Drs Amer and Qayyam recently published a study in the American Journal of Cardiology that pulled data on serum D and CRP from the Center for Disease Control (CDC) historical national survey. Their findings have raised the question in several large media outlets, does increased D increase cardiovascular disease risk?
Amer M, Qayyum R. Relation Between Serum 25-Hydroxyvitamin D and C-Reactive Protein in Asymptomatic Adults (From the Continuous National Health and Nutrition Examination Survey 2001 to 2006). Am J Cardiol. 2011 Oct 12.
The short answer is no. You can’t draw any conclusions about CVD risk because this was a correlation of two serum levels between two unmatched groups. It was not an intervention study, and it did not measure CVD. More details? Read on:
They divided their data into two groups at the median D level of 21 ng/ml:
They stated that in the single variable linear correlation (using a log of the CRP):
The authors found that vitamin D has a bigger effect in D deficient folks compared with those who had sufficient D levels.
However, when they ran the data as a multiple variable regression (and adjusted for demographic variables, obesity, hypertension, serum glucose, cholesterol, smoking, white blood cell count, and renal function), they concluded:
Remember, the big chunks in looking at a study are population, interventions (if applicable) and findings.
Since this was a retrospective study on serum D and CRP from historical files, there was no intervention, no subject randomization, and no objective health outcome (these were healthy adults and they did not look at actual CVD disease or death rates).
The Low D and High D groups were different by race, hypertension, gender, obesity, CRP, serum glucose, kidney function, and smoking (note: most of these affect CRP levels).
It should be no surprise to VDC readers that the overall D levels were very low, basically deficient and inadequate. Black, obese, and smoker D levels were worse than average. What this means is that the study didn’t have enough higher D scores to do a complete analysis (an inadequate ceiling). Hmm, what happens above 50 ng/ml (125 nmol/L)?
The American Heart Association’s (AHA) CRP average risk range is 1.0 to 3.0 mg/L. The Low D group’s mean CRP was 2.2 mg/L. The High D group’s mean CRP was 1.7 mg/L. So, both groups were in the middle of the average risk range.
It’s important to understand what CRP is. CRP is a liver protein that indicates general inflammation with a wide range of causes. Kruas et al found that,
“Serum hsCRP for predicting risk of CVD is confounded by obesity, ethnicity, gender and comorbidities.”
Kraus VB, Stabler TV, Luta G, Renner JB, Dragomir AD, Jordan JM. Interpretation of serum C-reactive protein (CRP) levels for cardiovascular disease risk is complicated by race, pulmonary disease, body mass index, gender, and osteoarthritis. Osteoarthritis Cartilage. 2007 Aug;15(8):966-71. Epub 2007 Mar 28. PubMed PMID: 17395501; PubMed Central PMCID: PMC2682321.
A CRP over 3 mg/L has been postulated as a CVD risk marker (it is associated with CVD), but it is not a risk factor (it does not cause CVD). This is the big distinction. The CDC and American Heart Association (AHA) Working Group states:
“hs-CRP for high- or low-risk adults… are not recommended (Class III). The lack of support for these tests may change as more evidence is acquired.”
What does all this mean to us? Is the sky falling? Is anyone who ventures into the sun doomed? Should we stop all vacations to Florida and Hawaii and deem them dangerous to our health?
No. This is at best a number drill. It is not an intervention study with controls. There is a difference between statistical significance and clinical utility. The multivariable High D group CRP mean change goes up by 0.06 mg/dL every 10 ng/ml increase in D (given 2 unmatched groups with an inadequate D range), but the single variable analysis shows CRP decreasing.
We don’t know why the adjusted multiple variable CRP went up because they did not control for population variables, other nutrients, or diseases. In theory, increased D might decrease magnesium levels that would increase the CRP. Or Rheumatoid Arthritis, etc…
The authors were correctly conservative in their conclusion:
“It is possible that the role of vitamin D supplementation to reduce inflammation is beneficial only among those with a lower serum 25(OH)D.”
In my opinion, they should have been more conservative in terms of their study’s limitations and the limits of CRP scores. I’d like to have seen the actual data plot.
But the press headlines were not that conservative:
I can’t say it often enough: correlation is not causation. I went to Hawaii a few years ago. My D went up, my CRP went up, my sleep hours went down, my carbohydrate consumption increased, and my blood alcohol levels went up. Did D cause my CRP to rise?