Obsessive-compulsive disorder (OCD) is characterized by obsessions (intrusive thoughts, images, or recurrent and persistent impulses that cause marked anxiety) and compulsions (repetitive behaviors or mental acts performed to reduce the anxiety caused by obsessions). It affects approximately 2–3% of the global population, regardless of age or gender. OCD has a strong neurobiological basis: the cortico-striato-thalamo-cortical (CSTC) circuits and the serotonergic system are central to its pathophysiology. First-line treatment is a combination of ERP (Exposure and Response Prevention, a specific type of CBT for OCD) and SSRIs (serotonin reuptake inhibitors, which often require higher doses than those used for depression). Nutritional psychiatry in OCD is an emerging field with preliminary but promising data.

The Serotonergic System, the Microbiome, and OCD

OCD has a strong serotonergic component: SSRIs (which increase serotonin availability) are the most effective medications, and polymorphism of the serotonin transporter gene (5-HTTLPR) is associated with OCD predisposition. Ninety-five percent of the body's serotonin is produced in the intestine (by enterochromaffin cells), regulated by the microbiome. Recent studies have documented differences in the microbiome of people with OCD compared to healthy controls: a study by Turna et al. (2020, Biological Psychiatry: Cognitive Neuroscience and Neuroimaging) found differences in microbiome composition associated with OCD symptom severity. Tryptophan (a precursor to intestinal serotonin) is absorbed by the intestine and converted to serotonin locally. A diet that supports the microbiome—rich in fermentable fiber and fermented foods—can influence intestinal serotonin production and, through the gut-brain axis (vagus nerve), brain neurochemistry. This doesn't mean OCD is cured with kefir: rather, the microbiome is a modulator of the serotonergic system, and diet is the primary modulator of the microbiome.

Inflammation in OCD and Anti-Inflammatory Diet

Several studies have documented elevated inflammatory markers in OCD (TNF-alpha, IL-6, IL-1 beta) similar to those found in depression. A meta-analysis by Abramovitch et al. (2021) found elevation of multiple inflammatory markers in OCD compared to controls. Inflammation can affect the function of cortico-striatal circuits through microglia and through effects on tryptophan metabolism (increased conversion toward the kynurenine pathway, which produces neuroexcitatory metabolites, rather than toward the serotonergic pathway). An anti-inflammatory diet therefore has a neurobiological rationale for OCD, not merely as general wellness. Specific nutrients with anti-inflammatory effects relevant to OCD: omega-3 EPA and DHA (documented anti-inflammatory effect on IL-6 and TNF-alpha), curcumin (inhibits NF-kB and reduces IL-6 and TNF-alpha: mechanism documented in laboratory studies), vitamin D (regulates pro-inflammatory cytokine production), probiotics and prebiotics (reduce systemic inflammation markers through the microbiome). The Mediterranean diet, with its combination of all these elements, remains the dietary model with the strongest evidence for inflammation and mental health.

OCD and the Relationship with Food: Food-Related Compulsions

For some people with OCD, food becomes the object of specific obsessions and compulsions. The most common manifestations: food contamination (obsessions about being poisoned or contaminated by food: leading to exhausting food-cleaning rituals, refusal to eat outside the home, avoidance of entire food categories), symmetry/order around eating (compulsions to arrange food precisely on the plate, to eat in a specific order, to have portions exactly equal), food scrupulosity (obsessions about foods that are "morally wrong," about food origins, about animal suffering: different from vegan ethics, which is a conscious choice), orthorexia nervosa (often comorbid with OCD: pathological concern about food "purity" and "healthiness" leading to increasingly severe restrictions). Note: food-related obsessions in OCD are different from classic eating disorders (anorexia, bulimia), though the two conditions can coexist. Differential diagnostic assessment is important for the correct therapeutic pathway.

SSRI and Nutrient Interactions: What You Need to Know

SSRIs (sertraline, fluoxetine, fluvoxamine, escitalopram, paroxetine) are first-line medications for OCD. Some food interactions are relevant. Fluvoxamine and caffeine: fluvoxamine is a potent inhibitor of CYP1A2 (the enzyme that metabolizes caffeine). People taking fluvoxamine have drastically slowed caffeine metabolism: even a small cup of coffee can produce very prolonged and intense effects (tremors, tachycardia, insomnia). Reducing or eliminating caffeine is often necessary with fluvoxamine. St. John's Wort (Hypericum) and SSRIs: St. John's Wort is an herbal supplement with SSRI-like effects. Combining it with SSRIs can cause serotonin syndrome (agitation, diarrhea, tremor, tachycardia, fever: a serious medical condition). Never take St. John's Wort if you're taking SSRIs. Serotonin syndrome from food and SSRIs: unlike MAOIs, with SSRIs the risk of serotonin syndrome from foods (tyramine) is very low. There are no mandatory specific food restrictions for SSRIs. Grapefruit inhibits CYP3A4 and can increase levels of some SSRIs (especially sertraline): it's prudent to avoid excessive amounts of grapefruit and grapefruit juice during therapy.

N-Acetylcysteine (NAC) in OCD: Emerging Evidence

NAC (N-acetylcysteine) is a precursor to glutathione and a modulator of the glutamatergic system. Glutamate is implicated in OCD pathophysiology (the CSTC circuits in OCD use glutamate as the primary neurotransmitter). NAC reduces glutamate release by modulating cystine/glutamate antiporters in neurons and glial cells. Evidence in OCD: a pilot study by Lafleur et al. (2006, Journal of Clinical Psychiatry) showed improvement in OCD symptoms with NAC in an open-label design (without a control group). A randomized controlled trial by Costa et al. (2017, Frontiers in Psychiatry) on 44 patients with treatment-resistant OCD showed that NAC as an add-on to SSRIs significantly improved Y-BOCS scores (OCD severity scale). Dosage: 1,800–3,000 mg/day in divided doses. Safe and well-tolerated. NAC is also found in foods rich in cysteine (eggs, chicken, legumes, broccoli, onion, garlic), but food amounts are much lower than therapeutic doses. Important discussion: NAC is not approved for OCD, and data are still preliminary. Its use should be discussed with the professional managing OCD treatment, not undertaken independently as a replacement for standard therapy.

Inositol in OCD: A Lesser-Known Nutrient with Interesting Data

Inositol (myo-inositol) is a carbohydrate compound naturally present in many foods (citrus fruits, legumes, nuts, whole grains: 1–2 g/day in a typical diet). It plays a role as a second messenger in serotonergic receptors (5-HT2) and glutamatergic receptors. Evidence in OCD: the most important trial is that by Fux et al. (1996, American Journal of Psychiatry): a randomized crossover study on 13 patients with OCD found that inositol (18 g/day for 6 weeks) significantly improved Y-BOCS scores compared to placebo. A subsequent trial (Carey et al., 2004) compared inositol with fluvoxamine: similar effects in the first months. Proposed mechanisms: inositol "resensitizes" serotonergic receptors (increases the sensitivity of 5-HT2 receptors to serotonin signaling, compensating for the impoverishment of serotonergic signaling in OCD). Dosage studied in OCD: 12–18 g/day (much higher than normal dietary intake). At these doses, inositol must be taken as a supplement (powder dissolved in water). Side effects: gastrointestinal (bloating, diarrhea) at high doses. Final note: data on inositol in OCD are promising but based on small studies. As with NAC, discussion with the psychiatrist or psychotherapist managing OCD is essential before any supplementation.