In the US, a surprisingly easy way to inadvertently cause mild depression is to decrease intake of iodized salt over time. The "natural" iodine content of many foods (bread, milk) seems to be dropping steadily due to organic and other preparation practices. Kelp-extract supplements with around 200mcg of iodine daily can go a long way towards preventing mild depression associated with borderline iodine deficiency.
If it's iodine deficiency that causes depression, then why would decreasing intake of iodized salt help?
He said decreasing intake would inadvertently cause mild depression, not help. (I read it that way too at first)