Over the past few decades, obesity has become a global public health problem [1], placing a heavy health and economic burden on people. As of 2017, there were approximately 712 million obese people in the world, and the incidence is still increasing at a rate of 0.3% to 0.8% annually. The number of obese people in China has surpassed that of the United States, with as many as 168 million affected [2]. In addition to increasing the risk of chronic diseases such as diabetes and cardiovascular disease [3], obesity also has many adverse effects on mental health [4,5]. An epidemiological survey showed that obesity and the incidence of abnormal mood regulation, such as anxiety and depression, promoted each other [6]. Dickerson et al. found that approximately half of people with mental illnesses such as anxiety are obese [7]. Moreover, recent studies have confirmed that a high-fat diet (HFD)- and gene-induced obesity in mice could cause abnormal emotion regulation and trigger anxiety-like behavior [8]. A meta-analysis of the incidence rate of anxiety disorders in obese and overweight people included 25 studies in which the results showed that the frequency of anxiety in obese and overweight people had a pooled odds ratios of 1.30 and 1.10 and the 95% confidence intervals of 1.20-1.41 and 1.00-1.21, respectively. Thus, anxiety occurs more frequently resulting from obesity or overweight [9]. However, it has not been reported whether weight loss through diet or exercise can reduce anxiety caused by obesity. Therefore, it is of great significance to explore an effective and synergistic method to treat anxiety disorder by regulating obesity.

In recent years, intermittent fasting (IF) has attracted increasing attention as an effective way to achieve weight loss, as well as other health benefits [10]. As one of the typical methods of IF, alternate-day fasting (ADF) refers to the 24-hour alternation of usual diet and restricted diet, which can reduce weight more than other forms of IF [11,12]. Currently, ADF has been shown to regulate energy metabolism, reduce the risk of age-related diseases, and extend lifespan [13], [14], [15]. Moreover, multiple previous studies have shown that fasting has neuroprotective and restorative effects in animal models of brain injury and neurodegenerative diseases [16], [17], [18]. However, the role of ADF in obesity-induced anxiety-like behavior is still poorly reported.

Sirtuin 1 (Sirt1) is a nicotinamide-(NAD)-(+)-dependent deacetylase involved in a wide range of biological activities such as chromosome remodeling and gene regulation [19], [20], [21], [22]. In addition, since Sirt1 can promote energy consumption and fat mobilization, it also plays a pivotal role in the physiological and pathological processes of obesity [23]. The AMP-activated protein kinase (AMPK) activation and the Sirt1 expression, induced by lactobacilli by regulating the composition of the intestinal flora, could reduce mouse obesity and HFD-induced anxiety [24]. Herskovits et al. described the profound link between Sirt1 and the neuroprotective effects in several aggregate-forming neurodegenerative diseases [25]. In addition, studies have also demonstrated that the activity of neuronal Sirt1 has a significant impact on emotion regulation and synaptic plasticity. Dietary energy restriction could prevent the decline in Sirt1 in the cerebral cortex of posttraumatic brain injury mouse models, thus effectively improving the cognitive impairment caused by brain injury [16]. From a review of the abovementioned research, it is reasonable to speculate that Sirt1 may be involved in regulating obesity-induced anxiety.

To our knowledge, the decreased release of 5-HT in the central nervous system is considered to be one of the pathophysiological foundations of various psychiatric diseases, including anxiety and depression [26]. 5-HT is an inhibitory neurotransmitter and a messenger of pleasure. MAO-A is the key enzyme regulating the level of brain 5-HT [27]. In vivo, 5-HT is catalyzed by MAO to 5-hydroxytryptamine aldehyde and 5-hydroxyindoleacetic acid, which are excreted with urine to degrade brain 5-HT. Sirt1 activates MAO-A gene transcription by deacetylation of the brain-specific transcription factor native helix-loop-helix 2 (NHLH2) on lysine 49, thereby affecting 5-HT metabolism and anxiety-like behavior in the mouse brain [28]. Taken together, the changes in Sirt1/MAO-A/5-HT are important in the occurrence of anxiety. Other studies in rodent models have shown that the brain contents of 5-HT increase after fasting [29,30].

However, few studies have explored the role of ADF on obesity-induced anxiety-like behavior. The present study aimed to uncover the effects of ADF on obesity-induced anxiety-like behavior and further explore the changes in Sirt1, MAO-A, and 5-HT levels in the hippocampus, which might provide new ideas for the treatment of mood disorders such as obesity-induced anxiety.