Brain fog and fatigue: when your gut makes your head tired

Healthy state

"Brain fog" is not a diagnosis but a description. It means that thinking becomes sluggish: concentrating is hard, words go missing, things get forgotten. When people with coeliac disease Coeliac disease An autoimmune disease in which gluten damages the lining of the small intestine. It is diagnosed with a blood test and usually a biopsy while you are still eating gluten. More in the chapter “Lactose, fructose, gluten: making sense of intolerances” described their experience for a questionnaire, the most common phrases were "difficulty focusing", "difficulty thinking" and "difficulty finding the right words".

For a clear head the brain needs enough oxygen, which means enough red blood cells and the iron to make them, plus vitamin B12 for nerves and blood formation, restful sleep and a calm immune system. The gut is involved at several points: iron and B12 are absorbed here, a large part of the immune system sits here, and signals constantly travel from here to the brain.

During an infection, tiredness is actually intended. Inflammatory messengers act on the brain and make you tired, listless and slow, so the body saves energy for defence. Experts call this sickness behaviour. Normally it fades once the inflammation is over.

Disrupted state: causes and effects

Fatigue is very common with gut conditions. A meta-analysis Meta-analysis An analysis that combines the results of several studies. It is only as good as the studies it includes. of more than 16,000 people found it in about 55 percent of people with IBS Irritable bowel syndrome Recurring abdominal pain linked to bowel movements, without tests finding another cause. Today it is understood as a disorder of gut–brain interaction. More in the chapter “The gut-brain axis”, 50 percent with Crohn's disease and 44 percent with ulcerative colitis. In inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation” almost three in four were affected during a flare, and almost half even in remission. Anaemia, sleep problems, anxiety and depression were most strongly linked to it.

Deficiencies are common and measurable causes. With gut inflammation, coeliac disease or blood loss, iron is often lacking. According to a meta-analysis, about one in 31 people with iron deficiency anaemia has coeliac disease. Vitamin B12 is more likely to run short after years of acid blockers, after small-bowel surgery, with small intestinal bacterial overgrowth Small intestinal bacterial overgrowth (SIBO) Too many bacteria in the small intestine, where normally few live. Bloating and diarrhoea are typical; it is usually diagnosed with a breath test. More in the chapter “When the small intestine is too densely colonised” or on a vegan diet without supplements. B12 deficiency can impair blood formation and nerves.

In untreated coeliac disease, brain fog is well described. In a small study of eleven newly diagnosed adults, attention and verbal fluency improved over a year on a gluten-free diet, in step with healing of the small-bowel lining. But: when a questionnaire was being developed, perceived brain fog hardly matched the results of cognitive tests. Experiencing and measuring are two different things, and both matter.

Sleep is often underestimated. A meta-analysis of more than 60,000 people found sleep disorders in about 38 percent of people with IBS, clearly more often than without. Pain, urgency at night and worry steal sleep, and poor sleep makes you more tired and more sensitive to pain the next day. In IBS, studies point more to attention being tied up, because pain and gut signals demand it, than to a general decline in thinking.

A widely shared study links brain fog to probiotics Probiotics Live microorganisms that are meant to provide a benefit in sufficient amounts. The effect depends on the individual strain. More in the chapter “Using fermented foods well”. Of 30 people with bloating and brain fog, all were taking probiotics, many had small intestinal bacterial overgrowth and raised D-lactate in their urine, an acid some bacteria produce. After stopping probiotics and taking an antibiotic, the fog cleared in 23 of 30. But the comparison group had only 8 people, there was no group that only stopped probiotics, and D-lactate was measured in urine rather than blood. It is a clue, not proof.

Many explanations from the internet have not been studied for brain fog, such as "toxins from a leaky gut" or "Candida in the brain". Believing them easily leads to expensive cures and strict diets that can themselves make you tired. More on this in the chapters on the gut barrier and on Candida.