Introduction
Chronic inflammatory diseases (CIDs), such as allergic and autoimmune
diseases, include a wide array of diseases in which the immune system is
thought to cause disease through excessive or dysregulated immune
reactions. Since these diseases are only partly genetic[1], many
environmental factors have been examined[2,3]. Environmental factors
implicated include occupational/environmental chemical exposures[4],
diet[3], infections[5–7] and stress[8,9].
Over the last 100 years, many of these CIDs have been increasing in
Western countries, and recently have increased in association with
westernization in developing countries[3,10,11] The term
westernization, as used here, refers to the adoption of practices
associated with Western culture that potentially impact health. The
westernized diet typically includes increased consumption of animal
products, fat (especially animal-derived fat and vegetable oils), sugar,
ultra-processed foods/beverages, salt, and food additives. New exposures
from xenobiotics/pollutants and pharmaceuticals are also included.
The hygiene hypothesis is one of the hypotheses proposed to explain the
westernization-associated rise in chronic inflammatory disease[12].
The hygiene hypothesis originally attributed the allergic disease
increase to decreasing rates of infections. These infections were
proposed to “train” the immune system to not react to harmless
allergens. Updates to the hygiene hypothesis have been proposed that
focus on the reduction of microbes that humans were exposed to
throughout evolutionary history[13–15]. These updates to the
hygiene hypothesis will be referred to here as the altered microbiota
hypothesis. In general, they propose that reduction in likely coevolved,
often commensal, microbes lead to microbial and immune system imbalances
that can give rise to chronic inflammatory diseases.