For all the attention paid to the negative effects poor dietary choices have on the body, the effects of diet on the brain are largely unexplored. However, emerging research is providing new insights to support the suggestion that food can have a profound influence on mental health and cognition.
From sugar and carbohydrates to fats and even, according to one controversial theory, whole grains, the list of dietary choices having potentially negative effects on the brain is growing by leaps and bounds.
And although the big caveat for the bulk of evidence is that most studies show an association with but not necessarily causation of mental health and cognitive deficits, many clinicians report seeing first-hand improvement in patients' mental health outcomes with the tweaking of a diet to eliminate some of the most notorious culinary culprits.
"While we don't want to send the message to patients that all they have to do is change their diet and their severe depression will be cured, I can say that I have absolutely seen dietary changes work to improve outcomes for a lot of patients, and there are a lot of reports of that," said Drew Ramsey, MD, an assistant clinical professor of psychiatry at Columbia University College of Physicians and Surgeons in New York City.
Dr. Ramsey, the author of The Happiness Diet: A Nutritional Prescription for a Sharp Brain, Balanced Mood, and Lean, Energized Body (Rodale, 2011) and Fifty Shades of Kale (HarperWave, 2013), asserts that the role of diet, so strongly emphasized in general practice, is simply too commonly overlooked in mental health practice.
"If someone has a severe mental illness, it is very important to talk to them about diet," he told Medscape Medical News. For example, he said, if a patient has certain nutrient deficiencies, it will be difficult for any medications to help until such deficiencies are treated.
"Yet I know clinicians who simply never talk to their patients about food because they're not taught to discuss the topic," he added.
"But certainly that will change over the next decade with the data that's coming out, including epidemiological data showing a clear signal that the risk of depression increases when you eat a diet of highly processed modern food."
In some of the strongest data along these lines, researchers led by Felice Jacka, PhD, of Deakin University, in Melbourne, Australia, have conducted a series of studies showing a poor diet to be associated with cognitive deficits.
In one longitudinal study of 2054 Australian adolescents, a diet consisting of "junk food," ranging from chips, chocolates, and sweets to pizza and soda, was associated with a worsening of mental health status during a 2-year period (PLoS One, published online September 21, 2011).
Another study of more than 23,020 women and children in the Norwegian Mother and Child Cohort Study showed that high consumption of junk foods during pregnancy and during the first 5 years of life predicted externalizing problems, such as aggression, hyperactivity, or tantrums, among children, independent of other confounding factors and of the childhood diet.
Children with unhealthy diets postnatally also were found to have had greater problems with externalizing as well as internalizing problems, such as worrying, sadness, and anxiety (J Am Acad Child Adolesc Psychiatry, published online August 17, 2013).
And in a third study of 5731 adult and older men and women, the research team found a lower risk for depression among participants with better-quality diets, and increased anxiety was observed with a higher intake of processed and unhealthy foods (Psychosomatic Medicine, 2011:73;483-490).
"A diet high in saturated fats and refined sugars has a very potent negative impact on brain proteins that we know are extremely important in depression ― neurotrophins, which protect the brain against oxidative stress and promote the growth of new brain cells," Dr. Jacka told Medscape Medical News.
"There also seems to be an impact of saturated fat on the stress response system, which is also important in both depression and anxiety."
The next step in the research will be to investigate the impact of dietary improvement among people who are already depressed, Dr. Jacka said.
"Until we have the results, we can't say definitively that dietary improvement will be a useful strategy in treating depression. However, on the basis of the observational studies from around the world, we do believe that dietary improvement should be recommended in psychiatric clinical settings, just as it is in other medical treatment settings."
Dr. Jacka is the president of the International Society for Nutritional Psychiatry Research (ISNPR.org), which has a Web site featuring an extensive list of links to the most recent research on the issue.
In yet another study on the topic, researchers in Spain showed a strong link between "junk food" and cognitive deficits. In the study of nearly 9000 adults, participants whose diets involved the consistent intake of fast food were as much as 40% more likely to develop depression than those who did not consume fast food, with the risk for depression increasing along with the amount of fast food consumed (Public Health Nutr, 2012;15:424-432).
The increased risk for depression was also seen among those who consumed higher amounts of baked goods, including croissants and doughnuts.
The research team in Spain also published a large study of 12,059 Spanish university graduates from 1999 to 2011 who were depression-free at enrollment, and results showed a detrimental relationship between transunsaturated fatty acid (trans fat) intake and the risk of depression.
The study also found weak inverse associations between depression and monounsaturated and polyunsaturated fatty acids, as well as olive oil (PLoS One, 2011;6:e16268).
"These findings suggest that cardiovascular disease and depression may share some common nutritional determinants related to subtypes of fat intake," wrote the authors, who were with the University of Las Palmas de Gran Canaria and the University of Navarra in Pamplona, Spain.
Saturated fat, notorious for its adverse cardiovascular effects, fares no better in mental health studies.
