If this were true people that ate yogurt would never suffer depression, which is not true.

Now if you can prove people eating yogurt duffer less fro depression either by the duration or severity than you've gone a long way to suggest depression can be remediated by diet.

TO some extent it can, the gut flora hypothesis has gained traction in the public mind and if you think back to what meal was 1000 years ago a cup (or two) of something fermented from apples or honey or hops and barley was an integral part of every meal

This was not pasteurized, you got all the correct gut flora (mostly acetobacter and Lactobacillus) 3 times a day from ingestion of them.

If you grow ginger beer then take half out and add freshly squeezed apple juice it will ferment into a gingery cider in your fridge.

But if you try to fermented apple juice without that it has the potential to produce deadly bacteria which is why the apple juice is always pasteurized first. But when theres an overgrowth of the correct bacteria that ferment the ginger and apple then the bad bacteria can never compete or take hold.

This is the same thing as your gut, if your gut is healthy you will be too an the opposite is true as well.

So with a 3x a day shot of bugs that prevent overgrowth of bad gut flora your chances of being healthy and absorbing nutrients properly (such as the enzymes the brain requires for proper functioning that we must eat and conno make, pyridoxine, ascorbate, citrate, niacin the key and least plentiful chemicals that the brain requires in some quantity.

But we don't eat these y more and instead we eat junk that promotes the overgrowth of the bad bugs - junk food, processed food, refined foods

So no bugs and damage instead. Is it any wonder things come off the rails?

This Is the reason mild alcoholic beverages were give to kids, it wasn't to get the drunk it was to make sure they sted healthy - bad food was more common then and the stronger the good gut flora is the more resilience will exist to overcome the bad ones especially if ingested.

Commercial yogurt is crap, it all contains a sweetener of some sort which is part of the problem. Ones you make yourself or unsweetened ones are ok.