Before You Start an Antidepressant:
What Nutrition Can and Can’t Do

This post covers the point where someone is about to start an antidepressant, or is being assessed for depression or anxiety. My aim is narrow. I want to set out which nutritional and blood findings are worth asking about, how good the evidence is for each, and where it runs out.

I’m not saying antidepressants don’t work, and I never advise anyone to stop or change one. Antidepressants beat placebo in trials, and for many people they help a great deal, sometimes saving lives. If you’re on medication and want to change anything, talk to your GP or prescriber first. Stopping suddenly can cause withdrawal effects and relapse.

The grades in this post: Strong means good randomised trials agree. Mixed means the trials are few, small, short, unblinded or conflicting. Mechanistic means the biology makes sense but hasn’t been shown to change outcomes in people. None means there’s no usable evidence. Practice means what experts, guidelines or I do, not tested in trials. Observational studies are labelled as such: they show a link, not proof of cause.

Nothing on this page reaches Strong for treating depression with a nutrient. That’s the honest headline.

Where the serotonin story stands

In 2022 an umbrella review in Molecular Psychiatry (Moncrieff and colleagues) found no consistent evidence that depression is caused by low serotonin. That’s a finding about the cause of depression, not about whether the drugs work. The largest network meta-analysis of antidepressant trials (Cipriani 2018, 522 trials, about 116,000 people) found all 21 antidepressants more effective than placebo, though the certainty of evidence ranged from moderate to very low.

So both things are true. We don’t fully understand why antidepressants help, and they do help many people. That leaves room to ask what else might be contributing. It doesn’t make medication the wrong choice.

For scale: in England in 2024/25, an estimated 8.89 million people were prescribed an antidepressant (NHS Business Services Authority).

The nutrients and markers: what the evidence shows

Vitamin B12

Observational for deficiency causing symptoms; Mixed for supplements if you’re not deficient.

True B12 deficiency can cause neurological and psychiatric problems before it causes anaemia. In a classic series of 141 patients with nerve or psychiatric problems from B12 deficiency, 40 (28%) had no anaemia or enlarged red cells. Their methylmalonic acid (MMA) and homocysteine were high, and nearly all improved with B12 (Lindenbaum 1988).

The other side: in people who aren’t deficient, B12 supplements didn’t improve depressive symptoms across 16 trials (Markun 2021). So the useful question is whether you’re deficient. A normal full blood count doesn’t settle it. If your symptoms fit, ask your GP whether B12 testing, and MMA or homocysteine where available, is appropriate.

Folate and methylfolate

Observational for folate levels; Mixed for methylfolate as an add-on.

People with depression have slightly lower folate on average. The difference is small and the studies vary a lot (Bender 2017).

L-methylfolate has been tested as an add-on for people who hadn’t responded fully to an SSRI. In two trials, one found no difference and the other found a benefit at the higher dose tested (Papakostas 2012). A later meta-analysis of three to four studies found a small effect (Maruf 2021). An international guideline taskforce rated methylfolate as a weak recommendation (“+”) for depression. It rated folic acid on its own as not recommended (Sarris 2022, WFSBP/CANMAT).

MTHFR. A common MTHFR variant (C677T) lowers the enzyme’s activity, and people with two copies (TT) tend to have higher homocysteine (Frosst 1995). In one large analysis, TT carriers were about 22% more likely to have had depression (Almeida 2008, a genetic association study, so Observational). I found no good evidence that MTHFR status predicts whether an antidepressant works. I also found none that folic acid is harmful for people with the variant. The NHS advises folic acid in pregnancy regardless of MTHFR. I don’t recommend methylfolate to everyone on the basis of a gene result. None for MTHFR predicting antidepressant response.

Homocysteine

Observational.

Homocysteine is on average about 2.5 µmol/L (0.34 mg/L) higher in people with depression, and high levels carry a modestly higher chance of depression (Moradi 2021). It’s a cheap blood test and it reflects B12, folate and B6 status, which is why I find it useful. I flag results from about 11 µmol/L (1.49 mg/L) in my own practice. That’s my working flag, not an official cut-off. Practice In the VITACOG trial, B vitamins slowed brain shrinkage in older adults with memory problems, mainly when homocysteine was above 13 µmol/L (1.76 mg/L) (Smith 2010). That trial measured brain atrophy, not depression.

Zinc

Observational for levels; Mixed for supplements.

