(Part 1) Beyond the Blues: Functional Medicine Approach to Depression

Depression is one of the most prevalent and disabling conditions in the world today. Yet for many people, standard treatments, antidepressant medications and talk therapy, provide only partial relief. Functional medicine offers a different lens: rather than asking “which drug matches this symptom?”, it asks “why is this person depressed?”

This root-cause approach recognises that depression is not a single disease with a single cause. It is a final common pathway of multiple, interconnected biological imbalances, and treating it effectively means identifying and addressing those upstream drivers.

The goal of Functional Medicine is to explore the provoking factors, collect data and create a personalized program for each patient.

The Limits of the Serotonin Story

For decades, depression has been explained primarily as a “chemical imbalance”, specifically, low serotonin. This framing has been central to the widespread use of selective serotonin reuptake inhibitors (SSRIs).

But this narrative is increasingly challenged by the science. Extensive reviews confirm there is no direct causal evidence that low serotonin causes depression.^[1] While serotonin plays a role in mood, depression is far more complex, involving inflammation, hormonal dysregulation, gut microbiome disruption, nutritional deficiencies, chronic stress, and more. Functional medicine embraces this complexity rather than reducing it to a single molecule.

The 7 Root Causes of Depression

Root Cause #1: Chronic Inflammation

One of the most compelling emerging frameworks for depression is the inflammatory hypothesis. Research shows a bidirectional relationship between inflammation and depression: inflammatory processes can drive depressive symptoms, and depression itself can increase systemic inflammation.^[2]

How it works: Pro-inflammatory cytokines are capable of crossing the blood-brain barrier, activating microglia (the brain’s immune cells), and triggering neurodegenerative changes. Chronic inflammation also diverts tryptophan away from serotonin synthesis and toward the production of neurotoxins such as quinolinic acid, which is linked to oxidative stress and neurodegeneration.^[3]

Meta-analyses of randomised clinical trials demonstrate that anti-inflammatory interventions reduce depressive symptoms.^[4]Functional medicine targets the sources of that inflammation: poor diet, gut dysbiosis, environmental toxins, food sensitivities, chronic infections, and unresolved psychological stress.

Root Cause #2: The Gut-Brain Axis

The gut is no longer considered a passive digestive organ. It is an active neuroendocrine system, housing some 500 million neurons and producing approximately 95% of the body’s serotonin. The gut-brain axis is a bidirectional communication network linking the gastrointestinal tract and the central nervous system (CNS) via the autonomic nervous system, the Hypothalamic-Pituitary-Adrenal (HPA) axis, and immune signalling pathways.^[5]

Accumulating evidence shows that gut microbial dysbiosis (an imbalance in the microbiome) is not merely a consequence of depression; it is a contributor to its development.^[6] In a demonstration of causality, transferring microbiota from depressed donors to non-depressed recipients has been shown to produce depression-like symptoms in animal models.^[7]

The mechanisms are multiple:

Neurotransmitter metabolism: Gut bacteria regulate the availability of serotonin precursors and produce neuroactive metabolites including short-chain fatty acids (SCFAs), indoles, and bile acid derivatives.^[8]
Immune modulation: Dysbiosis drives low-grade systemic inflammation via increased intestinal permeability (“leaky gut”), allowing bacterial endotoxins such as lipopolysaccharide to enter circulation.^[9]
HPA axis regulation: Gut microbiota influence cortisol release and stress reactivity through the HPA axis.^[10]

Functional medicine interventions targeting the gut-brain axis include:

Comprehensive gut testing (microbiome analysis, intestinal permeability markers), at OT&P we use the GI Map
Targeted probiotics: Lactobacillus and Bifidobacterium species combined with magnesium have shown clinically significant reductions in depression, anxiety, and stress scores^[11]
Prebiotic, high-fibre diets to support microbial diversity
Elimination of dietary triggers driving inflammation

Root Cause #3: The HPA Axis and Chronic Stress

The HPA axis, comprising the hypothalamus, pituitary, and adrenal glands, is the body’s central stress response system. Under chronic stress, sustained HPA activation leads to excessive cortisol production, which can cause hippocampal atrophy, impaired neuroplasticity, and neuroinflammation.^[12]

Elevated cortisol compromises the brain’s ability to modulate stress response. Studies show that patients who fail to respond to antidepressants often exhibit cortisol hypersecretion, suggesting HPA dysregulation as a key driver of treatment-resistant depression.^[12]

Chronic stress also disrupts gut microbiota diversity and promotes intestinal permeability, linking the stress axis directly to gut-brain dysfunction.^[9]

Functional testing (e.g., DUTCH test or salivary cortisol panels measuring the diurnal cortisol curve) can identify patterns of HPA dysregulation, from elevated morning cortisol and high evening cortisol (associated with anxiety and sleep disruption) to blunted cortisol awakening response (associated with burnout and depression).^[13]

Interventions addressing HPA axis dysfunction include mindfulness-based stress reduction, breathwork, trauma-informed therapies, sleep hygiene optimisation, and targeted adaptogenic botanicals where clinically appropriate.^[14]

Root Cause #4: Nutritional Deficiencies

The brain is a metabolically demanding organ, entirely dependent on an adequate supply of micronutrients for neurotransmitter synthesis, neuroprotection, and regulation of inflammation. Yet nutritional deficiencies are commonly overlooked in conventional depression care.

