A nutritional Therapist Explains
Brain fog is one of the most common complaints I hear in clinic. People describe it differently. Some say their head feels stuffed with cotton wool. Others talk about a fuzzy feeling that won’t shift, or a heaviness behind the eyes that makes everything feel like wading through treacle. A few have told me they genuinely worried they were developing early dementia.
It’s not a medical diagnosis. You won’t find “brain fog” listed in any textbook. But the experience is real: difficulty concentrating, poor short-term memory, mental fatigue, struggling to find the right word mid-sentence, reading the same paragraph three times and still not taking it in. For some people it comes and goes. For others it’s there every single day.
What frustrates most of my clients is that their blood tests come back “normal” and they’re told there’s nothing wrong. But something clearly is. The brain doesn’t just stop working properly for no reason.
Brain fog almost always has a physiological cause. Usually more than one. And most of those causes respond well to nutritional and dietary changes. I want to walk through the ones I see most often, because several of them are overlooked far too frequently.
Your gut is talking to your brain
This is the cause I find myself explaining more than any other. Your gut and your brain are in constant two-way communication through something called the gut-brain axis. The vagus nerve is the main physical link between them, but your gut bacteria also produce neurotransmitters that directly affect how your brain works. About 90% of the body’s serotonin is made in the gut, not in the brain.
When the balance of bacteria in your gut is disrupted (a state called dysbiosis), or when the gut lining becomes too permeable, inflammatory molecules can cross into the bloodstream and reach the brain. This triggers neuroinflammation. And neuroinflammation is one of the most well-documented drivers of brain fog.
I see this pattern all the time. Clients come in with brain fog as their main complaint and then mention the bloating, the irregular bowel movements, the reflux almost as an afterthought. They haven’t connected the two. But once we address the gut, the fog starts to shift.
I’ve written more about this in my article on the gut-brain connection.
Nutrient deficiencies your GP might not be testing for
Your brain is metabolically expensive. It uses roughly 20% of your total energy despite being about 2% of your body weight. It needs a reliable supply of specific nutrients to function properly, and when those run low, cognitive performance is one of the first things to drop.
These are the deficiencies I test for most often in clients with brain fog:
Iron and ferritin. Probably the most underdiagnosed cause of brain fog in women. Your GP will likely check haemoglobin, and if that’s fine you’ll be told your iron is normal. But ferritin (your iron stores) can be very low without haemoglobin budging yet. A ferritin level below 30 is enough to cause fatigue and cognitive problems in many people, even though the “normal” lab range starts much lower than that.
Vitamin B12. Essential for nerve function and the production of myelin, which insulates your nerve cells. B12 deficiency causes a specific kind of fog. Slow thinking, difficulty with recall, sometimes tingling in the hands or feet as well. More common than people realise, particularly if you don’t eat much red meat.
Vitamin D. The research linking low vitamin D to impaired cognitive function has become hard to ignore. It plays a role in neuroplasticity and helps regulate brain inflammation. In the UK, most people are deficient by the end of winter unless they’re supplementing.
Omega-3 fatty acids. Your brain is approximately 60% fat, and DHA (a type of omega-3) is one of its main structural components. Low intake of oily fish is very common in the UK and it genuinely does affect cognitive performance over time.
Magnesium. Involved in over 300 enzymatic reactions in the body, including many that directly support brain function. Chronic stress burns through magnesium quickly, and low magnesium makes the stress response worse. It becomes a cycle that’s hard to break without addressing the deficiency.
These don’t always show up with obvious symptoms beyond the fog itself. A thorough blood panel is worth doing. I can advise on which specific markers to request if you’re not sure what to ask your GP for.
Blood sugar crashes (even without diabetes)
You don’t need to be diabetic for blood sugar to be behind your brain fog.
When you eat a meal that’s heavy on refined carbohydrates and light on protein and fat, blood sugar spikes rapidly. The body overcompensates with insulin, which sends it crashing back down. During that crash, your brain is being starved of glucose. Its primary fuel. The result is foggy thinking, difficulty concentrating, irritability, sometimes a dull headache.
This tends to catch out people who skip breakfast and grab something carb-heavy mid-morning. Or those who start the day with toast or cereal and then feel like they’re moving through mud by 10am.
The fix is structural rather than complicated. Meals that combine protein, healthy fats and fibre release glucose slowly and keep the brain fuelled at a steady rate. Clients are often surprised by how much difference this single change makes.
Food sensitivities
Not the same as a food allergy. An allergy triggers an immediate immune response. A sensitivity produces a slower, lower-grade inflammatory reaction that can take hours or days to become noticeable. That delay makes it genuinely difficult to identify the trigger on your own.
