Can dietary fiber influence mental health?

Eating your favorite food might make you feel warm and fuzzy inside, while eating something you dislike might cause a minor dip in your mood.

But according to the latest research, the links between food and mood are much deeper than that.

There’s growing evidence that changes in your diet could significantly improve your mental health.

Researchers are still digging into the details, but for a summary of what they’ve found so far, have a listen to one of our recent ZOE Science & Nutrition podcasts on food and mood. 

The episode features Felice Jacka, a professor of nutritional psychiatry and director of the Food & Mood Centre at Deakin University in Australia.

In this article, we’ll concentrate specifically on the role of dietary fiber in mental health. 

Globally, around 4.4% of people have depression and 3.6% have anxiety. Medications are available, and talking therapies can help, too. But neither is perfect. 

For instance, medications can have side effects, and talking therapies can be expensive and time-consuming.

If we could change our diets to reduce the risk of depression, it could be a game-changer for society at large.

But does the evidence stack up?

Fiber and mental health

First off, what is fiber? Fiber is a type of carbohydrate that you can’t digest. Because we lack the relevant enzymes to break it down, it just moves through your gut until it reaches your gut bacteria.

Your gut bugs do have the right enzymes, and fiber is their main source of fuel.

Several studies have spotted links between a high-fiber diet and improvements in mental health. We’ll briefly cover a few of these studies here.

Japan: 2,000 participants

A study in Japan enrolled almost 2,000 people aged 19–69. The researchers looked for relationships between different types and sources of fiber and depression symptoms.

Fiber comes in many forms. Commonly, experts describe it as soluble or insoluble. 

As the names suggest, soluble fiber dissolves in water to form a gel-like substance, while insoluble fiber remains intact as it moves through your gut.

After analyzing the data, the team didn’t find any associations between depression symptoms and soluble, insoluble, or cereal fiber.

However, they did conclude that:

Korea: 3,000 participants

Scientists in Korea carried out a similar study. The researchers recruited almost 3,000 adults aged 19–64. They found that:

“Higher intakes of seaweed and mushroom fiber were associated with a lower likelihood of depression.”

According to the authors, seaweed and mushroom are two of the most common sources of fiber in the Korean diet. They explain that dried brown seaweed, in particular, is very high in fiber.

Interestingly, the researchers didn’t find any links between other types of fiber and depression, including fiber from cereal, vegetables, and fruit.

Similar to the Japanese study, this suggests that different sources of fiber might impact mental health in different ways.

China: 4,000 participants

A study in China involving almost 4,000 adults aged 65 or older looked at links between depression symptoms and a range of nutrients, including fiber.

They found that a higher fiber intake was linked to better depression scores.

Reviews

In 2021, scientists published a review looking at dietary fiber and symptoms of depression. They reviewed nine studies. 

Overall, they concluded that individuals with depression consumed less fiber and, conversely, individuals consuming higher levels of fiber had lower odds of experiencing depression.

Similarly, a review of 18 studies examined links between fiber, anxiety, and depression. Although they didn’t identify a relationship between fiber and anxiety, they did find links to depression.

The authors concluded that “Total dietary fiber intake was associated with a 10% lower odds of depression [...] in adults and a 57% lower odds [...] in adolescents.”

Also, each 5-gram increase in daily fiber intake was associated with a 5% reduction in depression risk. 

Much of the research into fiber and mental health so far has been “observational.” This means that the researchers looked at what participants were eating and their mental health during a defined period of time. But some scientists have used a different method.

A longer look

A large study on post-menopausal women took a different approach. This was a “prospective” study, which means they followed participants over several years. 

The scientists used data from 14,129 participants in the Iowa Women’s Health Study. 

They collected information about the participants’ diets and mental health at the start. Then, over the next 18 years, participants completed five more questionnaires that included questions about their mental health.

Overall, the researchers concluded that individuals eating more fiber reported better mental health scores than those eating the least fiber. 

They also found that the source of fiber mattered: Wholegrain sources of fiber were associated with better mental health, whereas refined fiber was not.

Caution, always

At this point, we should remind ourselves that correlation does not equal causation.

As an example, most people who are in traffic accidents are wearing a seatbelt. But that doesn’t mean that wearing a seatbelt causes traffic accidents.

Fruit and vegetables are good sources of fiber, for instance. They might also contain other beneficial compounds, such as polyphenols, vitamins, and minerals. In combination, these compounds could benefit mental health, too, or at least play a part.

How might fiber benefit mental health?

Scientists have a few theories about why dietary fiber might influence mental health. None are set in stone, and researchers need to do much more work, but these are all possibilities.

For instance, the authors of the 2021 review outline a few theories:

Neurotransmitters

Fiber fuels the bacteria in your gut. And some of these gut bugs produce neurotransmitters — compounds that send messages throughout your nervous system. 

For instance, some gut bacteria produce serotonin, which plays a part in depression. In fact, 95% of your serotonin is produced in your gut.

The most common antidepressants — selective serotonin reuptake inhibitors, or SSRIs — target this neurotransmitter and help increase its levels in the brain.

Increased fiber intake may nourish your gut microbiome, which then influences the serotonin system, potentially reducing symptoms of depression. 

Inflammation

There might be a role for inflammation, too. Inflammation is your body’s response to injury or infection. It’s healthy and can be lifesaving. However, if it continues for long periods, it can damage your body. 

Depression and other mental health conditions — along with many other diseases — are associated with inflammation. 

When you eat certain types of fiber, it nourishes your gut bugs. As they ferment the fiber, they produce short-chain fatty acids (SCFAs). Some research suggests that these compounds reduce inflammation, which could, in theory, help reduce depressive symptoms.

Glycemic index

The glycemic index estimates how large our blood sugar response will be after we eat a particular food. 

For instance, the carbs in white bread are quickly digested and can cause a spike in blood sugar. Conversely, wholegrain bread is broken down more slowly, so it produces a more moderate rise in blood sugar.

So, if someone has a high glycemic index diet, they will have more pronounced blood sugar spikes.

Some research suggests that a lower glycemic index diet might reduce the risk of depression.

Adding high-fiber foods like vegetables to a meal reduces its glycemic index score because it slows digestion. Therefore, sugar is released into the blood more gradually.

If glycemic index scores are linked to depression, fiber might reduce the risk.

Sugar spikes

Continuing on the same theme: Large blood sugar spikes after eating promote inflammation. 

As mentioned above, consuming fiber reduces these blood sugar spikes, so it may reduce the risk of inflammation and, therefore, depression symptoms.

Although these theories are all underpinned by science, the exact reasons why fiber might improve mental health are still unknown. It could involve all or none of the above.

A fiber boost

The links between dietary fiber and mental health are getting clearer. And the connections between a healthy diet and mental health are already well established.

Because most people in the United States and the United Kingdom don’t eat enough fiber, this is something we can all work on.

Whether the fiber in plant-based foods is the most important factor for mental health or not, eating these foods is a guaranteed way to make you feel better in the long run. So, stock up.