## 1 Introduction  
Diabetes mellitus (DM) represents a major health concern since its prevalence is continuously increasing worldwide. In 1980, the incidence of DM was 108 million, while in 2019, it was raised to approximately 463 million cases (adults older than 20 years). It is estimated that by 2045 the number of people suffering from the disease will reach 700 million, according to the World Health Organization [1] and the International Diabetes Federation [2]. Type 1 diabetes (T1D), or insulin-dependent DM, is a disease attacking mainly young people (children and adolescents) and constitutes about 5-10% of all cases of diabetes [1,3]. It is evoked by the autoimmune destruction of pancreatic \(\beta\)-cells, resulting in integral insulin deficiency and disturbed glucose metabolism [4]. Besides genetic predisposition, including particular haplotypes of the human leucocyte antigen [5], other factors, such as infections, caesarean section instead of normal childbirth, dietary habits and the overuse of antibiotics have been associated with T1D [6]. However, the exact mechanisms through which these parameters contribute to the onset and the progression of the disease are not well documented [7]. Diet is a modifiable factor of pivotal importance that can significantly contribute to accomplishing better management of the disease [8]. Pistachios (_Pistacia vera_ L.) are a mainstay of the Mediterranean diet due to their excellent sensory characteristics, as well as their high nutritional quality, attributed to their content in dietary fiber, vitamins, minerals and phytochemicals, known for their health-promoting effects [9]. Pistachios contain high amounts (73%) of monounsaturated fatty acids (MUFA), especially oleic acid (18:1\(\omega\)9) (70%), known for its cardioprotective, hypocholesterolem, hypoglycemic and anti-inflammatory properties [10; 11], as well as polyunsaturated fatty acids (PUFA) (16%) [12]. The presence of unsaturated fatty acids, which is accompanied by low saturated fatty acids (SFA) content (11%), renders pistachio an optimal choice for maintaining a healthy lipid profile [13; 14]. Several studies imply that SFA promotes insulin resistance and diabetes development, whilst MUFA and especially \(\omega\)3 PUFA can retreat these harmful effects [15; 16]. Dietary patterns influence the composition of gut microbiota, which is directly linked with host homeostasis [17]. Mediterranean diet-induced changes in intestinal microbiota give rise to greater production of short-chain fatty acids (SCFAs), particularly of butyrate [18]. The concentration of SCFAs in the gastrointestinal (GI) tract and peripheral circulation may predispose to or prevent pathological conditions, such as diabetes and other metabolic diseases. Furthermore, these acids exert pivotal functions on the anti-inflammatory responses in the intestinal region and participate in multiple signalling pathways of lipid and glucose metabolism [19]. A previous study [20] has demonstrated beneficial alterations in the gut microbiome of T1D rats after pistachio consumption, while favorable effects have also been supported by other researchers in obese mice [21], revealing the presence of underlying mechanisms involved in anti-inflammatory actions [22]. Adipose tissue is a complex endocrine organ that regulates metabolic homeostasis. Adipocytes synthesize and express several mediators, among them inflammatory cytokines, which have been suggested to be associated with the onset of T1D and the progression of its implications [23]. Recently, VAT inflammation and dysfunction were proposed to be linked with the progression of the disease of T1D [24]. From this point of view, VAT is considered a target organ and the identification of mechanisms linking metabolic pathways with \(\beta\)-cell autoimmunity is crucial for the development of new therapeutic approaches for the management of the disease.