This article provides information about ongoing scientific research and does not provide any medical advice. Consult your physician before changing your diet.
We tend to think of foods’ impact on our bodies, not our minds. After all, we can see our bodies and the changes that they undergo as our diets change. But the food you consume can alter your immune responses, your mood, and your ability to process information. And since the brain’s energy consumption far outpaces its size relative to the rest of the body, it makes sense that “the mechanisms that are involved in the transfer of energy from foods to neurons are likely to be fundamental to the control of brain function” (Gomez-Panilla). With this in mind, I reluctantly ditched my lunch of cheese sandwich with tomato, some variation of which I eat almost every day. Why? Everything I read about diet and cognition suggested that I should reduce my consumption of the foods that are associated with the “Western diet.”
The Western diet is defined as “high dietary intake of saturated fats and sucrose and low intake of fiber” which are all abundant in processed foods, like white bread and many packaged items. Cheese, too, is high in saturated fat, so it was hard not to conclude that what I’d thought was a decent meal was actually pretty unhealthy. Indeed, according to a recent book, the Western diet is associated not only with diseases like cancer, but with cognitive decline, because it contributes to inflammation and an unhealthy microbiome.
Inflammation is an immune response that kicks in when the body feels threatened, and while that’s useful in small doses, inflammatory responses damage cells and even entire organs when triggered too frequently. And the Western high-fat diet activates the immune system, causing inflammation. One way this might happen is as a result of macrophages—immune system cells that form in response to infection or cell damage—being initiated in fat cells, and then leaking inflammatory cytokines. Essentially, the body appears to recognize the fat as a disease.
Inflammation also causes the neurons that send signals to and within our brains to lose function. That may be why a recent study shows that, when deposited onto our bodies, fat appears to biologically age our minds. The study showed that, regardless of actual age and socioeconomic status, people in middle age who had more midsection fat suffered from a loss of fluid intelligence—the ability to problem solve in novel situations—over time. The study also showed that, since fat activates immune responses while muscle does not, the amount of fat we carry can be negatively impactful, regardless of the amount of muscle we have.
Another fat-inflammatory connection that affects the brain exists in the balance of fatty acids that we consume. Omega fatty acids are a set of nutrients responsible not only for brain health but brain structure. Fatty acids are necessary components of cell membranes in the brain, but our bodies are bad at synthesizing them, so we need to get them from the foods we eat. One of these fatty acids is DHA (Docosahexaenoic acid), an omega 3 that is thought to be responsible for the brain to body mass ratio in humans. As our ancestors adapted to consuming an increasingly shore-based diet including fish, we ingested more DHA, and that may have allowed for, or even prompted, our brains to grow bigger. 138 But that doesn’t mean that more is always better when it comes to omega fatty acid consumption.
Omega 3 and omega 6 fatty acids are both crucial to brain function, with omega 6 acids also reducing bad cholesterol and keeping blood pressure in check. But the balance of these fatty acids is askew in the meaty, heavily processed Western diet, which is chock full of omega 6 while being rather light on omega 3. Ideally, the ratio of omega 3 to omega 6 would be about 1:4, but for many Americans, it’s between 1:10 and 1:50. And, various studies have concluded that high omega 6 levels produce too much inflammation, which causes cognitive problems, like reduced attention and less responsive working and episodic memory, as well as longer term tissue damage, all associated with dementia down the road.
An even more direct connection between the diet and brain may lie in our microbiome. That’s the assemblage of gut bacteria, fungi, viruses and protozoa that live in our digestive systems. Understudied until recently, the health effects of the organisms that comprise the microbiome are so complex as to be difficult to summarize, but scientists do know that neuroactive chemicals are produced by the microbiome’s various organisms.
These chemicals impact not just our bodily health but personalities and what we think of as “intelligence.” As this review article by Fernando Gomez-Panilla notes, the gut not only affects systems “associated with synaptic plasticity and learning,” but the gut hormones leptin, threlin, GLP1 and insulin “influence emotions and cognitive processes…[and] the molecular mechanisms that determine the capacity for acquiring new memories and that control emotions.” The microbiome’s level of genetic diversity also affects our mental states: people with less diverse microbiomes showed higher levels of anxiety, stress and sleeplessness.
Probiotic foods can help introduce new strains of good bacteria into the gut, but interestingly—and perhaps inconveniently—probiotic supplements don’t seem to have much of an impact on cognition. Only foods with naturally occurring probiotic content like yogurt, kimchi, and pickles produced changes in the brain. Aside from contributing to genetic diversity in the microbiome, eating a variety of whole foods gives gut flora an environment to thrive in. For example, eating whole grains that cannot entirely be digested by humans provides sustenance for good bacteria. Processed grains whose husks have been removed (as in white bread), are digested quickly, leaving behind little for the microbiome’s organisms to consume.
If abandoning the Western diet sounds difficult or expensive—after all, it’s what’s available to a lot of us—some simple additions to our menus can counteract the Western diet’s negative impacts. For example adding in more berries has been well-established as yielding both general health and cognitive benefits, because while many fruits and vegetables contain antioxidant and anti-inflammatory polyphenols, berries have more than most. Eating more blueberries in particular had rather profound changes in school children. One study added 30g of freeze dried blueberry powder to the diets of 8-10 year olds, and found that they retained information better, performing better on verbal recall, word recognition, and complicated interference tasks. Likewise, blueberries and strawberries protected mice against the damaging effects of high fat diets, and even appeared to slow age-related cognitive decline in older women.
For more information on the types of foods whose nutrients are good for the brain, check out this chart, originally published in Nature Reviews Neuroscience.