Fog in the head, rapid mental fatigue, irritability, and the feeling that the brain is working against resistance are not always explained solely by lack of sleep or stress. Sometimes, there are quite earthly biochemical reasons behind this: energy depletion in the mitochondria, deficiency of B vitamins, inflammatory background, carbohydrate metabolism disorders, or overload from the intestines.
The analysis of organic acids is one of the deeper metabolic tests. It does not replace standard blood tests and does not diagnose on its own, but it can show where exactly metabolism is under strain. For the topic of fog in the head, this is particularly interesting because the brain is very sensitive to energy, oxygen, inflammation, and cofactor deficiencies.
What the organic acid test shows
Organic acids are intermediate products of metabolism. They are usually assessed in urine, and the result shows not the storage of substances in the body, but traces of how biochemical reactions are proceeding. Therefore, this analysis is better read not as a list of “bad” and “good” markers, but as a map of processes.
In the context of brain fatigue, the analysis can suggest several directions:
- how well cells manage energy production;
- whether there are signs of strain in the Krebs cycle and mitochondrial respiration;
- whether there are enough vitamins B12, B9, and B6 for the nervous system;
- whether there are markers of oxidative stress;
- whether intestinal fermentation can exacerbate fog after meals;
- whether there is an imbalance in the metabolism of tryptophan and dopamine.
When it makes sense to think about organic acids
Usually, it’s better to start not with an expensive extended test, but with simple things: sleep, nutrition, protein, ferritin, glucose, insulin, B12, folate, vitamin D, and thyroid function. Organic acids become particularly useful if the basic check has already been done, but the picture is still unclear.
This analysis may be relevant in the following situations:
- fog in the head persists for weeks or months, and regular rest does not help;
- after meals, there is a sudden urge to sleep or clarity of thought worsens;
- mental work quickly exhausts, although motivation is present;
- there is anxiety, irritability, poor sleep, and difficulty concentrating;
- there are borderline B12 levels, high homocysteine, or suspicion of B vitamin deficiencies;
- there is bloating, cravings for sweets, unstable stools, or pronounced fog after carbohydrates;
- after an infection, prolonged stress, or overwork, recovery is too slow.
Start with basic tests
Before testing organic acids, it is advisable to complete the basic level of diagnostics. It is cheaper, clearer, and often immediately reveals the main cause of fatigue.
| Test | Why it is needed for fog in the head | Example interpretation |
| Fasting glucose, insulin, HOMA-IR | Show whether there are spikes in sugar and insulin resistance | Glucose 5.6 mmol/L and insulin 14 µU/mL may explain drowsiness after meals and cravings for sweets |
| HbA1c | Reflects the average glucose level over the past months | HbA1c 5.7% and above is a reason to look at carbohydrate load and metabolic health |
| Ferritin, complete blood count | Help exclude iron deficiency and anemia | Ferritin 12 ng/mL with fatigue and shortness of breath may be more important than any “fine” biochemistry |
| B12, folate, homocysteine | Related to myelin, methylation, memory, and speech | B12 260 pg/mL and homocysteine 14 µmol/L may indicate functional deficiency |
| TSH, free T4 | The thyroid gland affects thinking speed and energy levels | Elevated TSH with drowsiness, chilliness, and swelling requires separate evaluation |
| 25(OH)D | Vitamin D is related to immune regulation, mood, and muscle weakness | 25(OH)D 18 ng/mL does not explain everything, but may exacerbate fatigue and poor recovery |
Organic acids and what they can indicate
If the basic picture does not provide answers, organic acids help look deeper. Below are not diagnoses, but practical logic for reading markers.
