How Processed Foods Can Make You Depressed

How Processed Foods Can Make You Depressed

By guest author Eirik Garnas, Darwinian Medicine

Eirik Garnas is a nutritionist (B.Sc. in Public Nutrition, M.Sc. in Clinical Nutrition), science writer, personal trainer, and health coach. He has several years’ experience with health/nutrition coaching and personal training and has written for a variety of different magazines and websites. On his website Darwinian-Medicine.com Eirik writes about health and fitness and talks about the origins and solutions to different diseases and health problems.



How processed foods cause our immune systems to light up

Let’s imagine that you sit down to eat a dinner composed of leafy greens, a variety of tuberous vegetables, and a piece of salmon. You chew the food and swallow, and consequently, bolus passes down into your gut. On its way through your digestive system, the food you’ve just eaten is broken down by digestive enzymes and nutrients – vitamins, amino acids, fats, and so forth – are released and absorbed into your body.

Some people seem to be under the belief that this is all that happens following the consumption of food. It’s not. Food is not just this benign thing that provides our bodies with nutrients. Its effects extend far beyond those directly related to our nutritional status. Among other things, the food we eat affects the composition of our gut microbiotas – the trillions of microorganisms that colonize our guts – and the expression of various receptors in our bodies, as well as the expression of our genes.

Let’s say that you sit down for a meal composed of highly processed foods – hamburgers, soda, and ice cream – the day after you ate the whole foods meal described earlier. This second meal will affect your body in a very different manner than the first one. It will shift the composition of your gut microbiota, including your oral microbiota, to a more proinflammatory state,5, 6, 14, 17, 18 it will cause your blood glucose and insulin levels to rise rapidly, and it will stimulate the pleasure centers of your brain in much more powerful way than the first meal.

Processed foods can cause inflammation via several different pathways, some of which have to do with the human microbiome.11, 13, 15, 17

One of the main problems with highly processed, low-fiber foods is that they are capable of changing the composition of our microbiotas in such a way that genome-microbiome mismatches develop.3, 5, 17, 18 The yeast Candida albicans, gram-negative LPS-containing bacteria, and the oral, caries-causing bacterium Streptococcus mutans are just some of the many problematic bugs that flourish in the presence of sugary, refined foodstuffs.

These sugar-loving microbes produce and shed a variety of compounds that can pass into our bodies and cause our immune systems to light up. In particular LPS (lipopolysaccharide) has been shown to be a very potent immune-stimulator. Our innate immune systems recognize this compound, which signals the presence of some potentially unfriendly bacteria, and activate various defensive strategies in order to protect our bodies from harm.

It’s a good thing that our immune systems get off the couch and get ready to work when they detect the presence of something that can harm us. That’s what they are evolutionarily programmed to do. There’s a problem though. The conditions in which we modern humans find ourselves differ markedly from the conditions under which the basic structures of the human immune system evolved. Highly processed foods, which are today ubiquitous in our environment, were obviously not present in our preindustrial ancestors’ milieu.

Most of the immune insults that our primal ancestors faced were short-term in nature. Many of the immune insults that we – modern humans – face, on the other hand, are long-term in nature.

The immune system of a person who eats highly processed foods on a regular basis is in a constant state of warfare. It is most likely subjected to a steady stream of immune-stimulating compounds originating from his microbiota, as well as the adipose tissue his calorie-dense diet has caused him to accumulate. If this person is also physically inactive and doesn’t get much high-quality sleep, the situation will be particularly dire.


Inflammation sets the stage for depression

Over the past decade it has become increasingly clear that inflammation is at the center of the pathophysiology of depression.1, 4, 7, 10 Is inflammation a cause of depression, or simply something that occurs as a result of depression? There’s no doubt in my mind that inflammation can drive the development of depression. In other words, I don’t believe for a second that inflammation is something that solely occurs secondary to moody feelings. I would go as far as to say that inflammation is likely the main cause of depression. This statement is based on everything I’ve read about health and medicine over the years, as well as my own experience and the observations I’ve made.

