by Gary L Wenk
In contrast to their lack of benefit for age-related cognitive decline, omega-3 fatty acids may have a beneficial influence on the outcome of depressive disorders. Chronic dietary supplementation with omega-3 fatty acids has produced antidepressant-like effects similar to those of common antidepressant drugs. The therapeutic approach of combining omega-3 fatty acids with low doses of antidepressants might lead to benefits in the treatment of depression, especially among patients with depression resistant to conventional treatments. Such an approach also could decrease the magnitude of some antidepressant dose-dependent side effects.
How does obesity affect brain function and development?
Scientists have demonstrated that obesity leads to hypertension, diabetes, sleep apnea, and numerous arthritic disorders. Obese individuals also perform worse in cognitive tests even when controlled for education level and evidence of depression. Furthermore, women who eat an unhealthy high-fat diet prior to and during pregnancy are more likely to give birth to children, particularly males, who are at risk of abnormal behaviors, predominantly anxiety, during adulthood. Physicians frequently warn pregnant women to monitor their caloric intake and maintain a healthy weight before and during pregnancy. Maternal nutritional status, infection, and physical or psychological trauma during pregnancy can all increase the risk of obesity, diabetes, and mental disorders in offspring. In the past, the concern was maternal malnutrition—that is, the developing fetus might lack critical nutrients for normal growth. Today, in the United States, the concern has shifted to overnutrition and obesity and the risks faced by the developing fetal brain. Maternal obesity leads to serious inattention problems in offspring and a twofold increase in the incidence of impaired emotional regulation that was still evident five years after birth. Maternal obesity also causes abnormalities in areas of the brain responsible for feeding behavior and memory. All of these changes were most noticeable in male offspring. How does maternal obesity impair fetal brain development? Once again, the damage is due to the fact that fat cells release inflammatory proteins, called cytokines, into the body and brain of both mother and fetus. The more fat cells the mother has, the more cytokines get released into her blood. As discussed earlier, the presence of these cytokines increases the likelihood of becoming depressed. Exercise can modestly reduce the level of cytokines in the brain, and if overweight, moms might find some relief from their depression by exercising.
Why do I like to eat?
Two different neurotransmitter systems, endogenous opioid peptides called endorphins and cannabinoids, make eating pleasurable. Endorphins enhance the sensory pleasure derived from food, and the consumption of foods high in fat and sugar stimulates the release of endorphins. Endorphins enable us to experience the deliciousness of food and ensure that we do not stop eating too soon; endorphins do not influence our decision to eat. Drugs that selectively block the action of endorphins reduce the intake of foods that are quite sweet or have a high fat content. Interestingly, these drugs that block endorphins only reduce the pleasure of eating these foods; they do not reduce the feelings of hunger.
Endorphins drive us to overconsume palatable foods by blunting the impact of feeling full. As we all know, while standing next to the buffet table we will engage in mindless eating. We know that we should stop eating and move away from the buffet line and let someone else get at the food. Our bellies are full to the point that it hurts to breathe. Belts are loosened another notch. So why can’t we stop eating? Neuroscientists have some interesting explanations. One of these is called “ingestion analgesia,” and it involves endorphins. The function of ingestion analgesia is to keep you eating. Even though continued eating has become unpleasant because the stomach is painfully stretched to its full capacity, we still keep eating. Essentially, we block out the painful feedback from these feelings by releasing endogenous opiates into our brain and body. Not surprisingly, our reaction to pain is reduced significantly when eating tasty foods, such as cheesecake or rich chocolate. This explains why we can indulge in a decadent dessert even after we have become fully satiated by a large meal. We have basically become insensitive to the pain of continued eating. Also, if the animal eating next to you tries to take away your portion of the food, having your body flooded with endorphins will lessen the pain of any injuries that you sustain.
Brains evolved when food was scarce; thus, we are compelled by our genetic legacy to eat whatever and whenever possible until everything that can be consumed, is consumed to completion. All animals have a tendency to eat a great deal of food when palatable food is readily available. Not only that, but we also subconsciously prevent others from taking our food source. Just watch people’s body posture at a buffet table. We defend our access to tasty food when it is within easy reach and is at risk of being consumed by other humans. Studies have shown that humans will eat more when more food is available even when the food is stale or otherwise unappealing (which is good news for bad cooks!). Furthermore, even if you point out to someone that the food is stale or that he has eaten more than his fair share, he will continue to eat. Our biological drive to consume tasty foods to completion outweighs any opposing cognitive or motivational factors.
Even after you have gained a lot of weight, your brain wants you to gain more. Research indicates that obese humans have elevated levels of endogenous endocannabinoids—marijuana-like chemicals—in the blood and brain. Remember “the munchies?” When we become overweight, our bodies induce a constant state of the munchies by bathing our brain in endocannabinoids. The endogenous marijuana neurotransmitters, the endocannabinoids, also contribute to the pleasure of eating. Scientists have discovered that marijuana increases the pleasurable response to eating sugar but has no effect on how much we dislike the taste of other types of foods. For example, if you hate eating peas or broccoli, smoking marijuana will not induce you to like eating them.
