The SG90 cohort comprised of subsets of the population-based prospective cohorts Singapore Chinese Health Study (SCHS) participants and Singapore Longitudinal Aging Study (SLAS) participants who were aged 85 years and above. The SCHS is an on-going prospective cohort study designed to evaluate the genetic, dietary, and environmental determinants of chronic diseases in Chinese adults living in Singapore. Detailed descriptions of the study have been reported previously [21]. In brief, 63,257 Chinese participants (27,959 male and 35,298 female) aged 45–74 (mean 53) years old were enrolled between April 1993 and December 1998. After recruitment, the participants were re-contacted for follow-up interviews in 1999–2004 (phone or in-person), 2006–2010 (phone or in-person), and 2014–2017 (in-person). In this SCHS-SG90 sub-cohort, due to limited recourses, the first 1,000 consenting participants who were aged 85 years and above were recruited to participate in SG90 between July 2017 and March 2021 [22]. SLAS is a population-based prospective and longitudinal cohort study on ageing and health of adults aged over 55 years residing in Singapore [23]. Participants were systematically identified through a comprehensive door-to-door census of community-dwelling older adults and then voluntarily invited to participate in the study. Individuals who were unable to participate due to severe physical or mental disabilities were excluded. The recruitment for SLAS-1 occurred between 2003 and 2009 in the South-East region of Singapore (n = 2,800), followed by SLAS-2 recruitment from the South and West regions of Singapore in 2010–2014 (n = 3,270) and SLAS-3 (aged over 80 years) from survivors and new island-wide recruitment in 2015–2021 (n = 611). Besides whole population sampling from these regions in Singapore, SLAS also used a nationally representative sampling list of households with oldest-old individuals provided by the Singapore Sampling Design Service, Department of Statistics. The SLAS-3 participants who were aged over 85 years were invited for further interviews and assessments according to the SG90 study protocol, which formed SLAS3-SG90 sub-cohort (n = 158). In total, 1,158 participants were included in SG90 cohort. The flow chart of SG90 recruitment is shown in Fig. 1.

Flowchart of SG90 sub-cohort from Singapore Chinese Health Study (SCHS) and Singapore Longitudinal Aging Study (SLAS) cohorts

The SG90 data was collected through extensive face-to-face structured interviews, clinical assessments, neuropsychological tests, and biospecimen sampling (Table 1). All procedures were conducted by trained interviewers, research nurses and research assistants at the on-site study senior activity centres or participants’ home.

Participants were visited for a face-to-face interview in their preferred language (e.g. English, Mandarin, Chinese dialects such as Hokkien or Cantonese). A standardized structured questionnaire was used to collect self-reported information on sociodemographics, family information, lifestyle (substances use, diet, physical and leisure-time activities), sleep quality, functional status, quality of life, medical conditions and healthcare economics.

Blood pressure was measured in supine, sitting and standing positions using an automatic digital blood pressure monitor HEM-705CP (Omron Corporation, Japan). Height (cm) was measured to the nearest 1.0 cm using a rigid, self-retracting metal tape measure, while weight (kg) was recorded to the nearest 0.1 kg with a Soehnle Exacta Comfort digital weighing scale (Model S63315 PSD). Circumference measurements (cm) were obtained using a soft, flexible plastic tape. Waist circumference was measured 2.5 cm (1 inch) above the participant’s navel, while hip, leg, and thigh circumferences were measured at the widest part of the respective region. Mid-upper-arm circumference was measured with the forearm bent at 90 degrees, positioning the tape between the head of the humerus and the olecranon. Supine total arm length (cm) was recorded to the nearest 0.1 cm from the acromion to the olecranon and from the olecranon to the styloid process of the ulna using a soft tape. Knee-to-floor height (cm) was measured with the subject seated, placing a tape measure from the fibular head down to the floor. Handgrip strength was assessed using a Jamar Plus + digital hand dynamometer (Pennsylvania, United States) with both hands tested alternately three times, while the subject was seated with the arm close to the side, elbow at 90 degrees, and wrist in 0-30-degree dorsiflexion and 0-15-degree ulnar deviation [24]. The Timed Up and Go test (TUG) was measured in seconds and conducted on a flat surface over a marked 3-meter distance, with subjects starting seated and instructed to stand, walk to the finish line, return, and sit [25]. The six-meter walking test was performed to measure gait speed (m/s) at fast pace and recorded in seconds [26]. Short Physical Performance Battery (SPPB) consists of a repeated chair stands test, a balance test and the 6-meter walking test at fast pace with a total score from 0 to 12 [27, 28]. Cognitive function was evaluated by Mini-Mental State Examination (MMSE) [29] which has been modified for the use in Singapore [30], Subjective Memory Cognitive Complaints (SMCC) [31], and Informant Rated Memory Decline (IRMD) [32]. The optimal cut-off scores of MMSE for no, primary and secondary and above education levels are 25, 27 and 29, which was validated in a large cohort of Singapore Chinese older adults [30]. The presence of depressive symptoms was assessed using the 15-item Geriatric Depression Scale (GDS-15), a tool validated for use among Singaporean Chinese, Malay, and Indian populations [33].