The natural question that arises from all of this is: How and why does inflammation cause feelings of despair? Let’s start with the whys and then see if that can help us figure out the hows

A person who’s inflamed is not mentally fit to take on the world. His or her body doesn’t prioritize reproduction, growth, and advancement; rather, its primary focus is on restoring homeostasis. In order to do so, it needs rest and sleep. Hence, it’s not surprising that people who are chronically inflamed typically feel tired, weak, and mentally down.1, 8, 9 In these cases, feelings of despair may actually be adaptive, in the sense that depression promotes rest, isolation (something that potentially lowers the risk of further immune insults), and recovery.12 Moreover, depression may perhaps cause one to reconsider one’s actions, thereby potentially facilitating behavioral change.

There’s no doubt in my mind that depression serves an adaptive function!

This leads us over to the hows. There are many potential mechanisms by which inflammation can cause depression. If depression is indeed adaptive in the context of immune restoration, which it most likely is, it seems highly plausible that some types of proinflammatory mediators are capable of crossing the blood-brain barrier and acting on centers of the brain that are involved in regulating mood, pleasure, and behavior. If these compounds are only transiently present at high levels in the body, the depressed state will be temporary in nature; however, if they are constantly there, chronic depression may ensue. Unfortunately, this seems to be the case inside the body of many modern humans.2, 13, 15

Perhaps needless to say, inflammation is not the only cause of depression. If you lose a loved one or get fired from your job, you will obviously feel down and dispirited, regardless of whether your body is inflamed or not. With that said, psychological factors are not the only factors that play a role in depression. Far from it. I would actually say that most cases of depression probably don’t originate in the brain, but rather in the gut.

Depression undermines reproduction; hence, the high rates of chronic depression in modern societies must have an environmental cause. We can’t simply blame our genes, seeing as it makes absolutely no sense for natural selection to drive up the frequency of “chronically moody alleles”. All of the reports I’ve seen indicate that hunter-gatherers and traditional, non-westernized people are happier and more content with life than the average westerner. There’s no doubt in my mind that this is largely because they live in an environment that is well-matched with the human genetic make-up and are not chronically inflamed.


Short-term pleasure vs. long-term satisfaction

We’re hard-wired to like sugary, calorie-dense foodstuffs, seeing as it was beneficial, fitness wise, for our ancestors to consume calorie-dense foods. The problem we have today is that calorie-dense, sugary foods are no longer rare. They are everywhere. Moreover, many modern foods have a nutrient configuration that doesn’t look anything like that of the foods our primal forebears ate.

When we consume highly processed foods such as ice cream, doughnuts, and pizza, the levels of the hormone dopamine in our bodies rapidly rise and the reward centers in our brains light up. We feel great. There’s a catch though. The pleasurable feeling is only temporary. Not long after the meal is over, it dissipates. In order to get it back, we have to return to the inner isles of the supermarket and pick up more crackers, chocolate, and ice cream.

The problem with these foods is that they are highly addictive. Moreover, they don’t make us fully satisfied. All we derive from them is short-term pleasure. Over the long-term, they don’t make us happy; on the contrary, they can make us fat, sick, and depressed.

The vast majority of contemporary humans consume highly processed foods on a regular basis. Moreover, a lot of people are chronically stressed, physically inactive, socially isolated, and disconnected from nature. Hence, it’s not surprising that so many people are depressed these days.

This is where the Paleolithic diet comes in. By giving our brains the type of diet they evolved to thrive on, we can ensure that they function at a high capacity. Over time, as one consumes a healthy diet, cravings for highly processed foods tend to gradually disappear, in part because microbiota-related dietary adaptations take place.




1 M. Berk, L. J. Williams, F. N. Jacka, A. O’Neil, J. A. Pasco, S. Moylan, N. B. Allen, A. L. Stuart, A. C. Hayley, M. L. Byrne, and M. Maes, ‘So Depression Is an Inflammatory Disease, but Where Does the Inflammation Come From?’, BMC Med, 11 (2013), 200.

2 M. M. Bosma-den Boer, M. L. van Wetten, and L. Pruimboom, ‘Chronic Inflammatory Diseases Are Stimulated by Current Lifestyle: How Diet, Stress Levels and Medication Prevent Our Body from Recovering’, Nutr Metab (Lond), 9 (2012), 32.

