The adage, “you are what you eat,” has never seemed more accurate than it is today. We all know that you feel better and more energetic after eating a salad than a burger or pizza slice. But the burgeoning field of nutritional psychiatry reveals that our food choices affect more than just our body—it also affects our brains and mental health. For example, research shows that what we eat may affect depression, cognition, sleep quality, hyperactivity and impulsivity in children, and overall happiness, to name a few things. And in general, the studies suggest that replacing a diet high in sugar, saturated fats, and processed foods with a plant-based diet, particularly the Mediterranean diet (MD), could help you feel better both physically and mentally. 

So if you’re tired of feeling mentally exhausted, maybe it’s time to change your diet. So what do we know today?

Microbes, Carbs, and Inflammation 

Your gut communicates with your brain through the gut-brain axis or connection. In the walls of the digestive system lies the enteric nervous system (ENS), which contains 100 million nerve cells lining the gastrointestinal tract. This system is responsible for controlling digestion—from swallowing to controlling blood flow for nutrient absorption and elimination—and communicates back and forth with the brain.

Within our guts live trillions of microorganisms—bacteria, fungi, viruses, archaea, and even parasites—collectively called the microbiota. The microbiota includes symbiotic and pathogenic species, which coexist with each other and their host (you). Our microbiota plays many vital roles in the body. For example, one important function is producing mood-regulating neurotransmitters like dopamine and serotonin (about 95% of your serotonin is made in your GI tract, and 90% of serotonin receptors are located in the gut), which reach your brain through the ENS.)

What you eat affects your microbiome, so it makes that sense that your food choices could also affect your mood through altered neurotransmitter levels. But other factors may also be at play, and one hypothesis points the finger at carbohydrates, which affect blood glucose and insulin levels. Repeated and rapid increases and decreases in blood glucose from eating a lot of processed carbohydrates could trigger the secretion of autonomic counter-regulatory hormones, such as cortisol, adrenaline, growth hormone, and glucagon, which may change a person’s anxiety, irritability, and hunger. Diets with a high glycemic index are also a risk factor for diabetes, a comorbid condition with depression. 

High glycemic foods can also increase inflammatory responses, affecting mental health. Heightened inflammation has been linked to biomarkers for various mental health conditions. For example, observational studies have shown that people who eat lots of inflammatory foods have higher depression scores. At the same time, randomized control trials of anti-inflammatory drugs found that the medications can significantly reduce depressive symptoms (note that the MD has been linked to lower inflammation.)

Food and Depression

Much of the research on how our diets affect our mental health has focused on depression. However, large population studies show a strong correlation between diets high in nutrient-dense foods, lowered depression scores, and increased happiness and well-being scores. For example, in one 2016 study, which followed nearly 12,400 people for seven years, researchers found that participants who ate more fruits and vegetables rated themselves substantially higher on general happiness and life satisfaction questionnaires than those who didn’t.  

Other studies have shown more of a causative relationship between food and mood. In the “SMILES” trial published in 2017, researchers had 67 clinically depressed patients meet regularly with a research assistant who provided social support; half the participants also met with a dietician who taught them about a Mediterranean-style diet. After 12 weeks, both groups saw lowered depression scores, but these scores improved significantly in the group that switched up their diets. Nearly one-third of the MD group were no longer classified as depressed, compared with 8 percent in the control group. 

Other trials have found similar results. One meta-analysis, which focused mainly on studies involving people with nonclinical depression, also found that dietary interventions can significantly reduce depressive symptoms.

Food and ADHD, Cognitive Function, and Sleep

Though depression is a hot topic in nutritional psychiatry, researchers have investigated the connection between diet and several other psychological issues. For example, some research shows a link between diet and ADHD. In a 2017 study involving 120 children and adolescents, researchers found that fast food, sugar, and soft drinks were associated with a higher prevalence of diagnosed ADHD. And the children who ate fewer items related to the MD (vegetables, fruit, fatty fish) were more likely to have ADHD symptoms. A 2019 meta-analysis found similar results, though the authors note that longitudinal studies are needed to understand this link. 

Research also shows a link between a high-quality diet and a reduced risk of cognitive decline. For example, in one randomized control trial, researchers found that an MD supplemented with nuts or olive oil could improve cognitive function in an older population, based on a few cognitive tests. One study even found that the MD could protect the brain against the protein buildup that leads to memory loss and dementia. Furthering the point, research using rodent and human models shows that a Western diet high in saturated fat and sugar is associated with cognitive impairments, particularly concerning memory.

Scientists have even found a link between sleep quality and diet. One study published in 2020 found that adherence to the MD—specifically, higher consumption of fruits and olive oil and lower consumption of red meat and subproducts—was associated with better sleep quality in pregnant women. Another recent study found that food insecurity (the inability to meet the body’s dietary needs) is associated with sleep disturbance. Importantly, people with insomnia may have a tenfold higher risk of depression than those who are free of sleep issues, according to Johns Hopkins University. 

Eat and Feel Happy

Though the field of nutritional psychiatry is relatively new, the research shows many ways our diet affects our mental health—and we’ve only just scratched the surface in this article. 

One connecting thread running through the research is that the Mediterranean diet confers numerous benefits, especially when compared with the Western diet. If you want to get all the mental health benefits from this diet, increase your consumption of fish, fruits, vegetables, and foods high in unsaturated fats, and decrease your consumption of dairy and red meat.

And if you want to double up on the benefits of this dietary change, consider changing when you eat as well. One study published this year found that restricting eating to a 10-hour block (intermittent fasting) may improve cognition in older adults. 

Happy eating, or perhaps more accurately, eat and feel happy.