Background: Prevention of coronary heart disease has included nationwide strategies to decrease blood lipid concentrations of New Zealanders. As the mean serum total cholesterol concentration of New Zealanders has decreased over time, these public health interventions appear to be somewhat successful. Time trends in young adults are useful to ascertain because at this age very few individuals take lipid-lowering medications, making the trends particularly informative on the effect of lifestyle changes. Time trends in triglyceride concentration have not been measured in New Zealanders.
Objective: The goal of this research project was to describe time trends in plasma total cholesterol and triglyceride concentrations in samples of young adult New Zealanders from 1991 to 2013, and to compare time trends in total cholesterol concentration to national trends in those of a similar age and sex.
Participants and Methods: Fasting plasma cholesterol and triglyceride concentrations of 2nd and 3rd year Human Nutrition students were compiled from class experiments conducted between 1991 and 2013.
Results: There was no difference in the proportion of males and females in each year (chi-square P = 0.161) or age between years (P = 0.264). However, there was a trend for body mass index (BMI) to decrease over time: BMI decreased by 1.1 kg/m2 (P = 0.005; 95% CI: -1.8, -0.3) per 10 year increment. BMI was significantly associated with both total cholesterol and triglyceride concentration. The unadjusted regression analysis showed plasma total cholesterol concentration decreased by 0.18 mmol∙L-1 every 10 years (P <0.001; 95% CI: -0.24, -0.11) and plasma triglyceride concentration decreased by 8.3% every 10 years (P <0.001; 95% CI: -11.2, -5.3). Adding sex, age and BMI to the regression model increased the reduction in total cholesterol and triglyceride concentration to 0.27 mmol∙L-1 (P = 0.029; 95%CI: -0.50, -0.03) and 12.0% (P < 0.012; 95% CI: -20.3%, -2.7%) per 10 year increment, respectively. However, this was based on a much lower number of observations (n = 506) and only included data from 3rd year classes.
Discussion and Conclusions: A downward trend in plasma total cholesterol and triglyceride concentration was observed in Human Nutrition students between 1991 and 2013. The decrease in total cholesterol concentration over time was considerable; however, it was smaller than the reduction seen in New Zealand females aged 19-24 years during a similar time period. Based on results from the Life in New Zealand survey in 1989 and the Adult Nutrition Survey in 2008/09, total cholesterol decreased by 0.84 mmol∙L-1 over the 20 year period. Regardless of the difference in magnitude, the consistency in the direction of these time trends indicates the decrease in Human Nutrition students is likely to be a true trend. Given the time trend in total cholesterol amongst Human Nutrition students paralleled national trends, it would be tempting to speculate about national time trends in triglyceride concentration. However, BMI is strongly associated with triglyceride concentration and trends in BMI amongst Human Nutrition students were opposite to those seen in young New Zealand females.