Omega-3 Fatty Acids

Low levels of omega-3 Polyunsaturated Fatty Acids (PUFAs), particularly EPA and DHA, are associated with increased depression risk. Observational data show that omega-3 supplementation significantly reduces depressive symptoms, with combination omega-3 and antidepressant therapy producing significantly greater improvements than either treatment alone.^[15] Both vitamin D and omega-3 fatty acids modulate depression pathways by influencing neurotransmission and reducing inflammation.^[16]

Vitamin D

Vitamin D deficiency is highly prevalent in Asia (including Hong Kong), and low serum 25(OH)D levels are associated with increased depression severity. Meta-analyses suggest vitamin D supplementation is effective in individuals with major depressive disorder and plasma levels ≤50 nmol/L.^[17]

B Vitamins and Folate

Deficiencies in folate (B9) and vitamin B12 are strongly correlated with depressive disorders and are associated with greater symptom severity and longer disease duration. Elevated homocysteine (a marker of B12 and folate insufficiency) is linked to cerebrovascular changes and neurotransmitter disruption. Combined folate and antidepressant therapy has been shown to enhance treatment efficacy and reduce homocysteine levels.^[3]

Zinc and Magnesium

Zinc deficiency is well-established as a risk factor for depression. In randomised controlled trials, zinc supplementation significantly reduced Beck Depression Inventory scores compared to placebo.^[18] Similarly, co-supplementation of probiotics and magnesium for 9 weeks has significantly reduced inflammatory CRP levels in individuals with obesity and depressed mood.^[19]

Root Cause #5: Hormonal Imbalances

Hormonal dysregulation is a frequently underdiagnosed contributor to depression. Functional medicine evaluates the full hormonal landscape:

Thyroid: Both hypothyroidism and hyperthyroidism can cause depression and anxiety. Even subclinical thyroid dysfunction can meaningfully affect mood, with thyroid disease severity generally correlating with mood disturbance severity.^[20]
Sex hormones: Oestrogen and progesterone fluctuations during the menstrual cycle, perimenopause, and menopause are significant drivers of mood disorders.^[20]Low testosterone (in both men and women) is associated with depression, fatigue, and poor concentration.^[21][22]
Cortisol: As discussed, chronic hypercortisolism impairs neuroplasticity and drives treatment resistance.

A thorough hormone panel, including thyroid function (including free T3 and reverse T3), sex hormones (oestradiol, progesterone, testosterone), and adrenal markers, forms part of the functional medicine workup for depression. The DUTCH test we use at OT&P can measure sex hormones and Cortisol.

Root Cause #6: Diet and Lifestyle

Evidence from clinical trials and epidemiological research consistently supports a dietary role in depression. The Mediterranean diet, emphasising seafood, olive oil, vegetables, fruits, nuts, lean protein, and whole grains, is associated with a lower risk of depression and may assist in its management.^[23]Whole-diet interventions rich in flavonoids are associated with the greatest reductions in depression scores, likely through antioxidative and anti-inflammatory mechanisms.^[24]

Exercise is among the most powerful lifestyle interventions for depression. It is effective both as a primary treatment and as an adjunct to medication or therapy.^[25] The mechanisms are well characterised: exercise reduces resting cortisol levels, boosts brain health, promotes endorphin and dopamine release, and improves sleep quality.^[26]

Sleep is foundational. Depression and sleep disruption are bidirectionally linked, and improving sleep hygiene is a core component of any functional medicine plan for mood disorders.^[27]

Root Cause #7: Low SAMe and Impaired Methylation

S-adenosylmethionine (SAMe) is synthesised in virtually every cell in the body from the amino acid methionine and adenosine triphosphate (ATP). It serves as the brain’s principal methyl group donor, a small chemical unit transferred to hundreds of biological targets in reactions collectively known as transmethylation.^[28]

In the context of depression, SAMe participates in at least three overlapping biological domains: neurotransmitter synthesis and catabolism, epigenetic regulation, and membrane phospholipid metabolism and neuronal fluidity. When SAMe levels fall due to nutrient deficiency, genetic polymorphisms, or chronic oxidative stress, all three systems are simultaneously impaired.^[29]

Dr Tim Trodd

Family Medicine, Functional Medicine, General Practice

MBBS (London)
DCH (London)
DRCOG (UK)
MRCGP (UK)
FHKAM (Family Medicine)

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References

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