases sleepiness and impairs sustained attention [2]. The effect on other cognitive functions was less clear-cut.

Adults are usually advised to get at least 7 hours of sleep a day, although individual needs vary. Sleep continuity matters as well. Someone who regularly wakes up during the night may feel tired despite spending enough time in bed. Loud snoring, pauses in breathing and marked daytime sleepiness call for considering tests for sleep apnoea.

Vitamin B12 and iron deficiency

Vitamin B12 takes part in processes essential for the nervous system to work properly. A deficiency can cause problems with memory and concentration, weakness, tingling in the limbs or balance problems. The 2024 NICE guideline stresses that B12 deficiency can occur without anaemia and without the characteristic changes in a blood count [3].

People who follow a vegan diet without supplements, take metformin long term, or have malabsorption or a history of gastrointestinal surgery need particular attention. In these groups, assessing B12 intake and blood levels may be justified.

Iron deficiency can also cause weakness and chronic fatigue. A review of 18 clinical trials found that iron supplementation in people with confirmed deficiency but no anaemia reduced subjective fatigue [4]. This finding concerns fatigue above all; it is not proof that iron improves concentration in everyone with brain fog. Treatment decisions should take into account, among other things, the blood count, ferritin and the cause of the deficiency.

Thyroid and glucose disorders

An underactive thyroid can come with sluggishness, sleepiness and difficulty concentrating. Additional symptoms such as feeling cold, constipation or dry skin help guide testing. When a thyroid disorder is suspected, the first-line test for most adults is TSH, and further testing is chosen by a doctor [5].

Another issue is hypoglycaemia, meaning blood glucose that is too low. It can lead to shaky hands, sweating, weakness and confusion. In people with diabetes, a reading below 70 mg/dL (3.9 mmol/L) calls for an appropriate response, especially during treatment with insulin or drugs that raise the risk of low blood sugar [6]. Feeling scatterbrained after lunch, however, does not on its own mean hypoglycaemia or insulin resistance.

Stress, medication and the period after an infection

Chronic tension, anxiety and low mood can affect concentration and working memory, which holds the information needed for the task at hand. Stress also often worsens sleep. When assessing symptoms it is worth reviewing medication too, because some sedatives, sleeping pills and drugs with anticholinergic effects can cause sleepiness or make thinking harder. Do not stop any medication on your own.

Brain fog is also one of the symptoms reported by people with long COVID. A large study published in 2024 observed, on average, poorer cognitive test results in some people with persistent symptoms after COVID-19 [7]. The mechanisms behind these difficulties are still being studied, and the course and recovery time vary between patients.

Can diet help with brain fog?

In nutrition, the key question is whether the way someone eats encourages deficiencies or an insufficient energy intake. Prolonged calorie restriction, skipping meals and a monotonous diet can worsen fatigue and make it harder to meet nutrient needs. There is, however, no single eating plan with proven effectiveness in treating brain fog regardless of its cause.

In practice, focus on regular meals, adequate energy intake and a variety of foods. Vitamin B12 comes from fish, meat, eggs and dairy; people on a vegan diet need a reliable source of it from supplements or fortified foods. Iron is provided by meat, pulses and seeds. Adding foods rich in vitamin C, such as peppers in a lentil dish, improves iron absorption from plant foods.

Simple everyday choices can help: breakfast with eggs and wholegrain bread, lunch with grains, vegetables and pulses, or dinner with fish. If you often skip meals, it is better to start by eating more regularly than by cutting out more foods. In people with diabetes, meal planning also has to be matched to their medication and their risk of hypoglycaemia.

If you suspect that your eating pattern is making things worse, you can review a food log from a few typical days. Include drinks, meal times and any symptoms after eating. A diary like this helps spot long gaps, eating too little or a monotonous diet. Feeling sleepy after a meal does not by itself justify cutting out gluten, dairy or carbohydrates. An elimination diet should be introduced when there is a medical reason for it. Otherwise, extra restrictions can make it harder to cover your energy and nutrient needs.

The 2026 WHO guidelines indicate that a balanced diet may support long-term protection of cognitive function [8]. They are, however, mainly about reducing the risk of cognitive decline and dementia. They do not show that the Mediterranean diet or any specific nutrient treats brain fog.

Supplements for memory and concentration

Popular products contain B vitamins, magnesium, omega-3s, plant extracts or blends of ingredients marketed as nootropics. When a deficiency has been diagnosed, replacing the missing nutrient makes sense. When nutritional status is normal, taking extra supplements does not guarantee better concentration. We cover this in more detail in our article on whether supplements can replace diet.