3 M. S. Desai, A. M. Seekatz, N. M. Koropatkin, N. Kamada, C. A. Hickey, M. Wolter, N. A. Pudlo, S. Kitamoto, N. Terrapon, A. Muller, V. B. Young, B. Henrissat, P. Wilmes, T. S. Stappenbeck, G. Nunez, and E. C. Martens, ‘A Dietary Fiber-Deprived Gut Microbiota Degrades the Colonic Mucus Barrier and Enhances Pathogen Susceptibility’, Cell, 167 (2016), 1339-53.e21.

4 T. G. Dinan, and J. F. Cryan, ‘Melancholic Microbes: A Link between Gut Microbiota and Depression?’, Neurogastroenterol Motil, 25 (2013), 713-9.

5 S. Duarte, M. I. Klein, C. P. Aires, J. A. Cury, W. H. Bowen, and H. Koo, ‘Influences of Starch and Sucrose on Streptococcus Mutans Biofilms’, Oral Microbiol Immunol, 23 (2008), 206-12.

6 C. Erridge, T. Attina, C. M. Spickett, and D. J. Webb, ‘A High-Fat Meal Induces Low-Grade Endotoxemia: Evidence of a Novel Mechanism of Postprandial Inflammation’, Am J Clin Nutr, 86 (2007), 1286-92.

7 John R. Kelly, Paul J. Kennedy, John F. Cryan, Timothy G. Dinan, Gerard Clarke, and Niall P. Hyland, ‘Breaking Down the Barriers: The Gut Microbiome, Intestinal Permeability and Stress-Related Psychiatric Disorders’, Frontiers in Cellular Neuroscience, 9 (2015), 392.

8 S. E. Lakhan, and A. Kirchgessner, ‘Gut Inflammation in Chronic Fatigue Syndrome’, Nutr Metab (Lond), 7 (2010), 79.

9 K. Louati, and F. Berenbaum, ‘Fatigue in Chronic Inflammation – a Link to Pain Pathways’, Arthritis Res Ther, 17 (2015), 254.

10 M. Maes, ‘Depression Is an Inflammatory Disease, but Cell-Mediated Immune Activation Is the Key Component of Depression’, Prog Neuropsychopharmacol Biol Psychiatry, 35 (2011), 664-75.

11 M. Martinez-Medina, J. Denizot, N. Dreux, F. Robin, E. Billard, R. Bonnet, A. Darfeuille-Michaud, and N. Barnich, ‘Western Diet Induces Dysbiosis with Increased E Coli in Ceabac10 Mice, Alters Host Barrier Function Favouring Aiec Colonisation’, Gut, 63 (2014), 116-24.

12 A. H. Miller, and C. L. Raison, ‘The Role of Inflammation in Depression: From Evolutionary Imperative to Modern Treatment Target’, Nat Rev Immunol, 16 (2016), 22-34.

13 I. A. Myles, ‘Fast Food Fever: Reviewing the Impacts of the Western Diet on Immunity’, Nutr J, 13 (2014), 61.

14 S. Pendyala, J. M. Walker, and P. R. Holt, ‘A High-Fat Diet Is Associated with Endotoxemia That Originates from the Gut’, Gastroenterology, 142 (2012), 1100-01.e2.

15 B. Ruiz-Nunez, L. Pruimboom, D. A. Dijck-Brouwer, and F. A. Muskiet, ‘Lifestyle and Nutritional Imbalances Associated with Western Diseases: Causes and Consequences of Chronic Systemic Low-Grade Inflammation in an Evolutionary Context’, J Nutr Biochem, 24 (2013), 1183-201.

16 E. M. Selhub, A. C. Logan, and A. C. Bested, ‘Fermented Foods, Microbiota, and Mental Health: Ancient Practice Meets Nutritional Psychiatry’, J Physiol Anthropol, 33 (2014), 2.

17 I. Spreadbury, ‘Comparison with Ancestral Diets Suggests Dense Acellular Carbohydrates Promote an Inflammatory Microbiota, and May Be the Primary Dietary Cause of Leptin Resistance and Obesity’, Diabetes Metab Syndr Obes, 5 (2012), 175-89.

18 R. Touger-Decker, and C. van Loveren, ‘Sugars and Dental Caries’, Am J Clin Nutr, 78 (2003), 881s-92s.



To learn more, visit Eirik Garnas’ website Darwinian-Medicine.com or his Facebook page @darwinianmedicine

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