Our bodies are our gardens; our wills are our gardeners. –William Shakespeare

At the dawn of the 21st century, it was hard to find a psychiatrist who didn’t work solely from the neck up. After all, mental problems, pretty much by definition, were all in the mind. But, in the early 2000s, there started to be some rumbles about the involvement of the rest of the body, especially the immune system. It was also becoming clear that more than two-thirds of the immune system is located in the gut.

In March of 2010, the cover of the American Journal of Psychiatry showed a food scale loaded with fruit to illustrate a study by Felice Jacka, Michael Berk, and colleagues from Deakin University in Australia. The article was titled “Association of Western and Traditional Diets With Depression and Anxiety in Women.” It opened psychiatry to the potential of diet to improve mood. The study showed that there was a significant association between junk food and depression.

Good food, good mood.

Midjourney/OpenAI

Doctors knew there was a connection between certain dietary components and mental health. In particular, a deficiency of B vitamins was known to affect cognition. But Jacka’s study showed that simply adding veggies and fruit to the diet had a large effect on mental states. Instead of specifying nutrients, Jacka referred to the quality of diet, a measure derived from Australian dietary guidelines. There was nothing fancy here. As Jacka points out, there is no need for exotic Peruvian goji berries, just foods containing fiber and polyphenols—substances that make plants colorful.

It had a big impact, shining a fresh light on the role of nutrition in psychiatry. Not everyone was sold. Jacka says, “There was a lot of eye-rolling, for many years.” But further studies buttressed the case that quality of diet was a predictor for depression. These diets were found to reduce the risk of depression by about 30 percent.

Still, Jacka and her team wanted more than just correlations, so they conducted another landmark study in 2017, called the SMILES trial. They recruited clinically depressed people and put them on a Mediterranean-style diet for 12 weeks. In the study, following a change in diet, 33 percent of the depressed patients went into complete remission, most with no residual symptoms. The more they adhered to the diet, the greater their improvement.

That is as good or better than the standard psychiatric cure rate, which caught the attention of psychiatrists. The results were so dramatic for some that they permanently adopted the diet and found that their all-round health improved, not just their depression.

A further study with younger people showed similar results, but it only took three weeks. For someone who is depressed, that’s a refreshingly swift recovery time.

Why does this happen?

In 2004, Nobuyuki Sudo at Kyushu University found that germ-free mice behaved differently than their normal germy cousins, but when they were fed a probiotic diet, they recovered normal behavior. Here was a weird connection between gut microbes and brain behavior and development. It offered an early peek into the gut–brain axis.

At least part of nutritional psychiatry is involved with the trillions of microbes in your gut called the microbiome. Humans evolve so slowly that we can’t possibly keep up with the rapid evolution of microbes. So, far back in the mists of time, we recruited our own bacteria to man the front lines.

These so-called commensal bacteria can join the fight against pathogens before our immune system is even aware of the intruders. These bacteria produce feel-good neurotransmitters like dopamine and serotonin to let us know they are doing their job. They also secrete short-chain fatty acids like butyrate that both nourish and heal the cells lining our gut, keeping it intact.

A “leaky gut” can allow bacteria to enter the bloodstream, where the heart blithely pumps them to every organ in our body, including the brain. Although the blood–brain barrier keeps pathogens and toxins out, it can be compromised by chronic inflammation. This is one of several ways that gut microbes affect our mood and cognition.

Our dependence on microbes is truly humbling.

The secrets unveiled

So, what are the secrets of nutritional psychiatry? Is this going to be time-consuming and cost us more money? Thankfully, no. There are many templates for quality diets, including those from Okinawa, Norway, and Greece. All of these have some important things in common:

• Veggies for fiber and polyphenols to feed your good bacteria, which then can protect you from pathogens.
• Grains for fiber, vitamins, and minerals.
• Lentils for fiber and protein.
• Nuts and seeds for healthy fat, protein, and fiber.
• Virgin olive oil for healthy fat and polyphenols.
• Fish for protein and omega-3 fats.

As well as adding these good foods to your diet, try to reduce junk food. It tastes delicious, of course, but it is designed to be addictive, and, by removing the fiber, it is killing your microbes.

Jacka and her team found that a quality diet actually costs less than a junk food diet. And, because they saw health care professionals less often, it saved them time, even if they had to cook a little more.

Perhaps the biggest upside is that these patients did it by themselves. They took on their own health care, and it was empowering.

What we eat can be fast medicine or slow poison. The choice is ours, and it’s on the plate in front of us.