## 1 Introduction  
Diabetes mellitus is defined as a group of disorders of carbohydrate metabolism, in which hyperglycaemia occurs because of alterations in gluconeogenesis and glycogenolysis, leading to both overproduction of glucose and its underutilisation as an energy source [1]. They are chronic metabolic diseases, which can be classified according to their pathophysiology and genetic and metabolomic characteristics into four different types: type 1 diabetes mellitus, type 2 diabetes mellitus, gestational diabetes, and specific types of diabetes due to other causes (e.g., monogenic syndromes such as type LADA or type MODY diabetes mellitus). Of these, type 2 diabetes mellitus (T2 DM) accounts for 90-95% of all diagnosed cases [1,2]. In T2 DM, there is progressive, non-autoimmune loss of insulin secretion, usually in the context of peripheral insulin resistance and metabolic syndrome (1). Furthermore, it is associated with a set of risk factors that lead to the development of macro- and microvascular complications, such as obesity, dyslipidemia, high blood pressure (HBP), and lifestyles such as excessive alcohol consumption, insufficient physical activity, and unhealthy diets (high energy and high fat) [3,4]. Therefore, cardiovascular diseases, including coronary artery disease, peripheral ischaemia, neurovascular disease, and heart failure, are the main causes of morbidity and mortality in individuals with T2 DM [2,3]. This is of great importance, given the continuing increase in their incidence and prevalence worldwide, especially in non-Caucasian and minority ethnic populations [5]. Once the diagnosis of T2 DM has been confirmed, it is necessary to assess each patient's comorbidity conditions and general health status, as well as the possible development of microvascular (retinopathy, nephropathy, and neuropathy) and macrovascular (coronary artery disease, cerebrovascular disease, peripheral arterial ischaemia, and heart failure) complications. Therefore, a continuous care plan should be established together with optimal treatment to prevent or delay the onset of these complications and improve patients' quality of life [6,7]. On the other hand, the microbiota is defined as the set of microorganisms (bacteria, viruses, fungi, and protozoa) that live both inside and on the surface of the organism. The highest concentration of these microorganisms is found in the gastrointestinal tract (mainly in the colon), although they are also found in the skin, mucous membranes, and urinary tract. The microbiota or microbiome is a complex system that can be considered an additional organ that interacts with the rest of the organism through the exchange of substances and metabolites produced by both parts, establishing a symbiotic relationship. This relationship is host-specific and varies throughout life, mainly in response to dietary, environmental, lifestyle, and ageing factors [8]. As mentioned above, the nutritional approach is the first step in the treatment of T2 DM, with diet being a relevant modulator of the composition and function of the microbiota [9]. At the intestinal level, four main bacterial phyla are observed: _Bacteroidetes_, _Firmicutes_, _Actinobacteria_, and _Proteobacteria_[10]. According to targeted metagenomic studies, approximately 90% of the bacteria present in the intestine of adults belong to the _Bacteroidetes_ (Gram-negative) and _Firmicutes_ (Gram-positive) phyla [4], with a relatively constant ratio between the two families, which when altered generates what is known as 'dysbiosis' or 'dysbacteriosis' [10]. Several studies link a decrease in the _Firmicutes_/_Bacteroidetes_ ratio to the development of obesity and metabolic disorders, such as T2 DM [4,11]. Under physiological conditions, the microbiome plays multiple roles.