There's a reason why doctors immediately put you on the scale when you visit. Weight changes are extremely indicative of how our bodies are faring physically, and the effects of unhappiness and depression can show themselves through appetite.
While for a long time we assumed that being overweight could make you more depressed, there's also some evidence that being depressed makes you gain weight (or at least gain abominable fat.) In a study conducted over 15 years, 5,115 men and women between 18 and 30 years of age took a survey every five years to assess depressive symptoms. The study held a surprising result: Those who had a higher BMI didn't get more depressed over time. But those who did report depression gained more belly fat as time went on [source: Needham].
Even more, there's now evidence that a healthy diet -- vegetables, fruit, whole grains, the whole shebang -- resulted in a lower occurrence of depression. About 2,000 middle-aged or older Finnish men took part in a study that surveyed both eating habit and depressive symptoms. Study participants with a high intake of processed and junk foods showed more prevalent depressive symptoms [source: Ruusunen]. So if you find that you're not so interested in feeding yourself well, you might be struggling with more than just proper diet.