There is an increasing interest in understanding how the ‘Western diet’ affects immunity. Many people across the globe have adopted this diet, and epidemiological studies have revealed that it correlates with a high incidence of chronic inflammatory disorders, including diabetes, multiple sclerosis and asthma. Nevertheless, because the Western diet includes a large proportion of red meat, sugars, fats and refined carbohydrates and relatively small amounts of vegetables, fruits and fish, it is likely that the causative component of its associated pathologies is not a single entity but a complex array of unbalanced abundance of micronutrients in the diet. A review paper in Nature Immunology takes some of the emerging data, and mixes it with history and future analytics to provide a good overview of where incidence of confidence lies, and where it is going.
Mechanism, mechanism, mechanism
The immune system is complex in its array of cell types and their functions, but is also highly integrated in all body systems. Beyond their classic role in fighting pathogens and pathogen products, cells of the immune system play important parts in tissue homeostasis. Furthermore, immunological functions are not restricted to cells of the immune system but are an integral part of most cell types. Immunological functions are always active, but they are called upon especially in times of tissue damage and the presence of invading microorganisms. Immune system activity is metabolically demanding, requiring a greater presence of metabolites and substrates. Hence, the typical epidemiological studies used to explore risks and benefits may not always establish a direct link between nutritional deficiencies and classic immunological functions. In addition, the effects of nutritional deficiencies may become apparent only upon challenge of an organism, such as an infection or vaccination, during stress on a particular tissue or when confounding environmental or genetic factors are at play.
Without basic science and mechanistic insights, any guidelines and health claims will remain compromised, open to speculation and of substantial concern to scientists, clinicians and decision-makers. Reliable scientific evidence underpins accurate nutritional advice and effective public-health policies that determine the optimal consumption of macro- and micronutrients in health and disease. Currently, even at the population-wide level, the optimal intake of many nutritional components is uncertain and is based on limited data, with undetermined baseline ranges for physiological concentrations. Nevertheless, some molecular pathways have been studied in sufficient detail to reveal nutritional sensing strategies that shape and define the fate of cells of the immune system and disease outcomes. Some amino acids, fats (such as omega-3 fatty acids) and vitamins (notably vitamin A and vitamin D) can be included in this category.
Plant phytochemicals, bacterial agents and others are gradually revealing their unique role in the generation and maintenance of a healthy human immune system and may also be regarded as having evidence of a substantive nature.
 Veldhoen M, Veiga-Fernandes H. Feeding immunity: skepticism, delicacies and delights. Nat Immunol. 2015 Feb 17;16(3):215-9. View Abstract