Season of birth, neonatal vitamin D status, and cardiovascular disease risk at 35 y of age: a cohort study from Sweden

Tornhammar P, Ueda P, Hult M, Simila H, Eyles D, Norman M. Season of birth, neonatal vitamin D status, and cardiovascular disease risk at 35 y of age: a cohort study from Sweden. Am J Clin Nutr [Internet]. 2014 [cited 2015 Oct 1];99(3):472-80. doi: 10.3945/ajcn.113.072520

A REVIEW OF THE LITERATURE• Vitamin D status may be associated with a higher risk of cardiovascular disease, impaired glucose tolerance and metabolic syndrome• Vitamin D status during the gestational period may influence future cardiovascular and metabolic health in their offspring• Indian study• United Kingdom study• Why is this study important?

BACKGROUND OF STUDY• Longitudinal cohort study- level 2 evidence• Objective: “to assess the link between season of birth, neonatal 25-hydroxyvitamin D³ [25(OH)D³] status and adult cardiovascular disease risk”• Hypothesis: low neonatal Vitamin D status, due to birth at the end of the winter would be associated with risk of cardiovascular disease and poor metabolic profile in adulthood•Dependent Variable(s): •Neonatal vitamin D status•Adult cardiovascular disease risk: • aortic stiffness, blood pressure, BMI, glucose tolerance, blood lipids, inflammation (C-reactive protein)

CRITICAL ANALYSIS•Multiple dependent variables….what’s missing?• New research- no study has ever examined early-life vitamin D status and cardiovascular disease risk beyond childhood

SAMPLE SIZE• 284 subjects (from a pool of 1305 subjects)• Chosen from Swedish Phenylketonuria (PKU) register in order in which they were listed• 35 years of age: born Feb. 9th- Mar 7th 1975; or Sept 14-29th 1975• born in Stockholm region• no sample size calculation

CRITICAL ANALYSIS• No sample size calculation – red flag! •Weakens external validity• Examined a specific population- Swedish PKU register• Born Feb. 9th- Mar 7th 1975; or Sept 14-29th 1975• Did not disclose why these dates were chosen• No randomization- weakens internal validity• Chosen in order in which they were listed reduces bias

STUDY DESIGN• Winter group: February 9-March 7, 1975 = 138 subjects; summer group: September 14-29 = 146 subjects• Subjects examined between July 5-August 30, 2010•Collected the following data: origin of parents; level of education, smoking, previously diagnose diabetes or hypertension, current medications, hours of physical activity/wk, weekly fish consumption, and family history of diabetes•Day of examination instructions•Collected data (measurements of dependent variables)

STUDY DESIGN• Blood spots stored for 37 years• Conducted a pilot study to assess feasibility of study• Compared samples from 1980 and 2010 • Reduction in 25(OH)D³ samples from 1980 compared to 2010 samples• 1980: 28.1 ± 11.4 2010: 58.6 ± 26.2 •25(OH)D³ was therefore not analyzed based on clinical cut-offs, but as a continuous variable

• Investigators blinded to season of birth, 25(OH)D³ measurement and clinical examination

CRITICAL ANALYSIS• Number of subjects in each group are similar• Collected data on several confounding factors• Confounding factors controlled for, but…• Degradation of vitamin D in dried blood spot samples proposes challenges of true concentrations•Also possibility of lower Vitamin D status in individuals in general in 1975

• Blinding of investigators reduces bias

STATISTICAL TESTS• Means and (±SDs) for normally distributed continuous variables• Geometric means and 95% CI for variables with skewed distribution• Proportions (%) for categorical variables•ORs and 95% CIs used to assess relationship between neonatal 25(OH)D³ concentration and risk of prehypertension/ hypertension, overweight, obesity and impaired fasting glucose/diabetes

CRITICAL ANALYSIS• Statistical tests described are appropriate for what they are measuring • Use of geometric means is questionable- why not a log transformation?• Disclosed the reason they did not adjust for multiple comparisons

RESULTS• Cohort characteristics: reported significant differences in maternal age at delivery between groups

RESULTS- SEASON OF BIRTH• Neonatal 25(OH)D³ concentrations significantly lower in winter group than summer group

RESULTS- NEONATAL 25(OH)D³

• Neonatal 25(OH)D³ concentration was significantly associated with weight, fasting insulin, cholesterol (women) and triglycerides. Once controlled for BMI, significance no longer existed• Reported a significant interaction between sex and neonatal 25(OH)D³ against adult height, BMI, serum cholesterol and HDL cholesterol. •Height and HDL increased at a higher rate per 1-nmol/L increase in neonatal 25(OH)D³ concentration in men•BMI and cholesterol increased at a higher rate per 1-nmol/L increase in neonatal 25(OH)D³ concentration in women

• Higher neonatal 25(OH)D³ concentration was associated with a higher adult BMI in women but not in men

• Reported in results and again in the discussion of the significant p-values. However the odds ratio suggest it is possible there is almost no association

STUDY LIMITATIONS Reported Limitations•Lack of data on body composition at follow-up, birth weight, and maternal characteristics, including BMI and vitamin D supplementation•Subjects not at a high cardiovascular disease risk age•Potential degradation of dried blood spots prevented stratification by clinically relevant cut-offs•Timing and place of study•Outcomes not detected due to sample size•Possibility those who chose to participate are more health conscious

FUTURE RESEARCH• Determine a sample size that would suggest clinically relevant results• Account for maternal vitamin D supplementation• Review the literature and justify choosing one cardiovascular disease risk factor as an outcome variable•Possibly triglycerides due to results of this study?

• Conduct the study when subjects are at an older age in which cardiovascular disease risk is more prevalent