Across 17 studies, blood zinc averaged about 1.85 µmol/L (about 12 µg/dL) lower in people with depression. The studies varied widely (Swardfager 2013). In a meta-analysis of trials, zinc lowered depression scores, but only when given on its own and not alongside antidepressants (Yosaee 2020). The WFSBP/CANMAT taskforce, looking at a wider set of evidence, gave zinc as an add-on to antidepressants a “++”. Those two readings don’t fully agree, so I grade zinc as Mixed.

Omega-3 (EPA)

Mixed This is the best-studied nutrient here, and its evidence is weaker than it’s often presented.

• The Cochrane review (Appleton 2021, 33 placebo-controlled trials, 1,848 people) found a small benefit. It judged the evidence very low certainty and said the benefit was unlikely to be clinically meaningful. It also found large differences between studies and signs that the result may be biased towards benefit.
• A meta-analysis of 26 double-blind trials (Liao 2019) found that EPA-dominant products (60% EPA or more) helped and DHA-dominant ones didn’t.
• The International Society for Nutritional Psychiatry Research (ISNPR) guideline is an expert consensus. Practice It favours pure EPA or EPA-rich products, and it lists people with high inflammatory markers as an area for future research, not an established target (Guu 2019).
• The WFSBP/CANMAT taskforce rated omega-3 well (“+++”) as an add-on to standard treatment. It didn’t support omega-3 as the only treatment (“+/−”).

My reading: if omega-3 is used, it belongs alongside treatment, not instead of it, and it should be EPA-rich. Ask your prescriber before adding it, especially if you take blood thinners.

Vitamin D

Mixed for treatment; Observational for low levels.

• VITAL-DEP (Okereke 2020) gave vitamin D or placebo to 18,353 adults aged 50 or over for a median of about 5 years. It made no difference to new or recurrent depression, or to mood scores.
• Meta-analyses of smaller trials find a small benefit. Mikola 2022 rated that evidence very low certainty and found large differences between studies.
• The trials disagree on who benefits. One meta-analysis found benefit only in people whose starting level was above 50 nmol/L (20 ng/mL), and none below it (Wang 2023). That’s the opposite of the common claim that the most deficient benefit most.
• The WFSBP/CANMAT taskforce gave vitamin D a weak “+” for depression.

The UK deficiency line is 25 nmol/L (10 ng/mL) (SACN 2016). My own working range is 75–125 nmol/L (30–50 ng/mL). That’s a clinical target I use, not official guidance, and no guideline sets a level for mood. In Scotland, winter testing is reasonable. But there’s no good evidence that vitamin D treats depression on its own, and there’s no evidence that seasonal affective disorder is a vitamin D deficiency.

Magnesium

Mixed

The 2017 trial often quoted (Tarleton 2017) was open-label: people knew what they were taking. It compared magnesium with no treatment, not with antidepressants. Depression scores improved, but an unblinded trial with no placebo can’t separate the magnesium from expectation. I found no replication. A meta-analysis of 7 small placebo-controlled trials (325 people in total) found a benefit and asked for larger, better trials (Moabedi 2023). The WFSBP/CANMAT taskforce did not recommend magnesium for depression (“−”).

Iron and ferritin

Mechanistic for the biology; Observational for the link with mood.

Iron is needed to make dopamine and noradrenaline. People with depression report iron-deficiency anaemia more often (Hidese 2018, a self-reported cross-sectional survey). In pregnancy, women with ferritin below 12 µg/L (12 ng/mL) had higher depression scores (Dama 2018). Neither study shows that iron treats depression. The best trial in women without anaemia measured fatigue, not mood. Fatigue improved with iron only in women whose ferritin was 50 µg/L (50 ng/mL) or less (Verdon 2003).

The bands I use (ferritin µg/L = ng/mL):

• 30 µg/L (30 ng/mL) or less: in anaemic patients, this identified iron deficiency far more reliably than 12 µg/L (12 ng/mL) (Mast 1998). With a normal blood count, a low-30s ferritin is a reason to look further, not a diagnosis. The American Society of Hematology’s 2026 guideline goes further, using 30 ng/mL (30 µg/L) or lower to diagnose iron deficiency whether or not someone is anaemic, but on low- or very-low-certainty evidence (ASH 2026).
• Below 15 µg/L (15 ng/mL): the WHO cut-off for iron deficiency.
• 50–100 µg/L (50–100 ng/mL): my target. Practice It’s informed by Verdon 2003, which tested fatigue. It’s not an official range, and not a mood threshold.

