Table 1.

Summary of included studies

Study	Country	Recruitment	Clinic/community	Diabetes/metabolic syndrome	Duration of FUP	Outcomes	Quality score (/10)
Artero [49]	France	Random sample recruited from French electoral roles	Community	Diabetes	4 years	All cause dementia	10
Ciudin [50]	Italy	Patients attending a memory clinic, Fundacio ACE, aged > 60 with type 2 diabetes	Clinic	Diabetes	2 years	All cause dementia	4
Exalto et al. [51]	Netherlands	Recruited from a memory clinic based Amsterdam Dementia Cohort of VU University Medical Centre	Community	Metabolic syndromea	0.6–4.6 years	All cause dementia	8
Li et al. [52]	China	Subjects were sampled from ten randomly selected communities in the city of Chongqing	Clinic	Diabetes	5 years	Conversion to Alzheimer’s dementia	8
Ma et al. [53]	China	Recruitment from six geographically convenient communities with high proportions of elderly residents within Tianjin city, China	Community	Diabetes	4 years	All cause dementia	7
Morris et al. [54]	USA	Data were obtained from ADNI on 5 January 2012. ADNI is conducted by the National Institute on Aging, the National Institute of Biomedical Imaging and Bioengineering, pharmaceutical companies, and nonprofits	Clinic	Diabetes	2 years	Conversion to Alzheimer’s dementia	8
Ng et al. [55]	Singapore	Participants were of Chinese ethnicity and recruited from five districts in the South East region of Singapore from September 1, 2003 to December 31, 2009	Community	Metabolic syndromeb and diabetes	4 years	All cause dementia	8
Prasad et al. [56]	Singapore	Retrospective analyses of a prospective clinical database comprising patients with cognitive impairment managed at the memory clinic of a tertiary neurology center between January 2008 and January 2011	Clinic	Diabetes	Minimum 18 months	Conversion to Alzheimer’s dementia	4
Ravaglia et al. [57]	Italy	Participants were recruited among the outpatients seeking medical advice for cognitive complaints at the Center for Physiopathology of Aging, University of Bologna	Clinic	Diabetes	From 6 months to 5 years	All cause dementia	4
Solfrizzi et al. [58]	Italy	A sample of 5632 subjects aged 65–84 years, independent or institutionalized, was randomly selected from the electoral rolls of eight Italian municipalities, after stratification for age and gender	Community	Metabolic syndromea	3 years	All cause dementia	7
Velayudhan et al. [59]	UK	Potential candidates were identified from general practice registers and invited to participate. Participants were assessed annually from 2001 to 2007	Clinic	Diabetes	4 years	All cause dementia	5
Xu et al. [48]	Sweden	Participants recruited from all registered inhabitants who were age 75 years or older and living in the Kungsholmen district of central Stockholm, Sweden, in 1987	Community	Diabetes	9 years	All cause dementia	6

^aMetS diagnosis: ATPIII criteria—3 or more of the following components abdominal obesity (waist circumference > 102 cm for men and > 88 cm for women); elevated plasma triglycerides (≥ 150 mg/dL); low HDL cholesterol (< 40 mg/dL for men and < 50 mg/dL for women); high blood pressure(≥ 130/≥85 mmHg) or being in hypertensive treatment; high fasting plasma glucose (≥ 110 mg/dL)

^bMetS diagnosis: International Diabetes Federation criteria—central obesity (waist circumference ≥ 90 cm for men and ≥ 80 cm for women) plus at least 2 CVRFs, including raised triglyceride levels (≥ 150 mg/dL) or specific treatment for this lipid abnormality; reduced high-density lipoprotein cholesterol level (< 40 mg/dL in men and < 50 mg/dL in women) or specific treatment for this lipid abnormality; raised blood pressure (systolic ≥ 130 mm Hg or diastolic ≥ 85 mm Hg or treatment of previously diagnosed hypertension) and raised fasting plasma glucose level (≥ 100 mg/dL or previously diagnosed type 2 diabetes mellitus)