Table 2 Quality Assessment of included studies using the Newcastle-Ottawa Scale

NOS QA for Cohort Studies (Max score: 4)

1. Representativeness of the exposed cohorts (score: 0–1)

2. Selection of the non-exposed cohorts (score: 0–1)

3. Ascertainment of exposure (score: 0–1)

4. Demonstrating outcome of interest not being present at the start of the study (score: 0–1)

NOS QA for Cohort Studies (Max score: 2)

• Comparability of cohorts on the basis of the design or analysis controlled for cofounders (score: 0–2)

NOS QA for Cohort Studies (Max score: 3)

1. Assessment of the outcome (score: 0–1)

2. Follow-up being long enough for outcomes to occur (score: 0–1)

3. Adequacy of follow-up of cohorts (score: 0–1)

NOS QA for Case-Control Studies (Max score: 4)

1. Case definition adequacy (score: 0–1)

2. Representativeness of the case (score: 0–1)

3. Selection of controls (score: 0–2)

4. Definition of controls

NOS QA for Case-Control Studies (Max score: 2)

• Comparability of cases and controls on the basis of the design or analysis (score: 0–2)

NOS QA for Case-Control Studies (Max score: 3)

1. Assessment of exposure (score: 0–1)

2. Same method ascertainment for cases and controls (score: 0–1)

3. non-Response rate (score: 0–1)

A Deeb 2017 (Prospective observational study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. B; Structured review (1 score)

4. B; No

B; Study controls for insulin regimen (1 score)

1. C; Self report

2. A; Yes (1 score)

3. A; Complete follow (1 score)

5:

• S: 2

• C: 1

• O: 2

Fair

B Afandi 2017 (Prospective observational study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. A; Secure records (1 score)

4. B; No

B; Study controls for HbA1c levels (1 score)

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. D; No statement

5:

• S: 2

• C: 1

• O: 2

Fair

A Deeb 2016 (Prospective observational study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3.B; Structured review (1 score)

4. B; No

B; Study controls for reduction of insulin dosage (1 score)

1. C; Self report

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

5:

• S: 2

• C: 1

• O: 2

Fair

AE Al-Agha 2017 (Prospective single arm pilot study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. A; Secure records (1 score)

4. B; No

B; Study controls for insulin regimen (1 score)

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

6:

• S: 2

• C: 1

• O: 3

Fair

NS Elbarbary 2014 (Prospective observational study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. A; Secure records (1 score)

4. A; Yes (1 score)

C; Cohorts are not controlled for confounders

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

6:

• S: 3

• C: 0

• O: 3

Poor

F Alawadi 2020 (Prospective single-armed interventional study without a control group; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. A; Secure records (1 score)

4. A; Yes (1 score)

B; Study controls for fasting status -before and after Ramadan- (1 score)

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

7:

• S: 3

• C: 1

• O: 3

Good

T Muammar 2022 (Retrospective cohort study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3.B; Structured review (1 score)

4. B; No

B; Study controls for insulin regimen (1 score)

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

6:

• S: 2

• C: 1

• O: 3

Fair

W Kaplan 2015 (Prospective observational study; assessed by NO QA for cohort studies)

1. B; Somewhat representative (1 score)

2. NA

3. A; Secure records (1 score)

4. B; No

C; Cohorts are not controlled for confounders

1. B; Record linkage (1 score)

2. A; Yes (1 score)

3. B; Unlikely to introduce bias (1 score)

5:

• S: 2

• C: 0

• O: 3

Poor

Sh Abdelghaffar 2022 (Case-control study; assessed by NO QA for case-control studies)

1. A/B; Yes (1 score)

2. A; Consecutive representative (1 score)

3. B; Hospital controls

4. A; No history of diseases/endpoint (1 score)

A; Study controls for fasting status (1 score)

1. D; Written self-report /medical history

2. A; Same method for cases and controls (1 score)

3. A; Same rate for both groups (1 score)

6:

• S: 3

• C: 1

• E: 2

Good