The Gut-Brain Axis
From the warm security of “comfort food” to the grouchiness and irritability of a sugar crash, most people know that what you eat affects how you feel. This isn’t just “common sense:” scientists are increasingly discovering that a healthy gut is crucial to mental well-being, going so far as to describe the roughly 100 million neurons embedded in your gut as a “second brain.” This “second brain” has local control over digestion, but that’s not its only role. Like all good partners, it communicates extensively with the brain in your cranium, through a neural pathway called the vagus nerve. Researchers are slowly uncovering the enormous impact of this communication on our emotions: one successful treatment for depression, for example, involves stimulating the vagus nerve to mimic the action of a healthy gut.
The communication between the two brains runs both ways. Stress (as felt by the upper brain) can prevent healthy digestion and set off a vicious cycle of physical health problems. But gut dysfunction (due to chronic disease, poor gut flora, or any other reason) also has negative consequences for your mental health. This makes gut healing invaluable for overall well-being: clean, nourishing food benefits your mind as well as your body. A Paleo diet can support your mental health in two essential ways: first, it avoids damaging your brain tissue (both above and below your shoulders) with harmful inflammation; second, it provides all the micronutrients and helpful bacteria that your brain needs to thrive.
Inflammation is the body’s natural response to injury. If you’ve ever gotten a splinter, you’ve seen inflammation in action: the body rushes blood to the site of the wound, carrying immune cells to prevent infection and heal the wound quickly. The large amount of fluid crowding into the small area of the wound causes it to swell up and become tender. Your splinter site is now inflamed.
An inflamed splinter wound heals fairly quickly, because the injury is a one-time event: if you irritated the site by continually poking it with splinters, the inflammation would never go down. This is what happens in your gut when you base your diet on foods that irritate the lining of your intestinal walls. Gluten, for example, causes gut inflammation in almost everyone; lectins, such as those found in non-gluten grains, pseudograins, and legumes, are also often problematic. These elements irritate the digestive tract, and can even pass through the gut walls into the bloodstream (a problem known as leaky gut syndrome). Other unhealthy factors of the modern diet (for example, an imbalance of Omega-3 and Omega-6 PUFAs) can also contribute to this. The constant gut irritation is like a constant splinter poking into your intestines, and provokes an inflammatory response in the “gut brain.”
Whatever the cause of the inflammation, it triggers the release of harmful molecules called cytokines. These cytokines are one of the reasons why gut inflammation is so harmful to your mental health. Unlike most molecules, cytokines can pass through the blood-brain barrier, the separation that keeps your brain safe from most common bacterial infections. Once in the brain, cytokines continue to cause inflammation in your neural cells: in addition to an inflamed gut, you now have an inflamed brain.
Gut and brain inflammation can unfortunately form a vicious cycle. Since the brain and the gut depend so heavily on constant communication through the vagus nerve, inflammation in the brain can disrupt important signals to the gut, causing poor digestion, which in turn increases gut inflammation, sparking the production of even more cytokines.
This inflammatory cycle contributes to a whole host of problems, especially depression. The precise mechanisms that cause this are still being studied, but the correlation is clear – in an extreme example, patients taking a proinflammatory drug (interferon) for hepatitis suffer from severe depression and higher suicide rates than patients on any other drug.
Inflammation, therefore, is a key cause of depression. However, it’s important to note that inflammation itself is not the problem; the problem is the toxic diet of modern processed foods that caused the inflammation in the first place. Anti-inflammatory drugs are increasingly used to treat depression, but eating a gut-healing, non-inflammatory diet in the first place is much more beneficial than filling your body with gut irritants and washing them down with a dose of chemicals to reduce their effects.
A Paleo diet forestalls the need for anti-inflammatories by preventing inflammation in the first place. People with severe digestive problems may see benefits from following a slightly stricter version of Paleo until their gut health improves, but a basic Paleo diet is inherently non-inflammatory because it excludes the food toxins that cause inflammation in the first place. When your gut is no longer being irritated by gluten, lectins, and other food toxins, it can recover just like your skin can heal from a one-time splinter wound.
Micronutrients and Brain Health
As well as not harming your gut, a Paleo diet also provides the micronutrients that a healthy, digestive system can absorb and use to support proper brain function. One of the most important of these is cholesterol. Cholesterol is a vitally important nutrient for humans because of the way our brains work. Humans only have a limited number of brain cells. We’re born with a finite number, and they begin to die immediately. Unlike muscle cells, we can’t make more of them. We can, however, maintain our cognitive abilities by making connections between the remaining brain cells. Cholesterol is the key to this process.
Under normal circumstances, the brain can synthesize its own cholesterol, but in some cases, it needs to get additional cholesterol from the blood. Since the only way for cholesterol to get into your bloodstream is if you eat it, this makes cholesterol a vital nutrient for maintaining brain health. As you grow older (and lose more brain cells, requiring your neurons to make more connections), cholesterol is increasingly important: one of the causes of Alzheimer’s disease may actually be a cholesterol deficiency.
Cholesterol is also extremely important for mental health because it allows you to synthesize Vitamin D from sunlight. Vitamin D supports mental performance and helps reduce harmful inflammation. Without cholesterol, you would only be able to get Vitamin D through foods – and while a few foods do contain it (especially wild-caught fatty fish), most people can’t get enough through diet alone.