The ability of sugar to induce a rewarding feeling is caused by the release of dopamine in the brain’s reward center. This brain region informs you that your brain likes this food and wants you to consume it more often. In the presence of marijuana, significantly more dopamine is released in response to the same amount of sugar-enriched food. So what does all of this mean? Your brain’s endogenous marijuana system ordinarily modulates how good a particular food tastes; smoking marijuana simply enhances this natural mechanism in the brain. Your brain’s main purpose is to help you survive and pass on your be-fearful-first genes. Eating is a critical and necessary behavior that the brain organizes and controls to allow daily survival. Therefore, the brain rewards itself for successfully consuming enough calories to survive by releasing these two powerful neurotransmitters—endorphins and endocannabinoids. Because of the manner in which evolution has shaped the response of our brain to food, overeating of calorie-dense foods has become a major health problem in the modern world. Our brains were shaped by evolution to be very efficient at instructing us to eat, but quite inefficient at stopping us from eating.
Why do I crave fat and sugar?
Fat and sugar are craved like heroin or methamphetamine. Why is this so? The answer is that these foods actually change how the brain functions. Day after day, year after year, the constant bathing of the brain in fats and sugar slowly changes how the neurons within our brain’s feeding center behave. Along with these changes, gradual modifications in brain circuitry also occur; ultimately, your brain rewires itself to eat more fat and sugar every day in order to feed the ever-more-powerful new programing that is evolving inside your brain. Scientists once assumed that obese people were simply addicted to food in the same manner that someone becomes addicted to heroin—that is, food produces happy, pleasant feelings, and, therefore, eating lots of food would produce extremely pleasant feelings. Not so. A few years ago scientists discovered just the opposite was true: The brain’s reward center decreased its response to eating tasty foods. In obese humans, dopamine function becomes significantly impaired in response to many years of poor diet. Consequently, ob
ese people consume ever greater quantities of fat and sugar in order to mitigate the diminished rewards that were once experienced by consuming only one scoop of ice cream or a small donut.
Are we born destined to become obese?
For some people, apparently, the answer is yes. Environment, determined by both geographical and societal forces, plays an important role. The genes we inherit from our parents also play a role. Many studies have shown that children who have two obese or overweight parents are four times more likely to become obese themselves. To be considered low risk, the parents of the adolescents needed to be lean, with a body mass index less than 25. When the children in the high-risk group were shown pictures of tasty-looking, high-calorie foods, the dopamine-dependent pleasure centers in their brains became highly activated, especially as compared with the response of the same brain regions in the low-risk children. Some children who are destined to become obese apparently inherit a dopamine system that becomes much more excited at the sight of a chocolate milkshake than does the dopamine system in the brain of a child who is not destined to become obese as an adult. Then, in adulthood, the brain switches the rules and begins to require more fat, salt, and sugar in order to achieve a similar level of dopamine-mediated reward. Once again, take note of the fact that your brain has only one goal: keep you alive long enough to pass on your genes to your offspring. Once that has happened the forces of evolution no longer care about your survival. Thus, your brain will regularly induce you to consume foods that bring it pleasure regardless of the long-term health consequences.
How do my gut bugs keep my brain healthy?
Your brain lives in a symbiotic relationship with the bugs in your gut. Whatever you eat, they eat. In return, they help your brain function optimally in a variety of ways. During the past few years, it has become increasingly apparent that in the absence of bacteria humans never would have evolved to our current level of cognitive performance. Our brains are profoundly dependent upon a wide range of chemicals produced by these gut bugs. For example, without these gut microbes our brains do not correctly develop the serotonin neurons that play a key role in the control of emotion.
If you were to count all of the cells on and inside of you that are not actually you, they would number in the trillions. These bugs were not simply along for the ride as we became the dominant species on this planet; they made the journey possible. As soon as individual cells evolved into fully multicellular organisms during the Cambrian period about 500 million years ago, they quickly discovered the fantastic survival benefits of fully integrating themselves; once there, they never left. The total weight of the many trillions of bugs that reside in your gut is over two pounds and they are multiplying constantly thanks to all of the nutrients you are providing them; they are also in a constant battle for survival. The viruses in your gut kill so many bacteria every minute that their carcasses account for about 60% of the dry mass of your feces (now you know what is in there!).
Gut bacteria produce many different chemicals that can influence brain function. They convert the complex carbohydrates in our diet to the fatty acids butyrate, acetate, and propionate. Butyrate can easily leave the gut and enter the brain, where it can influence the levels of brain-derived neurotrophic factor (BDNF). BDNF plays a critical role in the birth and survival of neurons and the ability of the brain to learn and remember. Reduced levels of BDNF are correlated with impaired cognitive function and depression. Accumulating evidence suggests that gut bugs play key roles in both the developing and mature nervous system and may contribute to emotional and behavioral disorders as well as numerous neurodegenerative diseases.
Recent animal studies have shown that eating a high-fat diet can negatively alter the diversity of your gut microbiome, leading to reduced plasticity in the brain and increased vulnerability to anxiety. Eating a diet high in sugar also altered microbial diversity and significantly impaired learning and memory abilities.
