In the movie "My Big Fat Greek Wedding," the lead character, Toula, brings her non-Greek fiance Ian to a family get-together, hoping her tight-knit clan will accept him.

When Aunt Voula offers to have Ian over for dinner, Toula intervenes: "Ian is a vegetarian. He doesn't eat meat."

Voula is stunned. She turns to Toula. "What do you mean he don't eat no meat!"

The room goes silent, then Voula says: "Oh ... That's OK! I make lamb!"

Ian's vegetarianism makes for a good joke, but for many vegetarians (about 5 percent of Americans), it's no laughing matter.

A study in the Journal of Affective Disorders looked at the mental health of nearly 10,000 men and found that vegetarians averaged higher depression scores than nonvegetarians.

And a 2007 study of Australian women in their 20s found that 7 percent more semi-vegetarians and vegetarians than nonvegetarians had depressive symptoms.

We know that vegetarians come out ahead in terms of heart health and longevity, but nutritional shortfalls in the diet may negatively affect mood: Vegetarians get less vitamin B-12, certain omega-3 fatty acids, zinc, and iron than meat-eaters.

Those nutrient deficiencies have been tied to depression.

So if you're vegetarian, choose vitamin B12-fortified soy and rice products. Walnuts, spinach, and flaxseeds contain omega-3s; nuts and wheat germ deliver zinc; and legumes dish up iron. And take a multivitamin (1/2 twice a day).

Whatever your diet, make sure you get the nutrients you need.