3.1 Study selection and characteristics

The search identified 23 studies for inclusion (Fig. 1), comprising two human studies and 21 animal studies. The two human studies included one longitudinal cohort [48] and one cross-sectional study [49]. The key characteristics are outlined in Table 2. In total, 29 participants underwent Roux-en-Y gastric bypass (RYGB) [48, 49] and 10 participants underwent laparoscopic adjustable gastric banding (LAGB) [48]. Participants had a mean age range of 42 to 52 years, 67% were female and none had diabetes. Time since surgery ranged from four months to 12 years. Taste receptor expression was analysed by quantitative polymerase chain reaction (qPCR) in oral (fungiform papillae) in one study [48], and jejunal/proximal alimentary limb mucosa in the other [49].

Fig. 1

PRISMA flow diagram illustrating the process of the literature search

Table 2 Characteristics of included human studies

Animal studies were conducted in rats (13 studies), mice (seven studies) or both (one study). The bariatric procedure was RYGB in nine studies [50,51,52,53,54,55,56,57,58], duodenal-jejunal bypass (DJB) in four studies [59,60,61,62], sleeve gastrectomy (SG) in seven studies [50, 63,64,65,66,67], and entero-gastric anastomosis (EGA) procedures [35], single-anastomosis duodenal-jejunal bypass (SA-DJB) [68], and ileal interposition (IIP) [69] in one study each. Five studies included analysis of animals with diabetes or insulin resistance [54, 61, 62, 67, 69]. Time between surgery and tissue harvest ranged between 11 days and six months. Taste receptor quantification was carried out using PCR for mRNA analysis in six studies [35, 50, 54, 57, 58, 61], protein analysis techniques (such as Western blotting, immunohistochemistry, or mass spectrometry) in another five [52, 53, 56, 68, 69], or both in 10 studies [51, 55, 59, 60, 62,63,64,65,66,67]. Key characteristics of animal studies are outlined in Table 3.

Table 3 Characteristics of included animal studies3.2 Risk of bias

Quality and risk-of-bias assessments are presented in Fig. 2. The overall quality of the human studies was rated as good. The internal validity of the 21 animal studies was limited in each case by lack of reporting on key bias reduction measures, therefore, most items in the risk-of-bias tool were assessed as ‘high risk’ due to omission. Although 12 studies mentioned randomization of group allocation, no study specified the method of randomisation. Similarly, three studies reported using a form of ‘blinding’ to reduce bias but provided minimal detail on the process. Most studies provided sufficient detail regarding pre-intervention characteristics of animals, and all studies adequately addressed the risk of reporting bias (Fig. 2c). The risk of conflicting interest was low for all articles.

Fig. 2

Summary of quality assessment results for: cross-sectional and cohort studies (a), pre-post observational studies without a control group (b), and animal models (c)

3.3 Changes to the gene or protein expression of taste receptors induced by bariatric surgery3.3.1 Sweet taste receptors

Lingual taste perception of sugars, artificial sweeteners and other sweet compounds occurs primarily through the activation of the T1R2/T1R3 heterodimer [70], although an alternative pathway utilising SGLT1 that is specific for glucose detection also exists [71]. All three receptors have been co-localised to enteroendocrine cells of the gastrointestinal tract [21, 72], where SGLT1 transduces information regarding the presence of glucose and its analogues [21, 24, 73] and T1R3 (with or without T1R2) responds only to artificial sweeteners [21].

Two human [48, 49] and 15 animal studies [50,51,52,53, 55, 57,58,59,60,61,62,63,64,65, 69] examined the effect of bariatric surgery on the sweet taste receptors SGLT1 (14 studies), T1R2 (four studies), and T1R3 (five studies).

SGLT1

One human study reported an increase in basal alimentary limb SGLT1 mRNA expression after RYGB, which was unchanged following a 30-min luminal glucose infusion [49].

Of the animal studies, five reported an increase in SGLT1 mRNA and protein expression in the alimentary limb after RYGB [50,51,52], and DJB [59] and in the interposed ileal segment after IIP [69]. No change to mRNA levels were observed in the alimentary limb of one study [58] or the biliopancreatic limb [50, 58], or colon [57] after RYGB. Two studies reported no change to SGLT1 mRNA in jejunal mucosa after SG [50, 64]. However, one of these studies did find a reduction in jejunal SGLT1 protein expression [64]. Five studies observed a decrease in the mRNA or protein of SGLT1 after bariatric surgery (n = 1 alimentary limb following RYGB, n = 3 alimentary limb following DJB, n = 1 jejunum and ileum following SG) [53, 60,61,62, 65]. Additionally, one study reported a decrease in the abundance of stomach, duodenal, jejunal and ileal SGLT1 mRNA and protein at 2 and 4 weeks after SG, followed by an increased expression above baseline at 8 weeks [63].

T1R2

Two human studies found no change in the mRNA expression of oral [48] or basal proximal alimentary limb [49] T1R2 after RYGB. However, following luminal glucose infusion, T1R2 mRNA expression decreased in the alimentary limb and remained unchanged in non-operated control groups [49]. There was no change to oral T1R2 mRNA expression following LAGB [48].

Two animal studies reported a decrease in the mRNA and protein expression of T1R2 in the alimentary limb [55, 62] and biliopancreatic limbs [55] after RYGB and DJB. No difference in the mRNA or protein levels of T1R2 in the common channel after RYGB was observed [55].

T1R3

No studies examined intestinal T1R3 mRNA or protein expression in humans after bariatric surgery, but one reported no change to oral T1R3 mRNA expression after RYGB and LAGB [