What's living inside you (downstairs round the back) might be affecting your ADHD
Do you ever have days when your ADHD seems to be loads worse and you can't figure out why? I call these my 'white noise days'. My mind is hovering around like a wasp on drugs and won't seem to land on anything long enough for me to get anything done. I've often wondered if it could have anything to do with what I ate the day before. In fact I've been researching for a book on how ADHD is impacted by what you eat. I found an article which I thought you might be interested in. Read on....
A 2024 study published in Scientific Reports (deets at bottom) tested the effects of giving supplements of specific bacteria. It was a really thorough test - double blind with a placebo group. And they used computerised cognitive testing, not unlike the ones I have designed on my tools page, to measure the results in a more quantifiable way, i.e. not just asking the volunteers how they felt at the end.
What the researchers actually did
Sixty college students with ADHD were split into two groups. One group took bacteria tablets every day for three months. The other took a placebo that looked identical and the person giving them out didn't know which is which. Attention, hyperactivity and impulsivity were measured before and after using the computerised test and validated questionnaire. Academic records were checked. Sleep and eating habits were tracked.1
The clever part was that they also measured the stress hormone cortisol to see how the bacteria affected that. And what is even cleverer is how they measured the cortisol. Rather than taking a blood sample on a single day which only gives an in the moment snapshot, the researchers measured levels in fingernail clippings. Cortisol is your main stress hormone. Because fingernails grow slowly the amount of cortisol locked into them reflects how your stress system has been operating over the past several weeks. It's a biological diary, not a snapshot.1
What they found
The people taking the bacteria showed a significant drop in hyperactivity. Their gut symptoms improved. Their academic grades went up. Younger participants got more benefit overall, but I'm still going to give this a try despite being the wrong side of 60.
The cortisol data added something important. People with the highest stress hormone levels - those whose stress response had been running hot for weeks - tended to have the worst ADHD symptoms. This makes total sense because your stress system and your brain chemicals interact directly. When your body keeps pumping out cortisol the part of your brain responsible for focus, planning and impulse control takes a hit (the prefrontal cortex). That's already the part under pressure in ADHD.1 Attention proved harder to shift than hyperactivity.
This wasn't an open-label trial with someone hoping for a result. It was double-blind, placebo-controlled, and published in a peer-reviewed journal. The improvements in hyperactivity and academic performance were real. That's a meaningful mark of quality.
But why would gut bacteria affect your ADHD at all?
Your gut and your brain are in constant conversation through your nervous system, your hormones, and your immune system. The bacteria living in your gut influence what chemicals your body produces, what gets absorbed, and what signals reach your brain.2
When your gut bacteria are out of balance, several things happen that are directly relevant to ADHD. Your body produces less serotonin and dopamine which are the two brain chemicals most associated with attention, mood and behaviour. It also produces less butyrate, a short-chain fatty acid that your brain uses as a kind of fuel. Drop either of those, and the cognitive effects are measurable.3
Add low-level gut inflammation to the mix, which tends to follow when the bacterial community is messed up, and you have a plausible mechanism for why sorting out your gut might nudge your ADHD symptoms in a better direction.3
This study doesn't stand alone
A separate 2024 study examined raw data from four adult ADHD case-control studies and found consistent differences in the gut bacteria of adults with ADHD compared to those without. One type of bacteria in particular - Ruminococcus torques - kept showing up in higher amounts, and it was specifically linked to hyperactivity and impulsivity. The bacteria associated with better outcomes were lower in the ADHD group. The researchers noted that the gut environment in adult ADHD appears to be a more inflamed one.4
A third 2024 paper looked at the relationship between diet, gut bacteria and ADHD in 153 adults. It found that the microbiome appeared to partially explain the link between diet and behaviour and not just sit alongside it.5 In other words, what you eat affects which bacteria thrive, which affects how your brain chemicals are regulated, which affects how your symptoms feel day to day.
What does this mean if you have adult ADHD?
It doesn't mean probiotics replace medication. If your medication is working, this research changes nothing about that. What it does suggest is that gut health is probably not irrelevant to how your symptoms feel and that a few low-risk changes are now supported by more than vague wellness logic.
The specific strains tested in the trial were Lactobacillus helveticus, Bifidobacterium animalis ssp. lactis, Enterococcus faecium, Bifidobacterium longum and Bacillus subtilis.1 You won't easily find a single product containing all five. But L. helveticus and B. longum specifically have the strongest track record across the wider stress and neuropsychiatric research - and they do appear together in some consumer products. Check the labels if you buy anything.
A diet that supports a healthy, diverse gut - plenty of fibre from vegetables, legumes and wholegrains; fermented foods like live yoghurt or kefir; less ultra-processed food — has strong general health evidence behind it and now has a more specific rationale for people with ADHD too.
The bottom line
Taking supplements of these specific bacteria and including things in your diet that are good for the bacteria will probably help reduce your ADHD symptoms.
The evidence points towards Lactobacillus helveticus and Bifidobacterium longum as the two strains most consistently linked to reduced stress hormone levels and better 'neuropsychiatric' outcomes in adults. These are the ones to look for on a label. If you can find a multi-strain supplement that includes both - ideally with a CFU count of at least one billion, and full strain names listed rather than just genus names - that is the most evidence-based starting point available right now.
The signal is consistent enough, and the downside risk small enough, to be worth acting on. Check with your GP if you're on medication, particularly anything that affects your immune system.
References
- Levy Schwartz M, Magzal F, Yehuda I, Tamir S. Exploring the impact of probiotics on adult ADHD management through a double-blind RCT. Scientific Reports. 2024;14:26830. https://doi.org/10.1038/s41598-024-73874-y
- Morais LH, Schreiber HL, Mazmanian SK. The gut microbiota-brain axis in behaviour and brain disorders. Nature Reviews Microbiology. 2021;19(4):241–255. https://doi.org/10.1038/s41579-020-00460-0
- Ramadan YN, Alqifari SF, Alshehri K, et al. Microbiome gut-brain-axis: impact on brain development and mental health. Molecular Neurobiology. 2025;62(8):10813–10833. https://doi.org/10.1007/s12035-025-04846-0
- Bundgaard-Nielsen C, Lauritsen MB, Knudsen JK, et al. The gut-microbiome in adult attention-deficit/hyperactivity disorder: a meta-analysis. European Neuropsychopharmacology. 2024. https://doi.org/10.1016/j.euroneuro.2024.07.006
- Jakobi B, Cimetti C, Mulder D, et al. The role of diet and the gut microbiota in reactive aggression and adult ADHD: an exploratory analysis. Nutrients. 2024;16(14):2174. https://doi.org/10.3390/nu16142174
This article is written for general interest and educational purposes. It does not constitute medical advice. If you have concerns about ADHD or any aspect of your health, please speak with a qualified medical professional.
