Examine.com is an independent website that documents clinical research on different supplements. I’ve looked through and managed to find 4 cheap natural supplements that are all described as having a “notable (moderate) effect on depression” and are supported by “high to very high level of clinical evidence”. The 4 supplements (with Amazon links) are:
All four are easily available and they don’t interact with each other so you can use all four of them daily if you wish. All four are available cheaply online.
(links to the supporting studies are available on the examine.com website under the corresponding supplement pages).
So what do they have in common?
All four supplements positively effect your digestive system.
- Curcumin is “a colon anti-inflammatory and can help with digestion”  and is considered a potential treatment for inflammatory bowel disease 
- Zinc helps tighten our gut lining reducing the amount of toxins and undigested protein that can get through 
- Creatine is typically used as a sports supplement. It gives our muscles energy, allowing athletes to perform for longer and increasing their strength. It turns out it also provides energy to something called the Epithelial cells in our stomach allowing them to reproduce faster . These cells protect the lining of our stomach
- Omega 3 is well known for its anti-inflammatory properties. What has been discovered more recently is that at least part of this anti-inflammatory effect is actually because Omega 3 has a positive effect on our gut bacteria  which then alters our immune system’s inflammation response.
A lot of emerging research is showing that our digestive health has a big impact on our overall health. Our digestion, including our gut microbiome alter our susceptibility to a range of autoimmune and inflammation driven diseases like heart disease, cancer and dementia. Depression is one of these autoimmune disease. These diseases, including depression, are the biggest causes of death and illness in the modern environment.
Even when our depression is driven by some external events the stress of these events alter our gut microbiome and our immune system creating depression and anxiety.
What Else Can You Do?
There are likely many other supplements and foods that also lower our depression but we don’t have enough clinical evidence yet. As mentioned in my previous article Foods for depression pharmaceutical companies are able to fund research on their drugs but it isn’t in the interest of commercial entities to pay to research cheaply available alternatives like the supplements mentioned above.
However based on the evidence that good gut health is strongly linked to reduced depression, chances are that most supplements that are good for your gut will probably help reduce depression too. Likewise a healthy diet with lots of veg and fiber will help a lot too. Along with avoiding foods that are bad for our digestion like foods high in processed sugar. Exercise was also shown to help our gut bacteria flourish too .