In this study, we investigated a Taiwanese geriatric population to evaluate the association between VAI and new onset T2DM and cardiovascular events. We found that older adults with higher VAI had a higher incidence of new onset T2DM.
Adipose tissue plays an important role in longevity.[10] Obesity, one of the most important global health care problems, is associated with many pathologies, such as metabolic dysfunction, insulin resistance, T2DM, and CVD.[11] It is also associated with high mortality,[12] and strategies directed at the management of adipose tissue may affect the life span.[10]
Increased risk of metabolic disease is closely linked to fat distribution, and different fat compartments may cause different metabolic risks.[13] Fat distribution varies with many factors, including ageing.[14] Circulating levels of fatty acids increase when subcutaneous fat cannot store lipids, which results in ectopic deposition and toxicity to most cells. They result in aged pre-adipocytes and potentially exacerbate fat dysfunction and contribute to metabolic dysfunction. WC alone cannot help in distinguishing between subcutaneous and VAT.[15] Although both subcutaneous abdominal adipose tissue and VAT are associated with metabolic risk factors,[3] VAT is more prevalent in obesity and T2DM because of chronic, low-grade inflammation.[16]
Epidemiologic studies have revealed that VAT is a common but preventable risk factor for metabolic disease. Huffman et al. reported that the mean and maximum lifespans in rats improved following surgical removal of VAT. This was the first evidence to suggest that VAT depletion may be a strong factor underlying improved life span.[14] In contrast, Klein et al.[17] surgically removed large volumes of subcutaneous abdominal fat in 15 participants and found insufficient improvements in their metabolic parameters. Therefore, subcutaneous fat may not be a key component of obesity-related metabolic disorders in humans.
The mechanism linking VAT and metabolic syndrome involves VAT’s anatomical location. VAT leads to a “portal” effect of glycerol release and greater blood levels of free fatty acids.[18] Adipose tissue is an active endocrine organ and secretes adipokines that can interfere with insulin action and promote inflammation.[19] VAT also drives cardiac aging through modulation of fibroblast senescence via osteopontin.[20] Cells of the immune system play varying roles in adipose tissue infiltrates. For example, macrophages were recognized in obesity-induced inflammation of the adipose tissue.[21] Ageing adipose tissue undergoes significant changes in the cellular composition, abundance, distribution, and endocrine signaling. Therefore, ageing plays a central role in metabolic dysfunction, insulin resistance, impaired regenerative capacity, and inflammation.[22] Accumulation of VAT is a hallmark of ageing in humans.[23] Ageing is associated with fat tissue redistribution from subcutaneous to visceral depots, which may cause not only insulin resistance but also metabolic complications as well as functional decline and mortality in older adults.
Amato et al.[24] demonstrated that VAI was closely correlated with body visceral adiposity estimated using MRI. They found that VAI was independently associated with cardiovascular events, which was not observed with BMI, WC, and other classical cardiovascular risk factors. VAI indirectly reflects other non-classical risk factors, such as elevated lipolysis, altered production of adipocytokines, and plasma free fatty acids. Therefore, VAI could be a valuable index of both fat distribution and function.[24]
Other studies have investigated the relationship between VAI and several health-related outcomes. A cross-sectional epidemiological survey in China revealed that higher VAI was positively associated with hypertension in both sexes.[25] Another cross-sectional analysis of 2,754 Chinese community-dwelling people, aged 20–50 years, revealed that higher VAI was positively associated with pre-T2DM and T2DM.[26] Bozorgmanesh et al. followed up 5,964 non-diabetic adult participants in Iran for a median of 6 years and found that VAI could be a prognostic tool for incident diabetic events.[27] Nusrianto et al. reviewed seven studies to determine whether VAI could be used as a predictor of T2DM in Asian populations.[7] The review included six studies from China and one from Iran. Four studies were prospective cohort studies and three were cross-sectional studies. They found that VAI was a predictor of T2DM in Asian populations with better prediction values compared with that in Caucasian populations.[7] The largest cross-sectional study included 7,639 participants.[28] The longest study lasted 15 years but only included 687 participants aged 35–65 years.[29] Therefore, most studies have included participants aged 18–65 years.
In our study, we included 12,875 patients aged 60–80 years over a 10-year period. Notably, this is the first study to demonstrate the relationship between VAI and new onset T2DM and cardiovascular events in geriatric patients. After adjusting for covariates, a higher VAI was associated with a higher risk of new onset T2DM in both males and females with up to 77% higher risk in males and 56% in females. Higher VAI was also associated with a higher risk of cardiovascular events; however, there was no significant difference between the sexes after adjusting for covariates. Only few participants developed CVD in this study.
There are several plausible explanations for this positive association between VAI and new onset T2DM in older adults. Fundamental ageing mechanisms occur in adipose tissue, including progenitor cell dysfunction and cellular senescence. A previous review[10] reported that ageing and obesity were related to elevated systemic inflammatory cytokines. Ageing and obesity are associated with adipose tissue dysfunction and reduced generation of anti-inflammatory factors such as adiponectin and interleukin-10. Recent evidence has demonstrated that ageing and obesity could aggravate metabolic dysfunction and that ageing is related to fat tissue redistribution and insulin resistance. Therapeutic interventions targeted at both obesity and ageing should be considered in the future. The risk of VAT accumulation increases with age and affects the health. Treatments aimed at depleting VAT should be considered to reduce the risk of diseases[14].
There were several limitations in this study. First, self-reported medical conditions may be affected by recall bias or misclassification or have led to over-reporting. Second, data regarding potential confounding factors, such as adiponectin and interleukin-10, were not available. Third, this was a mixed design study and may not demonstrate the cause-and-effect relationship. Finally, despite adjustments for several confounding factors, other unmeasured confounders cannot be ruled out.