Alternative medicine advocate Deepak Chopra says: "There are receptors ... in your immune system, in your gut, and in your heart. So, when you say, 'I have a gut feeling' or 'my heart is sad' or 'I am bursting with joy,' you're not speaking metaphorically. You're speaking literally."

Researchers from the University of Amsterdam might not put it that way, but their study in the journal Nature Communications looked at the relationship between the gut biome and depression, and found an overwhelming correlation.

The HELIUS study of more than 3,000 people shows that when your gut bacteria are less diverse or you're low on certain types of bacteria, you're at increased risk for depression. And those imbalances in your microbiome are as strongly related to feeling down as smoking, drinking alcohol, lack of exercise, and being overweight.

Another study confirms that 12 groups of gut bacteria produce neurotransmitters (glutamate, butyrate, serotonin, and gamma amino butyric acid) that can play a big role in the development of depression.

Want to know how to keep your gut bacteria in a happy mood?

Two studies in the journal Clinical Nutrition and The Journal of Nutrition reveal that adding an ounce of peanuts or about a teaspoon of herbs and spices such as cinnamon, ginger, cumin, turmeric, rosemary, oregano, basil, and thyme to your daily diet can promote healthy gut biome diversity, which helps fight depression.

Other great foods include high-fiber, colorful fruits and veggies, as well as probiotics in supplements or in foods such as yogurt, kimchi, and kombucha.