WHO does not recommend routine supplementation with vitamins B and E, omega-3s or multivitamins solely to prevent cognitive decline or dementia in people without deficiencies [8]. Research on nutritional interventions for long COVID also remains limited. A 2024 systematic review did not provide convincing evidence to routinely recommend the supplements studied as a treatment for persistent symptoms [9].

Which tests to consider, and when?

Persistent or worsening difficulty concentrating calls for a medical consultation. The choice of tests should follow from the history, symptoms and medication. Depending on the situation, a doctor may order a blood count, ferritin, vitamin B12, TSH, glucose or HbA1c. Ferritin should be interpreted with inflammation in mind, because its level can rise during infections and other inflammatory conditions. For a detailed look at when brain fog warrants testing, see our separate article.

Accompanying symptoms matter too. Heavy periods can raise the risk of iron deficiency, while chronic diarrhoea or weight loss may suggest malabsorption. New symptoms after starting a treatment are a reason to review the medicines being taken. Snoring and marked daytime sleepiness, in turn, should draw attention to sleep quality. Information like this helps avoid ordering large test panels at random and gets to the right diagnosis faster. If basic tests are normal and the difficulties continue, a doctor may consider further assessment of sleep, mood or cognitive function.

It helps to note down, for 1–2 weeks, your sleep times, how regular your meals are and the situations in which concentration problems are especially bad. If symptoms get worse after physical or mental exertion, particularly after an infection, discuss this with a doctor. Some people with long COVID experience post-exertional worsening, which requires matching activity to what the body can tolerate [10].

Sudden speech problems, weakness on one side of the body, marked confusion or sudden vision problems require urgent medical help: call 112 (or your local emergency number). These symptoms may indicate a stroke and should not be treated as an ordinary dip in concentration.

Brain fog has many causes and needs individual assessment. Better sleep, an adequate diet and treating confirmed deficiencies can help some people. If the difficulties persist, further testing makes it possible to tailor the approach to the actual problem.

Frequently asked questions

Can brain fog be caused by a vitamin deficiency?

Yes. Vitamin B12 deficiency is particularly relevant, as it can cause problems with memory and concentration. Diagnosis requires taking into account the symptoms, risk factors and test results.

Does brain fog mean insulin resistance?

No. Feeling scatterbrained is not a criterion for diagnosing insulin resistance. In people being treated for diabetes, however, episodes of hypoglycaemia should be considered.

Does coffee improve concentration?

Caffeine can temporarily increase alertness and reduce sleepiness. It does not remove the causes of a long-term decline in concentration, and drinking it late in the day can make it harder to fall asleep.

Do I need to take omega-3 supplements for brain fog?

Routine omega-3 supplementation has no proven effectiveness as a treatment for brain fog. It is, however, worth including fish in a balanced diet, in line with your individual needs.

Can brain fog go away on its own?

Yes, especially if it appeared after a short spell of poor sleep or overload. Symptoms that last for weeks, get worse or interfere with daily functioning need a medical assessment.

References

1. Denno P, Zhao S, Husain M, Hampshire A. Defining brain fog across medical conditions. Trends Neurosci. 2025;48(5):330–348. doi:10.1016/j.tins.2025.01.003
2. Wüst LN, Capdevila NC, Lane LT, Reichert CF, Lasauskaite R. Impact of one night of sleep restriction on sleepiness and cognitive function: A systematic review and meta-analysis. Sleep Med Rev. 2024;76:101940. doi:10.1016/j.smrv.2024.101940
3. National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE guideline NG239. 2024. (nice.org.uk)
4. Houston BL, Hurrie D, Graham J, et al. Efficacy of iron supplementation on fatigue and physical capacity in non-anaemic iron-deficient adults: a systematic review of randomised controlled trials. BMJ Open. 2018;8(4):e019240. doi:10.1136/bmjopen-2017-019240
5. National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE guideline NG145. 2019, updated 2023. (nice.org.uk)
6. American Diabetes Association Professional Practice Committee. 6. Glycemic Goals, Hypoglycemia, and Hyperglycemic Crises: Standards of Care in Diabetes–2026. Diabetes Care. 2026;49(Suppl 1):S132–S149. doi:10.2337/dc26-s006
7. Hampshire A, Azor A, Atchison C, et al. Cognition and Memory after Covid-19 in a Large Community Sample. N Engl J Med. 2024;390(9):806–818. doi:10.1056/NEJMoa2311330
8. World Health Organization. Risk reduction of cognitive decline and dementia: WHO guidelines, second edition. Geneva: WHO; 2026. (who.int)
9. Zeraatkar D, Ling M, Kirsh S, et al. Interventions for the management of long covid (post-covid condition): living systematic review. BMJ. 2024;387:e081318. doi:10.1136/bmj-2024-081318
10. Centers for Disease Control and Prevention. Long COVID Clinical Guidance. Updated 2026. (cdc.gov)