Cod for Anxiety: Why This Cheap White Fish Belongs on Your Keto Mediterranean Plate

Serotonin cannot cross the blood-brain barrier. The brain must synthesise its own, from scratch, starting with tryptophan — an essential amino acid that your body cannot produce and must obtain from food. Tryptophan is converted to 5-HTP (5-hydroxytryptophan) and then to serotonin in a two-step enzymatic process. No dietary tryptophan, no serotonin synthesis.

Cod provides 279mg of tryptophan per 100g — more than chicken breast (250mg), more than turkey (270mg), and comparable to eggs (167mg per egg). For a food that is not typically discussed in the context of mood or mental health, this is a meaningful amount. A standard cod fillet (150–180g) delivers 420–500mg of tryptophan — enough to meaningfully contribute to daily requirements alongside other dietary sources.

The catch with tryptophan is that it competes with other large neutral amino acids (LNAAs) for transport across the blood-brain barrier. This is why tryptophan from protein sources is most effective when eaten alongside carbohydrates — the insulin response from carbs preferentially moves competing amino acids into muscle tissue, leaving tryptophan with less competition for brain entry. In a keto context, this dynamic changes — but the baseline tryptophan from cod still contributes meaningfully to the pool available for serotonin synthesis.

Vitamin B12 is essential for myelin synthesis — the fatty sheath that insulates nerve fibres and enables efficient signal conduction throughout the nervous system. B12 deficiency causes demyelination, which slows and disrupts nerve signalling. In the context of anxiety, this manifests as heightened sensory sensitivity, exaggerated stress responses, difficulty regulating emotional states, and physical symptoms including tingling, palpitations and fatigue that independently increase anxiety load.

B12 deficiency is substantially more common than most people realise — particularly in women over 40, people with digestive issues affecting absorption (gastric acid reduction, IBS, Crohn’s), and anyone taking metformin (which depletes B12). The psychiatric presentation of B12 deficiency — anxiety, irritability, emotional lability — is frequently misattributed to stress or primary anxiety disorder rather than nutritional deficiency.

Cod provides approximately 1.2μg of B12 per 100g — around 50% of the recommended daily intake in a single portion. Combined with sardines, eggs and dairy (all strong B12 sources), a keto Mediterranean diet eliminates B12 deficiency as an anxiety driver almost by default.

Thyroid hormones regulate metabolic rate, temperature, heart rate and mood — through direct effects on brain tissue and indirect effects via the HPA (hypothalamic-pituitary-adrenal) axis. Both hyperthyroidism (overactive) and hypothyroidism (underactive) produce anxiety symptoms, through different mechanisms. Subclinical hypothyroidism — where TSH is slightly elevated but T3 and T4 are technically within range — is particularly relevant: it is common, frequently undiagnosed, and produces persistent low-grade anxiety, fatigue, cold sensitivity and cognitive fog that is often attributed to stress rather than thyroid function.

Iodine is the primary mineral required for thyroid hormone synthesis — T3 contains three iodine atoms, T4 contains four. Iodine deficiency directly impairs thyroid hormone production and can drive subclinical hypothyroidism even in the absence of autoimmune thyroid disease. Plant-based and dairy-free diets are frequently iodine-deficient; the keto Mediterranean diet, by contrast, is naturally high in iodine through fish and dairy.

Cod is one of the best dietary iodine sources available — providing approximately 110μg per 100g, around 73% of the recommended daily intake. This is substantially more than most other protein foods and makes cod genuinely valuable for thyroid support in a way that, for example, chicken and red meat simply are not.

Cod contains approximately 0.2–0.3g of combined EPA and DHA per 100g — meaningfully lower than sardines (1.5g), mackerel (2.5g) or salmon (2.0g). This means cod should not be your primary omega-3 source. But “not the primary source” is not the same as “irrelevant.”

In a weekly rotation where sardines appear twice and mackerel once, adding cod once provides an additional 0.3–0.5g EPA/DHA on top of the 3–5g already obtained from the oily fish — contributing to the cumulative omega-3 index without requiring a larger quantity of stronger-tasting fish. For someone building toward a 2–3x per week oily fish habit, cod provides a fourth fish meal that keeps the rotation varied without taxing the palate.

Cod also provides selenium — 33μg per 100g, around 60% of the RDI. Selenium is an essential cofactor for glutathione peroxidase, the brain’s primary antioxidant enzyme. Glutathione depletion is associated with oxidative stress in the brain, which both causes and amplifies anxiety. In this sense, cod contributes to anxiety reduction through an antioxidant pathway that is distinct from omega-3 and distinct from serotonin.