The science of happiness has turned up evidence that food can make you happy. However, a lack of certain foods -- or at least some of their essential ingredients -- can actually make you sad. A fatty acid called docosahexaenoic acid (DHA) is the most abundant fat found in the brain. This is good, since it's an essential building block for brain structure. It's also easy to get; two major sources of DHA are fish and shellfish. A study by the National Institutes of Health (NIH) uncovered a link between DHA deficiency and an increase in the prevalence of depression in the United States.
Though no direct causal link has been discovered, other studies support the correlative link the NIH found. One study turned up the fact that North American and European countries that don't eat a lot of fish have 10 times the prevalence of depression in their populations than does Taiwan, where fish is a staple of the popular diet [source: The Franklin Institute]. Although this doesn't prove causation, it's a pretty good reason to eat more fish and other foods containing DHA.
Oddly, while some foods have been shown to improve mood in the human brain, restricting food intake can have an even more pronounced effect on happiness. A hormone called ghrelin within the stomach heads over to the brain and tells it that it's time to eat. When ghrelin is produced, you feel hungry. After you ingest food, ghrelin production stops and your brain ceases to receive hunger signals.
When you don't eat, however, ghrelin production continues and the hormone builds up in the brain. While the increased ghrelin will prolong your hunger, researchers have found that the hormone also acts as a natural antidepressant. A 2008 University of Texas study found that rodents injected with ghrelin showed decreased symptoms of stress and anxiety [source: Lutter, et al]. Interestingly, rodents that were placed on a calorie-restricted diet (40 percent of normal caloric intake) showed the same results.
Whether it's psychological or physiological, it's clear that foods have a powerful effect on our moods. It would appear that eating only nutrient-packed foods that affect brain chemistry might be the best way to achieve happiness, but the occasional indulgence should make you just as happy. Perhaps a healthy balance of nutritious foods and comfort foods can help maintain the balance in a person's mood best of all.