Thyroid

Observational (modest).

Underactive thyroid and depression are linked, but more weakly than often claimed. In a 2021 meta-analysis of 25 studies, the link was clear for overt hypothyroidism, small for subclinical, and seen in women but not men (Bode 2021). In subclinical hypothyroidism, thyroxine didn’t improve depression compared with placebo (Zhao 2018). Thyroid antibodies are also linked with depression, but those studies vary widely and show signs of bias towards positive results (Siegmann 2018).

If you have symptoms that could be thyroid-related, it’s reasonable to ask your GP whether a thyroid test is right for you. I read TSH against the lab’s own range. I don’t use a tighter band, because the evidence doesn’t support one.

Inflammation (CRP)

Observational.

About a quarter of people with depression have CRP above 3 mg/L (0.3 mg/dL), and over half have CRP above 1 mg/L (0.1 mg/dL) (Osimo 2019). Because more than half of people with depression are above 1 mg/L, that cut-off doesn’t pick out a distinct group. The 1 and 3 mg/L cut-offs come from cardiovascular risk guidance (Pearson 2003), not psychiatry. Whether people with depression and higher CRP respond better to anti-inflammatory or nutritional approaches is still a research question. I’d treat a raised CRP as something to understand, not a diagnosis of “inflammatory depression”.

The gut

Mechanistic for serotonin; Mixed for probiotics.

Almost all of the body’s serotonin outside the brain is made by cells in the gut lining. But serotonin can’t readily cross the blood–brain barrier, so the gut and brain supplies are separate (Jones 2020). Gut serotonin mainly works on gut movement and metabolism. It isn’t a store your brain draws on.

Probiotics as an add-on to standard treatment were rated “++” by the WFSBP/CANMAT taskforce. A 2020 meta-analysis found a reduction in depressive symptoms and asked for larger, longer trials (Zagórska 2020). Which strains, at what amounts and for whom are all still open questions.

Diet as a whole

Mixed

• SMILES (Jacka 2017): 67 adults with moderate to severe depression, most already on treatment. Seven dietitian sessions were compared with social support. Remission was 32% versus 8%. It’s encouraging, but the trial was small, and participants knew which group they were in. Only the people rating their depression were blinded.
• HELFIMED (Parletta 2019): 152 adults; a Mediterranean-style diet plus fish oil reduced depression at 3 months. Only 95 people completed the 3-month assessment, and the fish oil makes it hard to tell what the diet alone did.
• MooDFOOD (Bot 2019): 1,025 overweight adults with low mood. Neither a multi-nutrient supplement nor food-focused therapy prevented depression over a year.

Improving diet alongside treatment looks reasonable and low-risk. It hasn’t been shown to replace treatment.

Medicines and nutrients: what to ask your prescriber

• Lithium: a low-salt diet, dehydration, heat or alcohol can raise lithium levels in the blood and the risk of side effects. Lithium can also affect the thyroid and kidneys, which is why they’re checked with blood tests (NHS). Don’t change your salt or fluid intake without talking to your prescriber.
• Carbamazepine and valproate: studies in people taking them for epilepsy found higher homocysteine, lower folate with carbamazepine, and lower B12 with valproate (Gorjipour 2013, Observational). If you take one of these as a mood stabiliser, it’s reasonable to ask whether these should be checked.

Don’t stop, reduce or add anything to a psychiatric medicine without your prescriber.

What I’d do

These are things to discuss with your GP or prescriber, not steps to take instead of them.

1. Safety first. If your mood is low enough that you’re struggling to function, or you’re having thoughts of harming yourself, see your GP now or call NHS 111. You can call Samaritans on 116 123 at any time.
2. Ask what’s already been checked. Many of these tests are routine when depression is assessed. Ask which ones were done, and what the results were in numbers, not just “normal”.
3. Tests worth asking about if they fit your symptoms: full blood count and ferritin; B12 (and MMA or homocysteine where available); folate; vitamin D; TSH; CRP; fasting glucose and HbA1c. Each has a reason above, with its evidence grade.
4. Correct a true deficiency. That’s the best-supported step here. It means correcting it under your GP’s care, alongside whatever treatment you and your doctor choose.
5. Food before supplements. Improve diet quality, and if you use omega-3, choose an EPA-rich product and tell your prescriber.
6. Be wary of anyone promising more than this. That includes me.

How strong is the evidence?

Sources

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