Vitamin D deficiency increases mortality risk and raising levels even slightly could decrease the risk, researchers examining data from the UK Biobank have found.
They used a Mendelian randomization approach, which uses genetic variants as “proxy indicators” for external factors that affect vitamin D levels, such as sun exposure or dietary intake. It allows for analysis of the relationship between deficiency and outcomes including mortality, which can’t be done in randomized clinical trials for ethical reasons.
Using this method, nutritionist Joshua P. Sutherland, PhD, of the Australian Centre for Precision Health, Adelaide, and colleagues found an association between genetically predicted vitamin D levels [25-(OH)D] and mortality from several major causes, with evidence of causality among people with measured concentrations below, but not above, 50 nmol/L. The findings were published online in Annals of Internal Medicine.
“Unlike other types of observational studies, we have overcome some of the methodological obstacles. What is special about this new study is we were able to look at people with very low vitamin D concentrations and what would happen if their concentrations were a little bit higher. Most randomized controlled trials don’t show much of an effect. That’s because most people have sufficient concentrations. Ethically you can’t do a trial of people with very low levels without treating them,” senior author Elina Hyppönen, PhD, told this news organization.
The data support the 50 nmol/L cut-off endorsed by the United States National Academy of Medicine and align with previous data suggesting the benefit of vitamin D supplementation is largely seen in people with deficiency.
“Everybody with vitamin D levels less than 50 nmol/L is recommended to increase their levels. Our results suggest there’s no need to go very high. The positive message is that if we are able to raise levels to just the current U.S. recommendations, that’s fine. There’s no need to use large supplement doses,” Dr. Hyppönen explained.
Thus, she advised, “Supplementation will clearly help, especially during wintertime or if a person isn’t getting enough vitamin D from the sun or in places where food isn’t fortified with vitamin D.”
But the data don’t support the approach of using large intermittent doses, she added.
“Sometimes doctors want to fix the deficiency quickly with a large ‘bolus’ dose, then continue with a maintenance dose. Increasing evidence suggests that’s not beneficial and might disturb the body’s metabolism so that it can’t get the amount it needs. It’s safe overall but might not work the way we want it to work.”
Rather, Dr. Hyppönen said, “My sense is that daily modest vitamin D dose supplementation when it’s needed is the best way forward.”
Genetic approach reveals causal relationship
The investigators analyzed data from 307,601 individuals in the UK Biobank, a prospective cohort of people recruited from England, Scotland, and Wales during March 2006 and July 2010. Most were of White European ancestry and were aged 37-73 years at baseline.
Genetically predicted vitamin D levels were estimated using 35 confirmed 25-(OH)D variants. Participants were followed for outcomes up to June 2020.
The average baseline measured 25-(OH)D concentration was 45.2 nmol/L, and 11.7% (n = 36,009) of participants had levels between 10.0 and 24.9 nmol/L. Higher levels were seen in people living in southern areas and nonsmokers as well as those with a higher level of physical activity, less socioeconomic deprivation, and lower body mass index.
During follow-up, 6.1% of participants died (n = 18,700). After adjustment for variables, odds ratios for all causes of mortality were highest among people with 25-(OH)D levels below 25 nmol/L and appeared to plateau between 50 and 75 nmol/L, with no further reduction in mortality at values of 75-125 nmol/L.