Metabolic Syndrome and Metabolic Syndrome Components in Young Adults

Abstract

Metabolic Syndrome represents a series of metabolic disorders most likely underpinned by insulin resistance, which increases the risk of developing clinical disease such as type II diabetes and cardiovascular disease. Metabolic syndrome is prevalent in 5-7% of young adults although estimates vary based on regional, ethnicity, and gender differences. The primary lifestyle behaviours that lead to development of metabolic syndrome, obesity and physical inactivity, increase substantially in the transition from adolescence to adulthood (18-25 years). Young adulthood is a key time period in developing behaviours that will either increase or decrease the future risk of developing clinical disease. Less than optimal risk factors as a young adulthood are associated with an increase in the lifetime risk of developing clinical disease. Therefore identifying young adults with metabolic syndrome components may represent an important strategic approach of identifying young adults with a high lifetime risk of clinical disease. However, investigations examining metabolic syndrome in young adults and the usefulness of metabolic syndrome to identify young adults with a high lifetime risk of disease are rare. A pooled analysis of 11 international studies (total n= 26,609) was conducted to examine the current prevalence of metabolic syndrome and metabolic syndrome components in apparently healthy young adults. Studies were included if participants were aged between 18-30 years old, and not sampled on the basis of any particular anthropometric characteristics. The key findings of this analysis were that 4.8-7.0% of young adults had metabolic syndrome, with low HDL-C being the most prevalent component (26.9-41.2%). Apparently healthy young adults (n=268, 45.9% male) attending the University of Auckland (n=124) or Western State Colorado University (n=144) were investigated for metabolic syndrome prevalence. Metabolic syndrome was present in 14.2% of all participants with significant regional and ethnic differences in the prevalence of metabolic syndrome (NZ 4.8%, USA 22.2%; Caucasian 2.4 – 9.5%, Asian 12.0%, Hispanic 53.3%, Black American 35.0%, Maori/Pacific Island 7.1%). There were also significant differences in the prevalence of metabolic syndrome components between countries and ethnicities with low HDL-C being the most prevalent metabolic syndrome component overall (HDL-C 30.2%, BP 22.0%, TG 17.9%, WC 14.5%, FBG 9.23%). Approximately 40% of young adults had at least one component of metabolic syndrome despite being “apparently healthy”. Using questionnaire data and submaximal estimates of cardiorespiratory fitness, predictive modelling suggested that young adults with increased body mass index, a low physical activity level, increased daily sitting time, and low cardiorespiratory fitness, had increased odds of having metabolic syndrome. An isotemporal substitution analysis indicated that replacing 30 minutes of sitting time with either walking or moderate physical activity for 30 minutes would reduce the odds of having metabolic syndrome. The results from the pooled analysis and prevalence study suggest that low HDL-C is the most prevalent component of metabolic syndrome, which requires further investigation to examine the significance of low HDL-C as a young adult. One of the potential early signs of cardiovascular disease may be microvascular endothelial-mediated dysfunction. Therefore, microvascular endothelial function was examined using laser Doppler flowmetry and iontophoresis in young adults with the low HDL-C component (male < 1.04mmol·L-1, female < 1.30mmol·L-1, n=15) of metabolic syndrome and normal levels of HDL-C (control group n=28) to determine if any impairment in endothelial-mediated vasodilation occurs in the microvasculature. The results indicated that there was no reduction in endothelial–mediated vasodilation when the low HDL-C component was present, suggesting no difference in endothelial function in the microvasculature despite having one of the components of metabolic syndrome. Therefore, further work is required to determine the role of HDL-C in metabolic syndrome and subsequent clinical disease. In summary, this thesis provides novel information that indicates that many apparently healthy young adults already have metabolic syndrome, or components of metabolic syndrome, with low HDL-C being the most prevalent component (approximately 30% of young adults). The impact of having low HDL-C as a young adult is unclear; however it does not appear to be associated with an impaired microvascular endothelium dilatory capacity. Further research should focus on determining the role HDL-C plays in the development of metabolic syndrome.