Obviously, you need to take good care of these bugs so that they will take good care of your brain. Consuming prebiotics and probiotics can help us to maintain a healthy diversity within the bug environment. For example, elderly and frail humans who have major cognitive impairments also have the lowest level of bug diversity in their guts.
Can a good diet make you smarter?
Given that a poor diet can impair cognitive function, can a good diet make you smarter? Recently, a group of scientists investigated whether eating fruits and vegetables for 13 years would actually protect against a decline in cognitive abilities that humans commonly experience with normal aging. Their answer? Yes, it does; this is how they proved it. The study involved about 2,500 subjects who finished the study and adequately completed all the dietary and cognitive evaluations. The subjects were between 45 and 60 years of age at the beginning of the 13-year study, and they were required to maintain careful and detailed records of their daily diets. The subjects were evaluated at the beginning and end of the study for a variety of cognitive abilities, including verbal memory and higher executive functions such as decision-making and mental flexibility, along with numerous other tests. There is good news and bad news in the results. First, their diets were composed of a variety of fruits and vegetables, but specifically excluded potatoes, legumes, and dried fruits (each of these foods would have introduced specific complications that might have interfered with the outcome). The adults were divided into four groups according to the following diets: folate-rich diets containing fruits and vegetables, beta-carotene-rich diets containing fruits and vegetables, vitamin C-rich diets of fruits and vegetables, and vitamin E-rich diets containing both fruits and vegetables. The individual consumption of specific nutrients—folate, beta-carotene, and vitamins C and E—also was monitored. The subjects were allowed to choose how much of each diet they wished to consume each day; therefore, daily intakes of each nutrient varied. This was allowed in order to more closely reproduce how most of us actually select our daily intakes. At the end of the study, this is what they found. Eating fruits and vegetables has significant beneficial effects on very limited aspects of brain function. When the specific diets were examined more closely, diets that consisted of only fruits or diets with fruits and vegetables rich in vitamins C and E selectively benefited only verbal memory scores. This test involved being told to remember 48 different words and then recalling them after a delay with distractions. The surprising finding was that eating fruits and vegetables had no significant benefit for other types of tasks that required alternative types of memory, such as learning motor tasks or recognizing familiar objects.
Clearly, each component of your diet may influence how well your brain works in unique ways. Natural antioxidants found in fruits and vegetables, like polyphenols, provide protective effects for the brain through a variety of biological actions. Polyphenols are everywhere in nature; more than 50 different plant species and over 8,000 such compounds have been identified in plant extracts. Obviously, investigating the multiple health benefits of these natural chemicals poses an enormous challenge. The most thoroughly investigated polyphenols are probably quercetin, which is found in apples, tea, and onions, and resveratrol, which is found in the skin of grapes. Grapes use resveratrol to defend against fungus. Tea contains a number of beneficial chemicals. In neurodegenerative diseases, administration of tea extracts reduced the production of mutant proteins and may prevent neuron cell death in Alzheimer’s disease. Although tea is not a cure for Alzheimer’s disease, its use is certainly justified given its safety and potential for long-term benefits.
What about an apple a day?
Are fruits good for you? After all, most fruits are full of sugar. Many popular diets recommend avoiding carbohydrates, especially sugar, in any form. There are some good arguments that could be made about avoiding sugar, but if this approach takes fruits out of your diet, you may be missing important nutrients that might make you healthier in the long term. One of these important nutrients is ursolic acid. Ursolic acid is found in apples (mos
tly in the skin), cranberries, and prunes, as well as in elderflower, basil, bilberries, peppermint, rosemary, thyme, and oregano. Eating fruits and spices that contain ursolic acid may enhance brain function and reverse some of the negative effects of obesity on the brain as you get older. Studies have shown that ursolic acid can improve cognitive functioning by increasing your brain and body’s sensitivity to insulin. The biological mechanisms now have been investigated fairly well, and it appears that ursolic acid is able to correct some of the errors of metabolism induced by long-term obesity. The real challenge is to discover how many apples, prunes, and cranberries you need to eat in order to achieve these benefits; studies on humans have never been performed.
Will you lose weight by eating these fruits?
Maybe; it depends on what else you are eating. Will you lose weight avoiding fruits and berries while only eating meat? Yes. Over the long term, however, it is unwise to do so. The benefits of an all-meat diet are more immediate than the benefits of eating apples, cranberries, and prunes, because their effects on your health take longer to notice. Essentially, most of the restriction diets that are often promoted in popular magazines have not been around long enough for medical science to determine the long-term risks. Dietary restriction, which is reduced caloric intake without essential nutrient deficiency (i.e., a state of undernutrition without malnutrition), is the only valid, scientifically proven dietary intervention that has been shown to slow the aging process and improve health. There are two reasons why we hear so little about this approach: First, no one stands to profit from all of us eating less food and more apples, cranberries, and prunes. Second, the effects of dietary restriction on longevity have never been demonstrated in humans because rigorous and well-controlled clinical investigations have never been attempted. The effects of dietary restriction on health and longevity have been compellingly demonstrated across numerous species from single-cell organisms to rats to primates. If you are not willing to restrict your calorie intake, some alternatives are discussed in the following paragraphs.