In December 2019 researchers from South Korea published their review on the effect of diet on the gut microbiota and how it is associated with obesity. Obesity is described as an accumulation of excessive fat mass and is itself associated with a low-grade chronic inflammation. Obesity has commonly been associated with the development of other metabolic disorders such as diabetes type 2, cardiovascular disease, and some types of cancer. Being overweight/obese has nearly tripled in the population since 1975. In 2014 over 1.9 billion adults were considered overweight which represented 39% of the world’s population and in 2016 over 340 million children and adolescents aged 5–19 years were also considered overweight or obese. By 2030 it has been estimated that over 50% of the world’s population will be considered obese. Gut microbiota has been studied extensively and it has been established that gut microbiota is an essential factor in the development and progression of obesity. In fact, scientific evidence has shown that dietary factors such as probiotics, prebiotics, fat, fatty acids, and fibre can all dramatically alter the gut microbiota profile as it is related to obesity.
The gastrointestinal tract contains at least 1014 types of bacteria, with the most abundant numbers in the large intestine. The number of genes in the gut microbiome is 150-500 fold greater than in human DNA. Various factors such as autoimmune disease, chronic disease, medications, antibiotics, smoking, stress level, and diet can affect the diversity and composition of the gut microbiota. Each individual has a unique gut microbiota profile which may affect nutrient metabolism. Research has shown that the composition and diversity of the gut microbiota is different between healthy weight and obese individuals.
The gut microbiota is made up of different microorganisms including bacteria, archaea, fungi, and viruses. The dominant bacterial species in human gut microbiota are Bacteroidetes, Firmicutes, Actinobacteria, Proteobacteria, Fusobacteria, Cyanobacteria, and Verrucomicrobia. Many studies have looked at the relationship between gut microbiota and obesity and found that the number of Firmicutes was increased while the number of Bacteroidetes decreased in obese individuals. Bacteroidetes have fewer genes involved in carbohydrate and lipid metabolism than Firmicutes. The increased Firmicutes/Bacteroidetes ratio plays a role in increasing energy storage in fatty tissue. In addition, studies have shown that there is decreased bacterial diversity in obese subjects.
Specific foods and diets can influence the number of different bacteria in the gut microbiota.
Prebiotics include inulin, fructo-oligosaccharides, galacto-oligosaccharides, and human milk oligosaccharides. The prebiotics usually found in fruits and vegetables may lead to various health benefits. Among the advantages of prebiotics is that they are able to promote trace element absorption, such as that of calcium, iron, and magnesium, and immune system regulation. Prebiotics can stimulate the growth of Bifidobacteria and Lactobacillus species, which can produce greater microbial diversity in the gut microbiome. Prebiotics have anti-obesity effects involved in improvement of lipid metabolism by a change in gut microbiota composition.
Recent studies have described how Bifidobacterium and Lactobacillus strains, alone or in combination, have led to reduced body weight, waist circumference, waist circumference/height ratio, BMI, and fat depots. High and low doses of probiotic mixtures (different strains of Lactobacillus and Bifidobacterium) have shown similar beneficial effects on weight, BMI, and body fat mass in obese postmenopausal women, but only in the high-dose group was there an improvement in lipid profile. However, the effectiveness of probiotic treatments is still unclear due to small numbers and short-term follow-up. Thus, further studies are required to identify which probiotic strains may produce effective changes in weight or body fat loss, either alone or in combination with other probiotic strains.
Diet is one of the critical factors in the progression of obesity and is linked to the composition, diversity, and metabolism of the gut microbiota. It has been seen that the composition and functionality of gut microbiota responds quickly to changes in dietary composition. Several studies have shown that 2 days after the start of a new diet, the gut microbiota responds and changes start to to be made to its composition. Therefore, a balanced diet is required to maintain the composition and proper function of the gut microbiota. Many dietary patterns such as Western diet, vegetarian diet, gluten-free diet, and the Mediterranean diet have been shown to affect the distinct diversity of the gut microbiota that may affect metabolism.
The Western diet consists of a high intake of saturated fats, refined grains, sugar, salt, and high fructose corn syrup and a low intake of fibre. It is significantly associated with obesity and metabolic disease. The Western diet promotes inflammation and changes the profile of the gut microbiota from a healthy to an obese pattern. It has also been shown to decrease the total bacteria amount as well as the beneficial Lactobacillus and Bifidobacterium species in the gut.
Vegetarian and vegan diets consist of plant-based foods which are rich in dietary fibre. Abundant fiber in these diets promotes a stable gut microbiota profile and increases the presence of lactic acid bacteria. Both the vegetarian and vegan diet have been shown to lower Bacteroides and Bifidobacterium species. It also decreased lipopolysaccharide producers (Escherichia coli and Enterobacter cloacae) that can induce inflammation.
The effects of a gluten-free diet on the gut microbiota is well known since a gluten-related disorder is closely associated with gut microbiota profile and metabolism. A gluten-free diet has been shown to reduce the abundance of Bifidobacterium, Lactobacillus, and Ruminococcus bromii and Roseburia faecis, whereas the Victivallaceae and Clostridiaceae are increased. Poorly digested fermentable carbohydrates (fermentable oligo-, di-, mono-saccharides, and polyols [FODMAPs]) are known to increase digestive problems and cause irritable bowel syndrome. Therefore, a low FODMAP diet can be used to reduce intestinal symptoms.
The Mediterranean diet consists of vegetables, olive oil, and fruits, a moderate intake of poultry, and a low intake of red meat and dairy products. It is well known as one of the healthiest dietary patterns. Gut microbiota composition in the Mediterranean diet is high in Lactobacillus, Bifidobacterium, and Prevotella which is closely associated with the prevention of obesity and improvement of lipid and cholesterol profiles. Furthermore, the Mediterranean diet appears to be able to lower Clostridium species which can induce inflammation. The Mediterranean diet provides high dietary fibre and polyunsaturated fatty acid content.
Kim B et al. Effect of Diet on the Gut Microbiota Associated with Obesity. J Obes Metab Synd 2019;28(4):216-224