Many factors have an impact on brain neurochemistry and endocrine balance such as age, genetics, circadian rhythms and various environmental factors including stress, diet, exercise and medications. Many people do not synthesize adequate serotonin, including diabetics and those on SSRIs for long periods of time. Dieting is known to lower levels of tryptophan, an amino acid needed to make serotonin.
Dieters and others often need help controlling food cravings, mood swings and addictions. Cravings for cigarettes are known to be controlled by dopamine receptors. The neurotransmitter dopamine is often low in those who are prone to addictive or erratic behavior.