Population features and study designs of the selected studies regarding CGM in GSDs are summarized in Table 2.

Most studies included both children and adults [15, 24, 25, 28,29,30]; and three studies specifically investigated adults [16, 26, 27].

The main types of GSDs represented were Glycogen Storage Disease type Ia (GSDIa) (95/130 = 45,4%), Glycogen Storage Disease type Ib (GSDIb) (6/130 = 6.2%), Glycogen Storage Disease type III (GSDIII) (10/130 = 10%), Glycogen Storage Disease type IX (GSDIX) (9/130 = 6.2%), and Glycogen Storage Disease type 0 (GSD0) (2/130 = 1.5%). Excluding the case report [31], seven studies focused exclusively on subjects with GSDI [15, 16, 23, 25,26,27,28].

The duration of glucose monitoring by Rt-CGM varied from 48 h to 7–10 days (Table 2).

All studies reported the use of medical foods as recommended by guidelines for GSD (such as UCCS or modified corn-starch (MCS) or glucose polymer) (Table 3), and most reported that research participants were already following an appropriate diet for GSD. All studies reported the use of UCCS, and eight studies provided the precise amount of UCCS (either in grams/day or grams per kg of body weight/day) [16, 24,25,26,27, 29, 30]. MCS use was mentioned in only five papers [15, 16, 26,27,28] with the precise amount provided (in grams/day or grams per kg of body weight/day) only in four papers [16, 26,27,28]. Peeks et al. [15] reported only a load test without the dose prescription. Glucose polymer use was used by White et al. [30], while Fukuda et al. [25] used a low-fat, lactose/fructose-free formula.

Most studies described dietary changes during Rt-CGM [15, 16, 23, 24, 26,27,28, 30] (Table 3). Two studies [26, 29] analyzed relevant nutritional parameters and dietetic therapy during Rt-CGM without making any changes in diet. Maran et al. [29] concluded that Rt-CGM could provide a rationale for changes in dietary treatment because CGM revealed unrecognized hypoglycemia, while Fukuda et al. [25]. assessed nutritional parameters and observed different dietary intake between ages in GSD homozygous group. Three studies assessed the effects on metabolic control of specific dietary interventions, in particular the use of UCCS vs. MCS [15, 27, 28] or the use of three different meals with an equal content of carbohydrates (CHO) consumed before bedtime [27]. In particular, Peeks et al. evaluated modification of the amount of nocturnal UCCS and the change from UCCS to MCS during nocturnal CGM [15].

Rossi et al. focused on CGM metrics in patients with GSDIa and provided only an observational description of different dietary treatments, such as UCCS, MCS, or continuous nocturnal gastric drip-feeding [16].

No studies have directly compared dietary interventions during Rt-CGM with blood/capillary glucose monitoring (Table 3). In subjects treated with MCS or other carbohydrate sources vs. UCCS at bedtime [15, 27, 33], Hsu et al. reported improvements in sleep time and fasting duration in individuals treated with MCS [28], and Hochuli et al. mentioned achieving stable nocturnal glucose control with the “pasta al dente” meal, and prolonged fasting times in some enrollees treated with MCS [27]; Peeks et al. reported less glycemic variations with MCS [15].

Rt-CGM combined with dietary modifications led to improvements in glucose values that were associated with better pre-set target in range (70–150 mg/dL) [24], pre-set target below range [15], glycemic variation/glucose variability [15, 26], morning glucose levels [28], and stability of nocturnal glucose [27].

The only case-control study documented lower pre-set target in range and higher pre-set time above range in GSDs vs. healthy subjects [16]. In this study, dietary modifications during Rt-CGM were found to be useful in preventing severe low glucose levels associated with mental or physical functioning impairment (level 3 hypoglycemia) [16].

Some authors reported improvement of additional biochemical parameters such as aspartate aminotransferase (AST), alanine aminotransferase (ALT) [23, 28], lactate [23, 30], triglycerides [23] and ketones [30] (Table 3).

Peeks et al. reported improved gastrointestinal absorption (better macronutrients and UCCS uptake) in GSDIb with empagliflozin [15]. In an observational study, Fukuda et al. underscored the prevalence of asymptomatic hypoglycemia in individuals with hepatic GSDs, with total cholesterol and lactate independently linked to hypoglycemia [25].

Two studies reported Rt-CGM as a safe, efficacious, and reliable system to identify asymptomatic hypoglycemia (Table 3) [23, 29].

Three studies compared Rt-CGM and blood/capillary glucose values: Herbert et al. and Rossi et al. reported good correlation between capillary glucose values and CGM [16, 24], even when glucose levels were < 70 mg/dL [24]; Hochuli et al. showed Rt-CGM was a reliable tool in outpatients, according to capillary blood measurements [27]. Herbert et al. stated that Rt-CGM is a useful tool both for patients to improve compliance with dietary treatment as well as for healthcare providers to facilitate home monitoring [24].

No studies evaluated individuals’ or parents’ perceptions of Rt-CGM in GSDs.

Two case reports investigated FGM in two adults with GSDIa [31] and GSD III [32]. One study used both Rt-CGM and FGM, including 14 GSDIa and 2 GSDIb subjects, with a mean age of 21 years [33]. The duration of glucose monitoring with FGM varied from 6 days to 4 months (Table 2).

All FGM studies included the consumption of UCCS (Table 4). Marcalo et al. and Massimino et al. [31, 32] reported precise amounts in individual case reports (grams per day or grams per kilogram of body weight per day), while Kaiser et al. [33] did not describe UCCS doses. Massimino et al. also reported the use of glucose polymer [32]. Kaiser et al. showed 44% of GSD individuals were fed using a gastric tube as an alternative to UCCS or MCS, not specifying if the feeding source was glucose polymer [33].

The two case reports described relevant dietetic modifications. By reducing UCCS dose, Massimino et al. gradually decreased the amount of CHO and increased fats [32]. Marcalo et al. revised the prescribed diet with stricter indications [31].

Massimino et al. showed a reduced number and duration of hypoglycemic episodes, and increased time in pre-set target range (70–140 mg/dL) [32] (Table 4).

Kaiser et al. found (micro)albuminuria in 59% of study participants, with more frequent events of low blood glucose and a trend for a higher AUC of low glucose in CGM measurements in patients with microalbuminuria (Table 4) [33]. The mean-total-serum-calcium was lower in GSDIb compared to GSDIa (statistically significant), while mean PTH concentration was slightly lower. Z-scores of bone density correlated negatively with serum lactate and positively with body mass index (BMI) [33].

Massimino et al. showed also a greater exercise capacity during dietary modifications and FGM [32]. The selected studies did not evaluate the concordance between the FGM and capillary or blood glucose monitoring nor individuals’ or parents’ perceptions.

Ten studies evaluated Rt-CGM in CH, involving a total of 159 subjects. The main characteristics of the selected studies are shown in Table 5.

Most studies included only children, with one study focusing solely on neonates [35], and only one investigation included both pediatric and adult subjects [43]. While most studies focused on individuals with CH, one study included CH treated with pancreatectomy [43], and another considered some uncharacterized hyperinsulinemic disorders [42]. The duration of glucose monitoring varied among the selected studies, ranging from 2 to 12 weeks. Two studies analyzed different periods of blinded or unblinded monitoring [37, 38], and two studies used only blinded Rt-CGM [38, 43].

Four studies reported the use of special medical foods in CH individuals during Rt-CGM. Skae et al. utilized dietary supplements with polyunsaturated fatty acids (PUFA) [41], Sivasubramanianan et al. described oral feeds or gastric tube