Research guide · Depression · Anxiety · OCD · Burnout
"It's all in your head." Your head is attached to a body, and research now measures that body in depression, anxiety, OCD and burnout: inflammation, stress hormones, gut bacteria, metabolism and brain chemistry. None of this is a character flaw. It is biology, and much of it is still being worked out.
Metabolic psychiatry (the study of how the body's energy systems affect mental health) is a young but growing field.
A 2026 review in Nature Mental Health describes how insulin resistance, blood fat imbalances, mitochondrial problems and inflammation turn up often in people with psychiatric illness. The authors say some metabolism-focused approaches look promising, but many have too little evidence yet, and more human trials are needed.
Insulin resistance means your cells respond less well to insulin (the hormone that moves sugar into cells). A meta-analysis of 18 studies and over 25,000 people found a small but real link between depression and insulin resistance. It shows the two occur together, not which one comes first.
Psychiatrist Christopher Palmer proposes that mitochondria (the parts of cells that make energy) underlie both metabolic and psychiatric conditions. It is a hypothesis, set out in his book Brain Energy and a 2025 commentary, not a proven explanation. He notes that randomised trials are still needed.
A 2023 meta-analysis of 1,400 people found higher levels of mitochondrial DNA (genetic material released from the cell's energy parts) in the blood of people with depression. Studies varied a lot and most were snapshots in time, so this hints at energy stress rather than proving it.
A 2024 pilot study followed 23 people with bipolar disorder or schizophrenia on a ketogenic diet (very low carbohydrate, high fat) for four months. Metabolic markers and symptom scores improved, but there was no comparison group. This is early research, not advice.
Low-grade inflammation means your immune system is running slightly "on" all the time. A meta-analysis of 37 studies found it in about 27% of people with depression, measured by CRP (a blood marker of inflammation). That is more common than in people without depression, but it is not everyone.
A 2019 meta-analysis of 36 trials found that anti-inflammatory medicines reduced depressive symptoms more than placebo. The authors rated all included studies at high risk of bias and called for larger, longer trials. This supports inflammation as one piece of the picture, not a treatment plan.
The HPA axis is your stress-hormone system, which releases cortisol. In a Dutch study of nearly 1,600 people, those with current or past depression had a slightly higher cortisol rise on waking. The difference was modest, but it is a real, measurable body change.
A systematic review searched for a biological marker (a measurable sign in the body) of burnout and found none that held up, largely because studies measured things differently. That is a gap in the science, not proof that your exhaustion isn't real.
The gut-brain axis is the two-way communication line between your digestive system and your brain.
Over 90% of your body's serotonin (a chemical messenger) is made by enterochromaffin cells (hormone-making cells in the gut lining), where it helps control gut movement. Gut serotonin does not travel into the brain, which makes its own. So the gut affects mood through other routes, not by supplying brain serotonin.
Gershon & Tack, 2007, Gastroenterology · Yano et al., 2015, Cell · Berger et al., 2009, Annu Rev Med
In over 1,000 people, two bacteria groups, Coprococcus and Dialister, were lower in people with depression, even after accounting for antidepressants. Coprococcus makes butyrate (a fatty acid that feeds gut cells). This shows a link, not cause and effect.
Short-chain fatty acids such as butyrate and acetate are made when gut bacteria break down fibre. A 2024 review found they are often linked to depressive symptoms, but which ones and in which direction varied between studies. It is not yet a consistent finding.
A meta-analysis of controlled trials found probiotics (live "friendly" bacteria) had a small effect on depression and a very small effect on anxiety. Effects looked larger in clinical groups, but there were few such trials. The authors asked for more research in people with diagnosed conditions.
Magnetic resonance spectroscopy (MRS, a scan that measures chemicals in the living brain) lets researchers look at glutamate (the main "go" signal) and GABA (the main "calm" signal).
A meta-analysis of 49 scan studies found slightly lower levels of glutamate-related chemicals in the front of the brain in depression. That contradicts the popular claim of "excess glutamate" in depression. The drop was clearest in people taking medication, so the picture is still being untangled.
Pooling 121 scan studies, researchers found GABA slightly lower across psychiatric conditions, and lower in one brain region in mood disorders. The differences were small to moderate and depended on brain area. They are averages across groups and can't diagnose you.
OCD was long thought to involve too much glutamate. A 2026 review of 20 scan studies in unmedicated people found limited support for that idea and pointed to GABA as a more likely lead. Your OCD is real; the exact chemistry is still being worked out.
MTHFR is a gene for an enzyme that processes folate (vitamin B9). One common variant, C677T, was linked to a modestly higher chance of depression in a review of over 11,000 people. It is one small risk factor among many, not a cause on its own.
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