Another extremely important group of micronutrients for mental health are the B vitamins. B vitamins include choline, which supports proper neurological function in a variety of ways. Choline even naturally occurs in cholesterol-rich foods like eggs and liver, giving you twice the benefit from that omelet. Thiamine, or Vitamin B1, is also crucial: your brain needs it to turn glucose into energy. Although scientists have not yet definitely determined the role of each one, two other B vitamins, folate and Vitamin B12, help prevent depression and maintain memory function especially in older adults. Vitamin B6 joins these two in lowering the levels of a neurotoxic amino acid called homocysteine.
Iron, which fills many of the same roles as the B vitamins, is also essential for mental health. Without iron, your brain can’t break down protein to make neurotransmitters, so iron deficiency can cause confusion, depression, and “brain fog.” Other crucial micronutrients for healthy brain function include selenium and magnesium.
A diet of nutrient-dense whole foods, therefore, can go a long way in supporting healthy brain function. A healthy Paleo diet will also include probiotics or probiotic foods to maintain the health of your gut flora. As well as helping you absorb the nutrients in your diet, several studies have shown that these beneficial bacteria help protect you from depression and anxiety. In mice, probiotic supplements reduced anxiety and depression, while introducing harmful bacteria caused anxious behavior. This does not necessarily indicate a straightforward cause-and-effect relationship (especially since so many factors influence gut health), but does point to gut flora as one of the many important elements in mental, as well as physical, well-being.
Food Reward and the Insulin Rollercoaster
Another way that a Paleo diet can support mental well-health is by eliminating two different high/crash cycles of stimulation that accompany the modern diet. The first is the food reward cycle of hyperpalatable processed foods. The food reward hypothesis of obesity argues that the human brain is only equipped to handle a certain finite amount of stimulation from food. Modern processed food overwhelms our pleasure receptors with stimuli too intense for them to handle, causing our regulatory pathways to become completely overwhelmed. This creates a kind of food addiction (gluten and opioid drugs like morphine are actually addictive in the same way), with the same mental highs and crashes that accompany any other addictive behavior.
Eating a diet of whole, unprocessed foods ends this cycle of highs and lows, because it doesn’t contain any artificial foods to overwhelm your neural processing centers. A diet based on fat and protein also avoids another unhealthy pattern: sugar-driven mood swings from too many refined carbohydrates. Although Paleo isn’t necessarily zero-carb (safe starches can be perfectly healthy, especially for active young people), Paleo carbohydrates come from whole foods and do not make up the bulk of dietary calories. Safe starches in the context of a diet rich in protein and fats do not cause the same metabolic problems and blood sugar spikes as toxic refined grain products in a nutrient-poor, low-fat diet.
By including only whole, nutrient-dense foods and plenty of healthy fats and protein to accompany unprocessed carbohydrates, Paleo avoids food-driven patterns of highs and crashes, leaving you operating on a much more even keel.
Paleo Lifestyle and Mental Health
Food is an important part of the Paleo lifestyle, but it’s not the only one – exercise is one of the most important ways that you can keep your brain in top condition. In the short term, exercise releases endorphins, special neurotransmitters that give you a boost of energy and motivation even after your workout ends. Exercise can also help reduce stress, which throws off several important hormonal balances in the brain, and leads to systemic inflammation even on a flawless Paleo diet.
In the long term, regular exercise can help prevent and treat depression: a study done on depressed adults found that even walking 20-40 minutes three times a week helped reduce symptoms. Resistance training (such as heavy weightlifting) and cardio exercise were both equally effective. Exercise also helps older adults, and people suffering from age-related diseases like Alzheimer’s, maintain mental function and keep making connections between neurons.
Exercise also supports your brain by helping you stay at a healthy body weight. Obesity puts a heavy stress on your internal organs, throwing you back into the vicious cycle of brain-gut inflammation. Maintaining a fit body can also help you feel better about yourself – who doesn’t get a confidence boost from looking in the mirror and liking what they see?
The Myth of the “Brain-Boosting Superfood”
One way that you won’t help your brain – even on a Paleo diet – is by relying on “superfoods.” Whether it’s blueberries, cinnamon, or green tea, each new issue of every popular diet magazine has another supposedly miraculous “superfood” that will raise you to new heights of mental acuity. The problem with these foods isn’t that they’re unhealthy in and of themselves. It’s that you can’t make up for an unhealthy, nutrient-poor diet by eating more of one particular “superfood.” A whole-foods, nutrient-dense diet will provide your brain with everything it needs even if you never touch a blueberry in your life; a steady stream of chemically processed junk food will leave your brain inflamed and unhealthy even if drink a gallon of green tea with every meal. There isn’t anything wrong with these foods if you enjoy them, but don’t rely on them to make up for the rest of your diet.
The mind and the body are not separate systems. The way you treat one will affect the other – chronic mental stress harms your body, and chronic physical stress harms your mind. This is not to say that Paleo is a magic bullet for depression, Alzheimer’s, or any other illness: if you have a serious mental disorder, Paleo is no substitute for a mental health professional. Instead, think of it as just one part of your efforts to maintain mental health. Consistently nourishing both your both your body and your mind with non-toxic, nutrient-dense foods will benefit your health in every respect.