How much does diet affect mental health

I would say - the diet affects mental health directly. In fact, the most efficient protocols for dealing with mental health and neurological issues (autism, epilepsy, etc.) involve radical (to some) changes in diet (for autism, for instance, high-fat and low-carb ketogenic diets prove very effective).

Several factors in our diet affects how our brain processes information and reacts to the stimuli. Just to name a few:

1.      Healthy fatty acids play a primary role in myelination of nerves in the brain - the better the myelin sheath, the less “interference” of the electrical signals you get, allowing your mental processing capacity to be more efficient. Loss of myelin (Multiple Sclerosis) is immune-mediated inflammatory process that may be triggered by consumption of too much omega 6 (like in vegetable oils) or non-celiac gluten sensitivity (from consuming grain products) triggering an auto-immune response.

2.      Poor nutrition (and consumption of pesticide-laden foods) can destroy healthy gut flora that normally protects against neurological disorders and promotes mental health (by secreting various neurotransmitters, such as dopamine, serotonin and gamma-aminobutyric acid (GABA))

3.      Any caloric restriction (which some people consider a “diet”) can lead to changes in mood, behavior and hormones that directly or indirectly regulate that behavior (read about the Minnesota Starvation Experiment mentioned here)

A brain - which is what defines mental health - is an organ. Like any other organ, it depends on proper nutrition. If you mess up that nutrition, the brain changes - sometimes irreversibly. So, yeah, I would say diet totally affects mental health!

 Refer; Stephanie Lyndy Komen