| Marker | What it may reflect | What it may correlate with in symptoms |
| Lactate and pyruvate | Strain on energy metabolism, shift towards less efficient energy production | Rapid mental fatigue, heaviness after exertion, feeling of “brain shutting down” |
| Kynurenine | Shift in tryptophan metabolism towards inflammation and stress response | Irritability, anxiety, poor sleep, decreased concentration |
| Arabinose | Used in some panels as a marker of intestinal fungal fermentation | Fog after meals, bloating, unstable mood, cravings for sweets |
| Glutaric acid | Disruption in passing certain stages of energy metabolism | Slow reaction, rapid attention exhaustion, poor tolerance to exertion |
| Glyoxylate | Oxidative stress and load on antioxidant systems | Poor recovery, worsening memory, feeling of overheating of the nervous system |
| Methylmalonic acid | Functional deficiency of B12 | Memory worse than usual, slowed speech, tingling, apathy |
| FIGLU | Possible deficiency of folate, i.e., vitamin B9 | Poor concentration, fatigue, decreased learning ability |
| Xanthurenic acid | Possible deficiency of B6 or disruption of B6-dependent reactions | Irritability, anxiety, sleep disturbances, sensitivity to stress |
| Homovanillic acid | Indirectly reflects dopamine metabolism | Low motivation, apathy, difficulty starting tasks, slowness |
Examples of values and interpretations
References for organic acids depend on the laboratory, method, and units of measurement. Therefore, it is safer to look not at “magical norms from the internet,” but at the result relative to the reference of a specific laboratory. However, the logic may look like this:
| Conditional example | What it may mean | What to check alongside |
| Methylmalonic acid 0.45 µmol/L with a reference of up to 0.26 µmol/L, B12 260 pg/mL | Possible functional deficiency of B12: serum B12 is still “seemingly normal,” but tissues may lack active B12 | Homocysteine, complete blood count, MCV, nutrition, stomach acidity medications, absorption issues |
| FIGLU twice above the upper reference limit, folate at the lower limit, homocysteine 13 µmol/L | Possible folate deficiency or increased need for B9 | B12, diet, intestines, pregnancy, alcohol, medication factors |
| Xanthurenic acid 1.7 times above the upper reference limit against a background of anxiety and poor sleep | May indicate a deficiency of B6-dependent reactions, which are important for GABA, serotonin, and dopamine | B6 in the diet, magnesium, protein, stress, homocysteine |
| Lactate and pyruvate above reference after a period of infection and overwork | May indicate energy strain and poor tolerance to exertion | Glucose, insulin, ferritin, thyroid function, sleep, recovery after exertion |
| Arabinose three times above the upper reference limit plus fog after sweets and bloating | May signal that intestinal fermentation is involved in the symptoms | Food diary, gastrointestinal symptoms, carbohydrate load, working with a doctor on intestinal issues |
| Homovanillic acid below reference with low motivation and apathy | May reflect decreased dopamine metabolism, but does not directly prove “dopamine deficiency” | Protein, iron, B6, sleep, stress, depressive symptoms |
| Kynurenine 1.5 times above the upper reference limit after chronic stress | May indicate a shift of tryptophan towards an inflammatory-stress pathway instead of calm sleep and stable mood | hs-CRP, infections, sleep, cortisol rhythm, protein deficiency, omega-3 status |
Why one marker cannot be read in isolation
The most common mistake is to see one high indicator and immediately treat it as a standalone disease. Organic acids reflect the flow of metabolism, not a static snapshot of an organ. The result is influenced by nutrition the day before, physical activity, infections, medications, supplements, alcohol, the intestines, sleep, and even the technique of sample collection.
For example, high methylmalonic acid does indeed prompt thoughts about B12, but one must also look at B12, homocysteine, complete blood count, MCV, and reasons for absorption issues. Elevated arabinose may coincide with gastrointestinal symptoms, but it is not a definitive diagnosis of candidiasis on its own. Low homovanillic acid may correspond to apathy, but it cannot be turned into a simple formula of “low dopamine.”
How to act after the result
The most effective course of action is as follows:
- First, correlate markers with symptoms: when does the fog appear, after what food, after what exertion, at what time of day.
- Check basic tests: glucose, insulin, HbA1c, ferritin, complete blood count, B12, folate, homocysteine, TSH, and vitamin D.
- Assess nutrition: is there enough protein, are there sharp carbohydrate swings, are calories and salt too low.
- Address not the number, but the cause: B12, B6, folate, magnesium, intestines, sleep, or recovery after exertion.
- Do not start aggressive schemes based on one marker: it is better to repeat the questionable result or confirm it with another test.
- Evaluate dynamics through well-being and repeated indicators, not through the desire to “bring everything to perfection.”
What this means for keto and low-carb diets
On keto and LCHF, fog in the head is most often related not to “lack of sugar,” but to adaptation, electrolytes, protein deficiency, too large a calorie deficit, poor sleep, or pre-existing deficiencies. However, if a person has been eating low-carb for a long time, is getting enough sleep, consumes enough protein and salt, and the brain still tires quickly, organic acids may provide additional clues.
It is especially important not to attribute everything to ketosis. If there are borderline B12 levels, high homocysteine, low ferritin, poor sleep, pronounced anxiety, or gastrointestinal symptoms, the brain may be “slowing down” not due to a lack of carbohydrates, but because the nervous system lacks normal conditions to function.
Conclusion
When experiencing fog in the head and brain fatigue, it is advisable to start with simple tests and basic physiology: sleep, protein, iron, B12, folate, glucose, insulin, thyroid function, and vitamin D. The analysis of organic acids is the next level when one wants to understand where exactly metabolism is failing.
It can highlight mitochondrial load, hidden deficiencies of B12, B9, or B6, oxidative stress, inflammatory shifts in tryptophan, or the connection of symptoms with intestinal fermentation. But it should be read calmly: not by one marker, but by the overall picture, together with symptoms and standard tests.