Trained research nurses performed phlebotomy to collect venous blood for both clinical laboratory tests and biological markers of ageing. A total of 48 millilitres of fasting blood was collected into different types of tubes for different research purposes. Blood collected in K2 ethylenediaminetetraacetic acid (EDTA) tubes (BD Vacutainer, NJ, USA), Serum-Separating Tubes (SST) (BD Vacutainer, NJ, USA), Sodium Fluoride tubes (BD Vacutainer, NJ, USA) was sent to the National University Hospital Referral Laboratory (NRL) for clinical laboratory tests including complete blood counts, lipid profiles, albumin, fasting glucose, creatinine, high-sensitivity C-reactive protein (hs-CRP), vitamin B12, folate, plasma total homocysteine, electrolytes (sodium, potassium, calcium), liver enzymes [alanine transaminase (ALT), aspartate transaminase (AST)], and thyroid function markers [free thyroxine, thyroid-stimulating hormone (TSH)]. The rest of blood collected in SST tubes, Tempus™ Blood RNA Tubes (Applied Biosystems, CA, USA) and CPT™ Mononuclear Cell Preparation - Sodium Citrate Tubes (BD Vacutainer, NJ, USA) was sent to Singapore Immunology Network (SIgN) to process into serum, plasma, peripheral blood mononuclear cells (PBMCs), sorted immune cells, and red blood cells and biobank according to established protocols. Additionally, saliva, stool, urine, toenails, hair, and skin tape strip were collected according to standard operation procedures to support a comprehensive view of the biological ageing process (Fig. 1). The other institutes Genome Institute of Singapore (GIS) and the Singapore Institute of Clinical Sciences (SICS) were also involved in ageing biomarkers analyses.

The rich collection of biomaterial supports extensive multi-omic and cellular analyses, which provide deep insights into the biological determinants of ageing. Specifically, DNA methylation analyses assess epigenetic markers associated with biological age, while immunophenotyping enables detailed profiling of immune cell populations and illuminates ageing patterns in immune systems. Multi-omics analyses including metabolomics, proteomics, lipidomics, transcriptomics, and genomics were performed to identify systemic biomarkers across molecular pathways relevant to ageing and resilience. Plasma toll-like receptors (TLR) stimulation and serum analytes assessments are essential to enhance the understanding of immune functions, metabolic health, inflammatory status and nutritional deficiencies. In addition, trace element and heavy metal analysis in toenails and hair samples reveals cumulative exposure to environmental toxins which potentially contribute to chronic diseases and accelerated ageing (Table 1).

Mortality data are obtained via record linkage through the National Registry of Disease Office (NRDO) at the Ministry of Health. Morbidity will be captured through Singapore’s National Electronic Health Record (NEHR) system, which provides comprehensive data on incident diagnoses and hospitalizations. To maintain cohort size and statistical power amid high mortality rates in the oldest-old, the SG90 study will implement regular follow-up assessments and consider replenishment recruitment of newly eligible participants.