The foods I see causing problems most frequently are gluten, cow’s dairy and eggs. Histamine-rich foods are another common culprit, especially in people who also experience headaches, skin flushing or nasal congestion alongside the fog.
The standard clinical approach is an elimination diet. You remove the suspected triggers for three to four weeks, track how you feel, and then reintroduce them one at a time. It takes some discipline but it’s one of the most effective tools I use in practice. When a food sensitivity is driving the brain fog, clients regularly notice a real improvement within the first fortnight.
If you think histamine might be a factor, I’ve covered histamine intolerance in a separate article.
Hormonal shifts
Brain fog is extremely common during perimenopause and menopause, and it catches many women off guard. Oestrogen plays a direct role in brain function. It supports memory, concentration and verbal fluency. When oestrogen levels start to fluctuate and eventually decline, many women experience cognitive changes that can be frightening.
What’s rarely discussed is the nutritional angle. Supporting healthy oestrogen metabolism through liver function, including phytoestrogen-rich foods like ground flaxseed and fermented soya, making sure B vitamin intake is adequate for methylation, and stabilising blood sugar (which affects cortisol, which in turn affects sex hormones) can all make a noticeable difference.
Brain fog also shows up frequently with thyroid dysfunction. Subclinical hypothyroidism in particular, where TSH is technically within range but sitting at the higher end, is worth investigating if the fog comes with weight gain, cold intolerance or thinning hair.
Chronic stress
Everybody knows stress affects the brain. But understanding the mechanism explains why the fog so often persists after the stressful period has ended.
Sustained stress keeps cortisol elevated. Over time, high cortisol damages the hippocampus, the part of the brain responsible for memory and learning. It disrupts sleep architecture, reduces nutrient absorption, and shifts the gut microbiome in an unfavourable direction. Stress doesn’t only cause brain fog on its own. It makes every other item on this list worse.
Nutritionally, the priority is replenishing what chronic stress depletes fastest: magnesium, B vitamins (particularly B5 and B6), vitamin C and zinc. Certain adaptogenic herbs can also help support the stress response, although I’d always recommend working with a qualified practitioner before supplementing with those rather than picking something off a shelf.
Poor sleep
This one seems obvious. But it’s worth mentioning because the relationship runs in both directions. Poor sleep causes brain fog, yes. But gut inflammation, blood sugar crashing at 3am, low magnesium and elevated cortisol all disrupt sleep quality. If you’ve worked through all the standard sleep hygiene advice and nothing has shifted, the root cause of your poor sleep might itself be nutritional.
Post-COVID brain fog
A significant number of people who’ve had COVID experience persistent brain fog for weeks or months afterwards. The research currently points to neuroinflammation, disruption of the gut microbiome and mitochondrial dysfunction as the primary drivers. Nutritional strategies that target inflammation (omega-3s, polyphenol-rich foods, cutting back on ultra-processed food) alongside gut restoration (diverse dietary fibre, fermented foods, and in some cases specific probiotic strains) appear to be the most promising dietary approaches. Rest and pacing remain important too.
How to start working out what’s behind yours
If brain fog has been hanging around and you want to get to the bottom of it, here’s where I’d suggest starting:
- Get proper blood work done. Not just the basic panel. Ask specifically for ferritin (not just haemoglobin), vitamin D, active B12, folate, a full thyroid panel including antibodies, and fasting glucose. If your GP won’t run all of these, a nutritional therapist can arrange private testing.
- Track what you eat and how you feel. Keep a food and symptom diary for a couple of weeks. Write down what you eat, roughly when, and how you’re feeling 1-2 hours after each meal. Patterns tend to emerge faster than you’d expect.
- Look at the structure of your meals. Are you getting protein at each meal? Are there fats and fibre in there too, or is it mostly carbohydrate? Are you going long stretches without eating?
- Try an elimination diet if food sensitivities seem likely. Removing gluten, dairy and eggs for three to four weeks is a reasonable starting point. Bring them back one at a time and pay close attention to what happens.
- Work with someone who can look at the full picture. Brain fog usually has more than one contributing factor. Sorting through the possibilities systematically with a practitioner who knows what to test for and how to read the results saves a lot of time compared to trying things at random.
You don’t have to keep living with this
Brain fog is not something you should just accept. In the large majority of cases, there’s a clear physiological reason it’s happening. Once you find that reason and address it, the fog lifts. I’ve watched it happen with client after client over the years.
If you’d like help working out what’s causing your brain fog, I offer a free 30-minute health support call. It’s a chance to talk through what you’re experiencing and figure out whether nutritional therapy is the right next step for you.
