“I am on a diet.” is probably one of the most commonly heard sentences these days. With or without a specific aim in mind, most of us have tried to be on a particular “diet” regimen. Diets, in nutrition, are defined as the sum of food consumed by a person. Nowadays, it often implies the intake or avoidance of particular food.
We can usually name the broad goals of a diet — weight loss, gut cleansing, or “healthy eating.” Yet, oftentimes, we are unaware of how these diets affect our bodies on a cellular level and impact how the cells get energy, function and behave. This article will focus on how diets affect our immune system specifically.
The Western diet is perhaps the most thoroughly researched diet. It is a modern-day diet rich in refined sugars, salt, processed meats, and saturated fats derived from animals while containing low fibre, vitamins, and minerals. Research from various disciplines has shown that the popularization of the Western diet and lifestyle accounts for the drastic increase in chronic inflammation and noncommunicable diseases worldwide, including most heart diseases and cancers.
A Western diet meal impacts both branches of the immune system: adaptive and innate immunity. Macrophages, a type of innate immune cell, are involved in engulfing and degrading infected cells and foreign pathogens. These cells are particularly susceptible to the stress induced by animal-derived fats in a Western diet. Obesity caused by excessive caloric intake also alters the natural localization of innate immune cells, which correlates to increased systemic inflammation.
As part of the adaptive immune system, lymphocytes, including T cells and B cells, are also impacted by obesity. The balance between two T cell subsets, Th1 and Th2, is broken when the body is in an obese state, rendering the body more susceptible to inflammation. B cell concentration also increases under obesity, worsening insulin resistance.
The Western diet also affects gut microbiota composition, causing microbial imbalances in the gut. This, in turn, sabotages gut-dependent processes and contributes to increased cardiovascular risks and autoimmune diseases.
In contrast to Western diets, Mediterranean diets are enriched in vegetable-derived protein, vitamins and unsaturated (healthy) fats. This diet has been associated with anti-inflammatory effects, a decreased rate of chronic diseases and increased longevity.
A study demonstrated that macaques on a Mediterranean diet, compared to those on a Western diet, had lower expressions of proinflammatory genes in monocytes, the precursors of macrophages. The diet also inhibited a specific type of monocytes, M1 monocytes, and their downstream M1 macrophages, known to be associated with chronic diseases.
Unsaturated fats, found in olive oil and nuts in the Mediterranean diet, can, in fact, dampen the deleterious effects caused by saturated fats and promote the functionality of innate immune cells. Fibres, another component enriched in the Mediterranean diet but lacking in the Western diet, are also crucial in regulating immune functions. Certain fibres have been found to promote the differentiation of a subset of T cells called regulatory T cells, essential for preventing autoimmune diseases. Moreover, a diet high in vitamins and fibres has also been shown to ameliorate intestinal inflammation and restore bacterial composition in mouse models.
The paleolithic diet, or the cavemen diet, is a diet that attempts to resemble what human hunter-gatherer ancestors ate during the Paleolithic era. This diet avoids any processed food, grains, legumes, vegetable oils and most dairy products and emphasizes the intake of fruits, vegetables, meat and fish. In fact, derivatives of paleo diets have been adopted and advocated for to alleviate the symptoms of some immune diseases.
The modified paleolithic Wahls diet is one of the two diets promoted for multiple sclerosis (MS). Despite the precise pathogenesis of MS being unknown, numerous studies have shown that the Wahls diet promoted mobility and improved symptoms of MS, including fatigue.
According to the Autoimmune Association, the Autoimmune Paleo (AIP) diet is suitable for people with autoimmune or inflammatory conditions such as psoriasis, celiac disease and inflammatory bowel syndrome. AIP diet also focuses on eating more vegetables and fish, aligning with the paleo diet principles, while avoiding food that people with autoimmune diseases commonly react to.
However, so far, no studies have attempted to uncover if or how a paleo diet alters immunity on a cellular level.
Eating Less or Not Eating at All
Fasting as a dietary option has gained popularity over the years, with the duration ranging from a couple of hours a day to multiple days as a “system cleanse.” The 16/8 intermittent fasting (IF) regimen is the most popular version of fasting. Dieters restrict their eating period to only an 8hour window, for example, from 8 am to 4 pm, and refrain from eating for the remaining 16 hours of the day.
A recent study showed that people going on a 14% caloric restriction over two years had a drastic increase in the generation of new immune cells and overall decreased proinflammatory gene expressions.
Studies have also investigated the effects of fasting during Ramadan, the month of fasting for Muslims. Over the month of Ramadan, the level of proinflammatory cytokines, the concentration of circulating leukocytes and the cell proliferation rates in the body were all significantly reduced compared to before Ramadan. This suggests that IF practiced during Ramadan effectively reduced the level of inflammatory processes in the body.
IF also promotes a cellular activity called autophagy, a natural degradation method to remove dysfunctional components of the cells. Autophagy plays an essential role in regulating the activation and responses of specific T cells. IF also reduces biomarkers of inflammation in the body and decreases fat mass, which correlates to lower levels of proinflammatory cytokines. Combining the effects of both reducing inflammation and promoting T cell activation, IF has been suggested as a possible defence mechanism against SARS-CoV2 infections.
So far, it seems like all the “healthy” diets mentioned have been shown to reduce the level of inflammatory responses in the body, on top of other pro-immune effects. However, as we will learn throughout this issue of IMMpress, gut microbiota composition and balance are essential for maintaining health. Sudden changes in diets can often cause drastic changes in the gut microbiota population and behaviour, leading to unpredictable or undesirable effects. We should always keep in mind that there is no one-size-fits-all diet, and we should all ensure we understand the impact of a diet before jumping into one.
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