How highly processed foods affect the gut microbiome and health

How highly processed foods affect the gut microbiome and health

Several studies have shown a possible link between ultra-processed foods (UPF) and chronic inflammation. A recently published review in The Lancet Gastroenterology & Hepatology describes the mechanistic link between UPF consumption and chronic diseases, particularly those affecting the gut microbiome.

Learn: Ultra-processed foods and human health: From epidemiological evidence to mechanistic insights.Photo credit: Dawid Rojek / Shutterstock.com

introduction

Over the past few centuries, many food processing techniques have been developed to preserve foods and improve their taste and digestibility. Modern industrially processed foods take advantage of the availability of salt, sugar, vegetable oils, animal fats and flour.

Typically, mechanical and physical techniques such as roll milling, extrusion and pressure recycling, as well as chemical processes such as hydrogenation, are used in the manufacture of these products. Artificial flavors and preservatives, anti-caking agents and other additives are also used to achieve the final desired texture, color and taste of these food products.

“These techniques enabled mass-produced processed foods to be produced and manufactured on a large scale, making them available year-round.”

Such highly processed foods tend to be hygienic, convenient, affordable and accessible, making them ideal products for many high-income countries (HIC). Recently, low- and middle-income countries (LMICs) have also become increasingly dependent on these foods.

Concomitant with this trend, the number of chronic inflammatory cases, including metabolic syndrome and inflammatory bowel disease (IBD), has increased. The current paper examines the evidence for a correlation between modern UPFs and chronic diseases mediated by the gut microbiota.

Types of processed foods

There are several classification systems used to describe foods based on how they are processed. The most commonly used is NOVA, which divides foods into groups 1 to 4.

Group 1 includes unprocessed or minimally processed foods such as fresh, chilled, dried, frozen, fermented or pasteurized products. Group 2 consists of processed ingredients such as vegetable oils, sugar, salt, butter or other food extracts added to Group 1 foods.

Group 3 includes all typical processed foods such as salted preserves, candied dried fruits, cured meats, cheese and fresh bread. In short, Group 3 describes Group 2 products added to Group 1 foods.

Group 4 includes UPFs that have undergone one or more of the above industrial processes. These may contain non-domestic ingredients, including those used for flavoring, coloring, sweetening or emulsifying.

UPF intake has increased HICs, accounting for up to 30%, 50%, and nearly 60% of caloric intake in France, the United Kingdom, and the United States, respectively.

Study results

Nearly 50% of the 100 prospective studies analyzed in the current study examined the association between UPFs and various health or mortality outcomes.

mortality

Seven studies showed an association between UPFs and death from all causes, with an increase in risk from 20% to 60% in the highest UPF use category compared to the lowest. Five studies showed an increased risk of illness or death from cardiovascular disease or stroke. Four studies showed a higher risk of type 2 diabetes, while one reported an increased risk of gestational diabetes.

hypertension

Four studies reported an increased risk of high blood pressure of up to 30%, one with cancer risk and several with overweight and obesity. Many of these studies also reported an association with increased weight, waist circumference, and increased serum lipid levels in children.

IBD

In the 2021 Prospective Urban Rural Epidemiology (PURE) study, UPF use was associated with a higher risk of IBD, particularly Crohn's disease, but not ulcerative colitis. Other studies have linked UPF consumption to an increased risk of depression, abnormal lipid levels, a decrease in kidney function, and fatty degeneration of the liver.

Weight gain

Short-term randomized intervention trials are currently being conducted in which UPFs and unprocessed foods are assigned to different cohorts for short periods of time. The researchers then observed an average increase in energy intake of over 500 kcal per day and an average weight gain of 0.8 kg in the UPF group. Conversely, an average weight loss of over 1 kg was reported in the unprocessed food cohort.

The mechanisms behind chronic inflammation with UPFs

There are numerous mechanisms attributable to the weight gain and chronic inflammation observed with higher UPF consumption. One example is the poor nutritional quality of many UPFs.

