Low serum levels of vitamin D were linked to increased blood pressure, hyperglycemia, and obesity in an analysis of more than 3,500 American teenagers, a link previously seen in adults.
In addition, the prevalence of vitamin D deficiency has increased among all Americans, based on data collected in a national survey during 1988–2006 and presented in a separate report.
“Vitamin D plays a useful role in general human health. We are just now beginning to understand the role that vitamin D may play in cardiovascular health,” said Dr. Jared P. Reis, who presented a poster on the associations between vitamin D levels and cardiovascular risk factors at a conference on cardiovascular disease epidemiology and prevention sponsored by the American Heart Association.
His study analyzed data collected from 3,528 adolescents aged 12–19 during 2001–2004 in the National Health and Nutrition Examination Survey (NHANES). Their average age was 15, and 52% were boys.
The average serum vitamin D level for all participants was about 25 ng/mL, but a quarter had a level less 15 ng/mL, while another quarter had a level greater than 26 ng/mL.
The teens in the quartile with the lowest vitamin D level had an 11% prevalence of hypertension, 2.3-fold more common than those with the highest vitamin D levels. A fasting serum glucose level of 100 mg/dL or greater (impaired fasting glucose) was seen in 6% of those with the lowest vitamin D levels, 2.5-fold more often than those with the highest levels. Adolescents in the low-vitamin quartile were greater than fivefold more likely to have a BMI in the 95th percentile or higher, compared with teens in the highest vitamin D quartile. All of the differences between the lowest and highest vitamin D quartiles were statistically significant, reported Dr. Reis, a researcher in the School of Public Health at Johns Hopkins University, Baltimore.
Those in the lowest vitamin D quartile also had a significantly higher rate of low levels of HDL cholesterol, high triglycerides, and metabolic syndrome, compared with the teens in the highest vitamin D quartile.
Vitamin D reduces activation of the renin-angiotensin system, thereby lowering blood pressure. It also reduces insulin release from pancreatic beta cells and raises insulin sensitivity. Low vitamin D levels produce opposite effects, activating renin-angiotensin to raise blood pressure, boosting insulin release, and lowering insulin sensitivity, he said.
Low vitamin D levels in blood are usually caused by lack of adequate sun exposure and poor diet. Another factor is obesity. Vitamin D is fat soluble, which means it tends to leave the blood and accumulate in fat, Dr. Reis said in an interview.
Low dietary levels of vitamin D were linked to elevated percent body fat and visceral adipose tissue among teens in a separate talk given at the meeting. The study included 659 healthy teenagers, aged 14–19 years, who resided in or near Augusta, Ga. Their average age was 16, half were girls, and nearly half were African American. Vitamin D intake was calculated based on several 24-hour diet recall surveys for each participant.
The results showed an inverse relationship between the amount of vitamin D in the diet and both percent body fat and amount of visceral adipose tissue, reported Inger Stallmann-Jorgensen, a research dietitian at the Medical College of Georgia, Augusta.
“Other researchers have reported that dairy intake appears to have a stronger, negative relation with body fat and visceral fat than calcium intake alone. Thus, other constituents in dairy foods, such as vitamin D” are potentially involved in the relationship, she said in an interview. It's also possible that low dietary vitamin D is a marker for poor diet in general.
“Intervention studies giving vitamin D supplements to teens are needed to determine causality,” Ms. Stallmann-Jorgensen said.
The time trend for vitamin D deficiency among American adults was tracked using serum specimens collected in several cycles of the NHANES. The analysis assessed specimens collected from more than 18,000 people during 1988–1994, and from about 8,000 people during each of three subsequent collection periods, 2001–2002, 2003–2004, and 2005–2006. There was an even balance of women and men; about 70% of those surveyed were white, about 12% were African American, and about 7% were Mexican American. About 45% were normal weight, about 30% were overweight, and about 25% were obese.
The prevalence of vitamin D deficiency depended on the serum-level cutoff applied. With a conservative cutoff of less than 11 ng/dL, the prevalence of deficiency rose from 3% in 1988–1994 to 7%, 6%, and 9% in the subsequent three sampling periods, reported Dr. Sandy Saintonge, a public health and pediatrics researcher at Cornell University, New York. With a higher cutoff of 20 ng/mL, the prevalence was about 20% in 1988–1994, and about 28%–40% during the three more recent sampling periods.