The MicroFit Study – The effect of very-low energy diets for weight loss on gut microbiota composition and associated health in women with obesity: a randomised controlled trial

Obesity affects 36% of adult men and 38% adult women globally, with individuals with obesity experiencing disproportionate levels of mental illness compared to those with normal-weight. The overarching aim of the MicroFit Study is to promote dietary interventions for weight loss in obesity that address desired weight loss, while avoiding possible adverse effects on gut microbiota and overall health.

Diet and the huge number of bacteria living within the gut, are associated with obesity. Food that undergoes many processes and has many added ingredients (e.g., salt, sugar, fat, additives, preservatives and/or artificial colours) is considered ultra-processed. Common examples of ultra-processed food include lollies, soft drinks, pizza and chips, all of which contain high amounts of calories and don’t contain enough of the nutrients needed for general wellbeing. When comparing these types of ultra-processed foods with minimally processed foods, ultra-processed food has been shown to have a negative impact on our health and may be linked with a change in the number, type and function of gut microbiota. However, ultra-processed food can also be made into products that are low in calories and high in essential nutrients, such as protein, fats, carbohydrates, vitamins and minerals.

Little is known about the impact of these relatively ‘healthier’ ultra-processed foods on gut microbiota, and in turn, how gut microbiota may influence health outcomes when these foods are eaten. Could people with obesity lose weight with an ultra-processed diet that is low in calories and full of nutrients without harming their gut bacteria? The MicroFit Study aims to answer this question by comparing the impact of an ultra-processed versus minimally-processed diet for weight loss on gut microbiota and health outcomes in women with obesity.

Preliminary research suggests dietary intake based on ultra-processed food may be linked with a change in gut microbiota composition and function compared to unprocessed or minimally processed food. Recent large-scale studies also demonstrate direct links between consumption of ultra-processed food and chronic disease states that commonly occur with obesity, such as some cancers, cardiovascular disease, heart disease, depressive symptoms as well as overall mortality.

While these associations have primarily been made between ultra-processed food that is typically high in energy density and low in nutritional quality, ultra-processed food can also be engineered to form ‘health’ and ‘slimming’ products for weight loss in obesity. Examples include ultra-processed very-low calorie diet consumables, such as meal replacement and ready-to-eat shakes, bars, soups and desserts. While these products are considered safe in terms of their effect on human physiology, their impact on gut microbiota is yet to be established.