Review Fast food fever: reviewing the impacts of the Western diet on immunity Ian A Myles http://www.nutritionj.com/content/13/1/61 Abstract While numerous changes in human lifestyle constitute modern life, our diet has been gaining attention as a potential contributor to the increase in immune-mediated diseases. The Western diet is characterized by an over consumption and reduced variety of refined sugars, salt, and saturated fat. Herein our objective is to detail the mechanisms for the Western diet’s impact on immune function. The manuscript reviews the impacts and mechanisms of harm for our over-indulgence in sugar, salt, and fat, as well as the data outlining the impacts of artificial sweeteners, gluten, and genetically modified foods; attention is given to revealing where the literature on the immune impacts of macronutrients is limited to either animal or in vitro models versus where human trials exist. Detailed attention is given to the dietary impact on the gut microbiome and the mechanisms by which our poor dietary choices are encoded into our gut, our genes, and are passed to our offspring. While today’s modern diet may provide beneficial protection from micro- and macronutrient deficiencies, our over abundance of calories and the macronutrients that compose our diet may all lead to increased inflammation, reduced control of infection, increased rates of cancer, and increased risk for allergic and auto-inflammatory disease. Introduction The Western diet is characterized by a high intake of saturated and omega-6 fatty acids, reduced omega-3 fat intake, an overuse of salt, and too much refined sugar [1]. Most are aware that this type of eating, if not in moderation, can damage the heart, kidneys, and waistlines; however, it is becoming increasingly clear that the modern diet also damages the immune system. The modern lifestyle is also typified by reduced exposure to microorganisms, increased exposure to pollutions, heightened levels of stress, and a host of other exceptionally well reviewed variables that likely contribute to immune dysfunction [2]. Therefore, while dietary effects on immunity should not be thought of in isolation, herein we focus on the body of evidence detailing the mechanisms for the Western diet’s impact on immune function. Total nutrient intake Intake of adequate calories and micronutrients is vital for optimal immune function. Deficiency in total calories and/or protein, as seen in parts of the world stricken with starvation, severely reduces the immune system’s ability to respond [3]. As one example, inflammatory cytokines are themselves proteins, and thus infection during starvation can lead to the production of these cytokines at the expense of other proteins from blood and tissues [3]. However, the obesity epidemic clearly outlines that today’s diet contains an over abundance of nutrients [1]. While the Western world is not free from micronutrient deficiencies, since this review aims at detailing the immuno-nutrition of a Western diet not typically characterized by micronutrient deficiency, they are thus considered outside the focus. Therefore, we encourage interested readers to seek out lovely review articles on the immune impacts and mechanistic understandings of dietary minerals and vitamins [3-6]. Adipocytes release inflammatory substances including interleukin (IL-) 1, IL-6, and tumor necrosis factor (TNF) [7]. In animal models, it appears that these signals can act as false alarms that, over enough time and in large enough amounts, cause the entire system to dial down its responsiveness – analogous to a person removing a battery from a twitchy smoke detector that frequently alarmed when no signs of fire were present [7,8]. When an actual infection comes along, the response may be delayed because the early warning system was silenced – just as deactivating that smoke detector leaves a home more susceptible to fire [7]. While human verification is lacking, this concept is not unique to immunity, for example anabolic steroid abusers down regulate their steroid responses [9] while opioid abusers down regulate innate opiate responses [10]. Obese individuals have fewer white blood cells to fight infection and those cells they do possess have reduced phagocytosis capability [11,12]. While a complex interplay of hormonal, metabolic, and immunologic processes contribute to the biologic responses in the obese the resultant immune dysfunction increases the risk of infections of the gums, respiratory system, and of surgical sites after an operation [13-17]. Even routine interventions like immunizations may not work as well simply due to the inability of the vaccination needle to reach the muscle tissue of the arm [18]. One possible mechanism for obesity’s harmful effects on the immune system could be the increased levels of leptin in the blood. All mononuclear immune cells have a receptor for leptin and activation leads to an increase in IL-1, IL-6, and TNF [19]. Leptin stimulates NK cells, activates the transcription factor STAT3, and reduces the anti-inflammatory T-regulatory (Treg) cells [20]. In general, adiponectin has opposing effects on immunity and interestingly the ratio of the two can predict the development of coronary artery disease in diabetics [21]. Hypothetically, as with the development of resistance to leptin’s appetite suppressing functions [19], patients with obesity may, overtime, down-regulate the immune activation attributable to leptin [7]. However intriguing the in vitro impacts or correlation evidence of leptin may be, the precise mechanism by which excess calories impact the immune system has yet to be fully elucidated and will be very difficult to separate from the underlying mechanisms of the macronutrient sources of calories. Eating disorders currently attributed to image obsession are also an unfortunate part of modern dietary habits and their immune impacts have gone relatively ignored compared to obesity. While the nutrient deficiencies seen in eating disorders are not nearly as severe as for the starvation seen in developing nations, subtle deficiencies appear to lead to subtle immune defects [22]. Both anorexia and bulimia may reduce neutrophil and monocytes numbers, T-cell number and function, anti-bacterial complement proteins, as well as effects attributable to any micronutrient deficiency that may develop [23]. Yet surprisingly, despite infection being a major cause of death in patients with anorexia, most are infection free until the late stages of their disease; speculation on the cause of this finding proposes that either the severe iron deficiency and sequestration render the blood a harsh environment for pathogen survival or that anorexic patients are, typically, deficient in carbohydrates and calories but may be only moderately deficient in proteins and fat [22,23]. Further investigation into the immune impacts of eating disorders is warranted.
After reading all these threads I am ready for a glass of red wine and a Ribeye cooked (mr) with a loaded baked potato and keylime pie for dessert.