Out with the bad: Lipid levels declining in U.S.

Serum cholesterol levels in U.S. adults continue to improve, according to a study that tracked trends over a 22-year period. The analysis adds to the growing evidence that these cardiac risk factors are on the decline, the lead author said in an interview with Cardiovascular Business.  

In an analysis of three cross-sectional National Health and Nutrition Examination Surveys (NHANES), researchers found that levels of total cholesterol, low-density lipoprotein cholesterol (LDL-C) and non-high-density lipoproteins (non-HDL-C) have been declining across almost all adult population segments over the past 22 years, and continue to decline. Meanwhile, serum levels of high-density lipoproteins (HDL-C) have held steady across all segments of the U.S. adult population. High levels of total cholesterol and LDL-C and low levels of HDL-C are major risk factors for coronary heart disease. 

"This favorable trend had been identified in prior studies, but these results—using the gold standard for health-related data—confirm that the trend is real," lead author Margaret D. Carroll, MSPH, said in an interview. The study was published in the Oct. 17 issue of the Journal of the American Medical Association.

Carroll, of the National Center for Health Statistics at the Centers for Disease Control and Prevention (CDC) in Hyattsville, Md., and colleagues analyzed data collected through NHANES over 22 years, from 1998 until 2010. In addition to determining the mean levels of serum lipids and lipoproteins throughout the survey population, the authors also evaluated trends in participants receiving lipid-lowering medication, participants not receiving lipid-lowering medication and obese participants.

The CDC surveys the non-institutionalized adult population continuously and since 1999 data have been released in two-year cycles. The surveys included in the study were conducted from 1988 to 1994, 1999 to 2002, and 2007 to 2010. Each survey included an interview and an examination. Participants were randomly divided into morning groups who fasted for at least eight hours prior to examination, and afternoon groups who did not fast. Triglyceride measurements were obtained only for the morning group, while both groups were tested for total cholesterol and HDL-C.

The study cohort included 37,810 participants in the three NHANES surveys analyzed for the study. Participants self-identified their race and their lipid-lowering medication history.  Body mass index was used to categorize the participants as underweight, healthy weight, overweight or obese.

The age-adjusted mean total cholesterol level for adults dropped from 206 mg/dL in the 1988-1994 to 196 mg/dL in 2007-2010. Levels of decline were similar for men and women. “Statistically significant declining trends ... were observed in all sex and race/ethnicity classifications except for Mexican American men," the authors wrote.

The results also showed that the mean LDL-C declined from 129 mg/dL in 1988-1994 to 116 mg/dL in 2007-2010 and the mean non–HDL-C declined from 155 mg/dL in 1988-1994 to 144 mg/dL in 2007-2010. Mean serum triglyceride levels increased between 1988-1994 and 1999-2002 but then dipped in 2007-2010.

The authors observed a linear upward trend in mean levels of HDL-C for men and women overall, but when broken down by race this trend was not observed in non-Hispanic blacks or Mexican Americans.  They also noted an increase in the age-adjusted percentage of adults receiving lipid-lowering medications, from 3.4 percent in the 1988-1994 survey to 14.7 percent in the 2007-2010 survey. This trend was maintained across all subgroups.

Favorable trends in serum levels were also observed among the population not receiving lipid-lowering medications. "This indicates that other factors may account for the decrease," Carroll said.

Among obese participants, the authors found statistically significant declines in age-adjusted mean total cholesterol, non-HDL-C and LDL-C over the course of the study. The mean HDL-C did not increase significantly for all obese adults or for obese men, but did trend upward in obese women.      

Carroll suggested that changes in smoking habits, dietary shifts away from trans fats and increasing use of lipid-lowering medications may be driving the decrease, but emphasized that further research must be done to identify the causes of the favorable trends. "They are unlikely to be the result of changes in physical activity, obesity or saturated fat," the authors wrote, noting no decline in saturated fat intake as a percentage of calories or obesity over the corresponding period, and no increase in leisure time physical activity of adults.