SSRIs work reasonably well for a lot of people to help with depression. But that depends on how much serotonin you have in reserve and whether or not the depression is actually caused by low serotonin. One of the major places your body stores serotonin is in the intestinal lining. If you eat a diet that is more conducive to intestinal health, you’ll store serotonin better. Meanwhile, 5-HTP supplements are like eating pure serotonin (there’s actually a two step conversion process from 5-HTP to Tryptophan to Serotonin, IIRC, but 5-HTP passes through the blood-brain barrier much more easily than Tryptophan). And if you are too low in serotonin, an SSRI won’t help, because there just isn’t enough serotonin to reputake inhibit.
Some people are low on norepinepherine too, so an MD might prescribe an SSNRI. Tyrosine (which you can also get in pill form) is a precursor to dopamine, norepinepherine, and epinepherine. Another way to boost dopamine is low-dose (i.e. 4.5mg) naltrexone (LDN), prescribed for a variety of things including chronic fatigue and autoimmune diseases; it’s a dopamine receptor antagonist that causes the brain to produce a net surplus of dopamine. Some people with mood problems also benefit from supplementing GABA, but that never worked for me or my wife, so I don’t know much about it, except that GABA is inhibitory in some parts of the brain and excitatory in others, making it have the opposite of the desired effect for some people.
Another mood enhancer is Theanine. You can get it in pill form, but a great source of that is Kombucha, which is fermented tea. It’s also loaded with antioxidants and probiotics. The probiotics and possibly the moderate amount of vinegar are also helpful for digestion problems.
Getting back to intestinal health, some people have a mild sensitivity to things like dairy (casein, lactose), wheat (gluten), and/or soy. Removing those from your diet may reduce tissue inflamation that interferes with good intestinal function. My kids can’t have dairy in winter. That’s when all these colds and other infections go around. Dairy causes just enough additional inflammation that when they pick up a bug, they much more prone to ear infections that require antibiotics (which tend to also kill off a lot of good bacteria). In small children, eustachian tubes aren’t fully developed and tend to have drainage problems. If we keep them off dairy (they get calcium and protein from other sources), proper drainage prevents ear infections from getting out of hand, and although they probably pick up various infections anyway, the symptoms are so mild that there’s no need to take them to the doctor. IIRC, when I was a kid, my parents observed that if I had too much dairy, I’d get phlemmy and have more trouble with colds and such. The dairy might also directly interfere with immune function. Anyhow, removing that may seem like a mild food irritant can actually have a substantial positive impact on intestinal function due to reduced inflammation and as a result better mucosal lining and better serotonin storage.
Other amino acids people often take to enhance intestinal health (e.g. people diagnose with celiac disease who require a great deal of gut rebuilding) include glycine and glutamine. Google that for more.
Not to get mystical or anything, but everything in the human body is a lot more connected than is suggested by what you learn superficially in high school biology. Why would the human body store serotonin (an important brain neurotransmitter) in the intestinal lining? I don’t know. Because there was no selective pressure not to? Perhaps the mucosal lining that partly serves to protect your tissues from getting digested themselves just happens to be good at suspending other things the body needs to store. Either way, the link is well established (see http://www.jneurosci.org/content/21/16/6348.full.pdf in the Journal of Neuroscience, for instance). Some things may seem obvious, like maintaining proper blood sugar levels (prefer low glycemic foods) and making sure you get enough protein are good for mental function; in fact, the link goes much deeper. Eat well, and you’ll think well.