According to the French Open Food Facts database, only one in five UPFs had high nutritional value. Even when energy intake from UPFs is standardized for comparison with unprocessed foods, the negative health effects continue to show a strong association, suggesting that “factors beyond nutritional aspects play a role.”

Another concern is the presence of potentially toxic substances in UPFs, such as polycyclic aromatic hydrocarbons, furans, advanced glycation end products, trans fatty acids from the hydrogenation of fats, and acrylamide from cooking starchy foods at high temperatures. The latter chemical is found in common foods such as french fries, chips and cookies, whether made at home or industrially; However, the acrylamide levels in these industrial products are often higher.

Other contaminants such as phthalates, bisphenols, mineral oils and microplastics can leach into food from packaging, especially if the food is left in contact for long periods of time. This is likely to occur with food products that have a long shelf life.

Prepared foods are typically UPFs and require microwave heating, which can increase leaching of bisphenols from polycarbonate packaging or increase acrylamide formation.

The outcome of such exposures is unknown; However, previous research suggests a link with cancer, cardiovascular disease, insulin resistance, type 2 diabetes, obesity and endocrine disorders.

Another concern is the structural change that occurs in the various components of a food product due to processing, which can affect its bioavailability through altered digestibility, satiety, eating and chewing speed, all of which can contribute to increased energy intake with UPFs. This area is severely under-researched.

There are over 300 food additives approved in Europe, some of which may be linked to chronic inflammation through their effects on the gut microbiome. Landmark studies have shown that dietary patterns influence gut microbiota profiles, which can subsequently alter host metabolism and promote obesity.

Gut bacteria are often able to absorb and process simple sugars when exposed to high dietary fats. Gut stressors include certain food colors, emulsifiers, artificial sweeteners and nanoparticles such as E171 (titanium dioxide).

These additives alter the ratio of important bacterial genera in the intestine, affecting the protective mucosal layer and the expression of important defense molecules such as β-defensins, allowing microbes to reach the sterile zone of the mucosa. This triggers endotoxin accumulation, which subsequently leads to meta-inflammation and IBD.

At the same time, changes in the molecular profile promote increased energy extraction from ingested food, causing metabolic dysregulation and obesity.

This links calorie extraction from ingested food with metabolic aberrations caused by diet. This dysregulated metabolism is characterized by mild inflammation and changes in the intestinal microbiome. The result is a leak of bacterial products such as lipopolysaccharides (LPS) from the intestine into the host system.

LPS from gram-negative bacterial cell walls contain lipid A, a molecule that passes through the intestinal mucosa. Once lipid A reaches the blood, it can trigger inflammation of various target tissues such as the liver and fat depots of the body.

This cycle is called “metaflammation.” Metainflammation can be defined as a metabolic inflammatory state defined by chronic low-grade inflammation generated by metabolic cells and stress sensors.

This suggests the need for interventions in UPF uptake to induce a beneficial change in the gut microbiome that promotes the production of “good” bacterial metabolites through appropriate gene expression by intestinal mucosal cells. These include anti-inflammatory short-chain fatty acids (SCFAs).

Conclusions

The study results highlight the need to improve current understanding of how food processing affects human health. Large-scale studies are needed to identify components causally associated with metainflammation and obesity.

Public education is also crucial to support the consumption of healthier foods, preferably those that are minimally processed and without additives. Such dietary choices are emphasized by the recommendations of the Food and Agriculture Organization of the United Nations (FAO) as well as several national dietary guidelines.

Federal policy must shift to promoting the production and distribution of healthy, high-quality food products through measures such as financial incentives, laws, and mobile phone applications that provide evidence for food choices.

“Governments and the food industry should combine their efforts to develop policies to promote a healthier food environment for consumers to effectively combat the increasing incidence of chronic inflammatory diseases.”

Reference:

• Srour, B., Kordahi, MC, Bonazzi, E., et al. (2022). Ultra-verarbeitete Lebensmittel und menschliche Gesundheit: Von epidemiologischen Beweisen zu mechanistischen Erkenntnissen. The Lancet Gastroenterologie & Hepatologie. doi:10.1016/S2468-1253(22)00169-8.

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