In one recent study of 6183 older participants in the Women's Health Study, higher intakes of saturated fatty acids were shown to be associated with worse global cognitive and verbal memory trajectories, whereas higher intake of monosaturated fatty acids was related to improved trajectories ( Ann Neurol , 2012;72:124-34).
The association has not only been seen in older women ― another recent study of 38 younger women (25 to 45 years old) showed higher intakes of saturated and trans fats as well as fasting insulin to be associated with cognitive and memory deficits after adjusting for age, activity, and verbal IQ (Front Hum Neurosci, 2013;7:838).
Interestingly, in that study, carbohydrate intake was not found to be significantly associated with cognitive performance.
In terms of dementia and Alzheimer's disease, a high-fat diet was linked to the disruption of the body's process in clearing the brain of amyloid, the toxic protein associated with the development of the disease (JAMA Neurol, published online June 17, 2013).
The role of sugar in mental health has been well documented, with recent research linking intake with cognitive deficits.
In the study of 737 participants without diabetes in the Boston Puerto Rican Health Study, higher sugar intake was significantly associated with lower cognitive function, as indicated on the Mini–Mental State Examination ( Br J Nutr , 2011;106:1423-32).
Another recent study involving 937 initially cognitively normal adults aged 70 to 89 years linked higher intake of carbohydrates with a risk of developing mild cognitive impairment that was as much as 3.6 times greater than those on low-carbohydrate diets, and the risk was also greater in those with diets high in simple sugars (J Alzheimers Dis, 2012:32).
"Grain Brain" Controversy
One of the most recently vocal proponents of a low-carbohydrate diet as a means of optimal cognitive and overall mental health is neurologist David Perlmutter, MD, whose book Grain Brain: The Surprising Truth About Wheat, Carbs and Sugar (Little, Brown, 2013) has been on the New York Times' bestseller list for weeks.
In his book, Dr. Perlmutter argues that processed carbohydrates, sugars, and even whole grains increase the risk for dementia by raising the glycemic index. In a recent Expert Interview with Medscape Medical News, Dr. Perlmutter noted that the glycemic index of whole-grain bread is as high as 72 to 74.
"It's higher than that of white bread. It's much higher than that of many candy bars," he said.
To avoid the potential highs and lows of "grain brain," Dr. Perlmutter recommends, among other things, a diet including healthy, unsaturated fats and limited to 60 to 80 grams of carbohydrates a day.
The "grain brain" argument has its critics, however – including Dr. Ramsey.
"The book has gotten a lot of buzz, but the argument seems to me to be an unfortunate oversimplification on things," he told Medscape Medical News.
"It's good that we want people to focus on the health of their brain and how their dietary choices affect the risk of mental illness, but eliminating whole grains just isn't supported in the evidence for most people, and I doubt that it will be."
He called the argument on glycemic index "misleading" in its involvement of glucose.
"The argument is that a piece of wheat bread has a higher glycemic index than sugar, but this is really very slight of hand because half of sugar is fructose, and fructose doesn't register on the glycemic index because it doesn't raise blood sugar insulin ― it raises fructose levels."
In terms of nonnatural fructose effects, numerous studies have shown links with mental health issues, such as cognitive decline and memory, according to Shaheen E. Lakhan, MD, PhD, MEd, executive director of the Global Neuroscience Initiative Foundation, who recently published an extensive review on the topic in Nutrition Journal (Nutr J, 2013;12:114).
"There are several lines of studies, including cell based, animal model, and human studies, to suggest that fructose intake from nonnatural sources impair neuronal connections, spatial memory, and other cognitive functioning," Dr. Lakhan told Medscape Medical News.
"A growing pool of data on nonnatural sources of fructose such as sugar-sweetened beverages suggest increased inflammatory load impairing memory consolidation."
Other potential mechanisms may be neural or vascular, Dr. Lakhan explained.
"Mechanisms may also include impaired hippocampal synaptogenesis, or the ability of brain cell connectivity in an area of the brain implicated in memory," he said.
"The other major potential pathway is the vascular hypothesis, in that cerebrovascular risk factors are elevated, directly and indirectly, with fructose consumption."
Dr. Lakhan has been at the forefront of some of the newest research on mechanisms between diet and mental health, specifically suggesting a relationship with the microbiota in the gut.
"The emerging evidence suggests the pathway to be diet-microbiota-inflammation-mental health," he said.
The research suggests gut dysfunction, particularly in the context of obesity-related inflammation (J Transl Med, 2011;9:202).
"The neural network in the gut, called the enteric nervous system, acts as a regulatory housekeeper of the cellular border of our gut," Dr. Lakhan said.
"Interestingly, food-related 'brain drains' may be more pronounced in those with gut dysfunction ― including obesity."
Lifting Brain Fog
Although all of the evidence, theories, and countertheories on the foods that can drag down mood and cognition may be tough to relay to patients, Dr. Ramsey says he takes the approach of recommending a simple "brain food" swapping system.
