Three studies reviewed why adults widely take vitamin supplements, and that they sometimes are associated with severe side effects.
All three research letters were published online Feb. 4 at JAMA Internal Medicine.
The first review found that physicians prescribed less than one-quarter of supplements used by adults.
Researchers looked at data from nearly 12,000 adults in the 2007-2010 National Health and Nutrition Examination Survey. People responded that they used supplements to improve (45%) or maintain (33%) overall health. Women took calcium supplements for bone health (36%) and men took them for heart health or to lower cholesterol (18%).
More than half of adults in the survey took some kind of supplement, spending more than $30 billion on dietary supplements in 2011, most frequently for multivitamins, followed by calcium and omega-3 or fish oil supplements.
But, cause and effect were difficult to disentangle, researchers noted, because supplement users were more likely to report very good or excellent health, have health insurance, use alcohol moderately, not smoke and exercise more frequently than nonusers.
Researchers wrote, "These data lend credence to the 'inverse supplement hypothesis' that many supplement users are healthy individuals who want to take an active role in their own health, and who perceive supplements as a type of 'insurance' against poor health."
But, there's no cure that doesn't have a side effect. Two more studies outlined associations with diseases from supplements.
One study reported that high intake of supplemental calcium was associated with an excess risk of cardiovascular disease death in men but not in women.
Researchers conducted a prospective study of dietary and supplemental calcium intake from 1995 through 1996 in six states and two major cities among more than 388, 000 adults ages 50 to 71 from the National Institutes of Health-AARP Diet and Health Study.
More than half of men took calcium supplements. In men, supplemental calcium intake was associated with an elevated risk of CVD death (relative risk [RR] more than 1,000 vs 0 mg/d, 1.20; 95% confidence interval [CI], 1.05 to 1.36). The risk of heart disease death was 1.19 (95% CI, 1.03 to 1.37) while the risk of cerebrovascular disease death was 1.14 (95% CI, 0.81 to 1.61).
Seventy percent of women took calcium supplements, but it wasn’t associated with death from cardiovascular disease (RR, 1.06; 95% CI, 0.96 to 1.18), heart disease (RR, 1.05; 95% CI, 0.93-1.18), or cerebrovascular disease (RR, 1.08; 95% CI, 0.87 to 1.33).
The authors wrote, "[B]eyond calcium's established role in prevention and treatment of osteoporosis, its health effect on nonskeletal outcomes, including cardiovascular health, remains largely unknown and has become increasingly contentious."
Another research letter looked at whether ascorbic acid supplements of about 1,000 mg were associated with kidney stones in a population-based, prospective cohort of about 49,000 Swedish men.
Researchers found that ascorbic acid use was associated with a statistically significant two-fold increased risk of kidney stones. (Specific rates and confidence intervals slightly varied after adjusting for demographic and lifestyle factors.)
Those who took fewer than seven ascorbic acid tablets per week had a relative risk of 1.66 for developing kidney stones (95% CI, 0.99 to 2.79) compared with those who didn't take supplements in the full multivariate-adjusted model. Those who took seven or more tablets per week showed an increased relative risk of 2.23 (95% CI, 1.28 to 3.88) compared with those who didn't take supplements in the full multivariate-adjusted model (P for trend=.001).
Multivitamin use was not associated with kidney stones (relative risk, 0.86; 95% CI, 0.62 to 1.19).
Researchers wrote, "Our findings need to be confirmed by other studies but may have important implications for the clinical advice given to kidney stone formers. Currently there are no well-documented benefits of high-dose ascorbic acid supplement use, and, therefore, it seems prudent to advise that high-dose preparations be avoided, particularly by those with a history of kidney stones."