The ratio of total cholesterol to HDL is reported to be linked with greater cardiovascular risk than other lipid measures, including “bad” LDL-cholesterol, explained researchers from Beth Israel Deaconess Medical Center, Johns Hopkins, Baylor College of Medicine, and the University of Minnesota.
Data from 13,039 participants in the Atherosclerosis Risk in Communities (ARIC) study indicated that the associations were still robust even after the researchers accounted for a range of potential factors, including age, gender, race, BMI, physical activity, smoking, and more.
“This study is one of the first studies to show the relationship between 25(OH)D levels and lipid measures over time,” wrote the researchers in Nutrition.
Correlation v. Causation
However, the study shows correlation and not causation, and it is not known if vitamin D supplements would improve HDL levels.
“Our results […] complement recent findings from several small studies that suggest vitamin D supplementation may improve HDL-C levels in certain populations,” wrote the researchers. “However, vitamin D supplementation has not yet been definitively shown to decrease the risk of cardiovascular events or improve the lipid profile, but prior studies of vitamin D supplementation have been limited due to factors such as inadequate power, reliance on post hoc analyses, and low dose supplementation.
“Results from further studies including large randomized clinical trials such as the ongoing VITAL study are needed to definitively determine the benefit of 25(OH)D supplementation on cardiovascular outcomes.”
The sunshine vitamin
Vitamin D refers to two biologically inactive precursors - D3, also known as cholecalciferol, and D2, also known as ergocalciferol. Both D3 and D2 precursors are transformed in the liver and kidneys into 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form, and 1,25-dihydroxyvitamin D (1,25(OH)2D).
While our bodies do manufacture vitamin D on exposure to sunshine, the levels in some northern countries are so weak during the winter months that our body makes no vitamin D at all, meaning that dietary supplements and fortified foods are seen by many as the best way to boost intakes of vitamin D.
The researchers assessed data from over 13,000 people with an average age of 57 and a mean vitamin D level (measured as 25- hydroxyvitamin D (25(OH)D), the non-active 'storage' form) of 24 ng/ml. Deficiency was defined as 25(OH)D levels of less than 20 ng/ml, while optimal levels were defined as 25(OH)D levels of at least 30 ng/ml.
Results showed that, after adjusting for a range of potential confounding variables, vitamin D deficiency was association with average HDL-cholesterol levels 3.02 mg/dl lower than in people with optimal D levels. In addition, the average total cholesterol (TC) to HDL ratio was 0.18 higher in deficient participants.
On the other hand, no overall associations were observed for triglyceride or LDL-cholesterol levels, although an association between vitamin D deficiency and higher LDL levels were observed when participants on lipid-lowering therapies were excluded from the analysis.
It is unclear why vitamin D levels were associated with some lipids but not others, but they noted that vitamin D-dependent metabolic pathways may preferentially impact HDL-cholesterol, with a “possible role for vitamin D in reverse cholesterol transport with increased formation of these larger, more cardioprotective HDL-C particles”.
Could supplements help?
“Given that we consistently observed both cross-sectional and longitudinal associations between deficient 25(OH)D and certain measures of dyslipidemia, our findings suggest that the cardiovascular risk associated with low 25(OH)D may indeed be mediated through interactions with lipid pathways,” they wrote.
“Therefore, a final major implication of our study is the possibility of modifying cardiovascular risk through Vitamin D supplementation. Though our findings are observational only and did not examine any effects of treatment, Vitamin D supplementation remains an intriguing potential intervention for individuals with 25(OH)D deficiency and dyslipidemia at increased cardiovascular risk given its widespread availability and low cost.”
Published online ahead of print, doi: 10.1016/j.nut.2017.01.008
“Serum Vitamin D and Change in Lipid Levels over 5 years: the Atherosclerosis Risk in Communities (ARIC) study”
Authors: K.F. Farid et al.