"For instance, if a patient really wants a chocolate-covered pretzel, I suggest trying to make it dark chocolate ― and ideally make it a whole wheat pretzel."
"If it's the salty crunch they're looking for, maybe swap out the pretzel for a dark chocolate–covered almond ― and if they do that, all of a sudden they're eating 2 of the most healthy brain foods ― nuts and dark chocolate."
Dark chocolate is in fact the only food shown to acutely improve mood, he noted, citing a recent small but intriguing randomized controlled trial demonstrating positive effects of cocoa polyphenols on mood in healthy participants (J Psychopharmacol, 2013;5:451-8).
But whether it is recommending a food swap or another strategy, the important ― and often overlooked ― thing is to bring diet into the discussion with psychiatric patients, Dr. Ramsey underscored.
"I really believe people should be able to enjoy food, and the intervention we are missing in mental health is helping patients with what to eat," he said.
"It's a very easy intervention when you think that taking a simple dietary history takes about 3 minutes, and giving them some good foods to eat takes about the same."
"If you can encourage people to change what they eat to include more nutrient-dense foods, that actually is very helpful and gives patients something to do that is under their control until their medication kicks in."
What Not to Eat
The experts agree that although certain foods such as white pasta may epitomize the components shown in research to affect mood and cognition, what matters most is the diet as a whole.
"There is no one food that should be eaten or not eaten to reduce the risk for mental disorders or any other health disorder," said Dr. Jacka. "The way that dietary components interact in our bodies to support or reduce health is highly complex."
"If I had to focus on just 1 or 2 elements, I would say the most detrimental would be refined carbohydrates ― foods with refined grains and added sugars ― and trans fats. Diets high in sugar, and hyperglycemia itself, are linked to increased inflammation and oxidative stress, which in turn is linked to psychiatric disorders," she added.
Dr. Ramsey agreed, adding that the idea of an exclusionary approach to foods likely is not going to go over well with patients.
"I encourage my patients to think of dietary patterns ― it's not that you can't eat a piece of cake, it's just that that cake comes with no nutrients that your brain needs to deal with all of the calories."
That said, Dr. Ramsey offered a sampling of some of the worst offenders in terms of mood- and cognition-sabotaging foods:
White pasta/bagels: "Instead try choosing a whole wheat or quinoa pasta, or choose less pasta and more vegetables."
White chocolate: "It has none of the benefits of dark chocolate, none of the polyphenol flavonoids that make dark chocolate increase blood flow to the brain and decrease inflammation," he said.
Anything with partially hydrogenated vegetable oil: "Even though it's just correlational data, there's nothing good for you about trans fats, and they do appear to increase inflammation," said Dr. Ramsey.
Iceberg lettuce: "It's very low in phytonutrients and vitamin K, which is very important for brain health. A better choice would be something like kale."
When it comes to benefiting cognitive function, the diet that may be the most beneficial is the Mediterranean diet, which is rich in seafood, omega-3 fatty acids, and plant-based foods.
In one of most recent studies on the topic, the same research team behind the trans fat study, led by Miguel A. Martinez-Gonzalez, MD, PhD, University of Navarra, Spain, found small but statistically significant improvements in cognition scores in patients who consumed the Mediterranean diet after 6.5 years, compared with those who consumed a low-fat diet, (J Neurol Neurosurg Psychiatry, published online May 13, 2013).
Dr. Ramsey said that he has indeed observed improvements in psychiatric patients with such dietary changes.
"People will ask if they can simply swap out their Prozac for kale, and that's of course impossible to answer."
"But if you think about a patient eating the modern American diet ― empty carbs, industrial fat like vegetable oil, and very few phytonutrients ― and you help them shift to a whole-food, nutrient-dense diet, consisting of leafy greens, seafood, whole grains, and nuts, there certainly are a portion of patients who will have an improvement in mood. I have definitely seen that."
Meanwhile, as reported by Medscape Medical News, the bulk of evidence concerning cognition and factors associated with the development of Alzheimer's disease is well represented in guidelines issued last year by the Physicians Committee for Responsible Medicine (PCRM):
The 7 Dietary Principles to Reduce Alzheimer's Risk
Minimize saturated fats and trans fats.
Vegetables, legumes (beans, peas, and lentils), fruits, and whole grains should be the primary staples of the diet.
One ounce of nuts or seeds (one small handful) daily provides a healthful source of vitamin E.
A reliable source of vitamin B12, such as fortified foods or a supplement providing at least 2.4 μg per day for adults, should be part of the daily diet.
Choose multivitamins without iron and copper, and consume iron supplements only when directed by your physician.
Avoid the use of cookware, antacids, baking powder, or other products that contribute dietary aluminum.
Engage in aerobic exercise equivalent to 40 minutes of brisk walking 3 times per week.
Dr. Ramsey, Dr. Jacka, and Dr. Lakhan report no relevant financial relationships.
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Cite this: Culinary Culprits: Foods That May Harm the Brain - Medscape - Jan 30, 2014.