Vitamin D studies are now a dime a dozen. They provide enough material for vitamin D fetishists, but also for their opponents. The suspicion consolidated here is that vitamin D studies are now more a question of faith than they are a scientific process.
DocCheck has placed one recent meta-study and one cohort study of vitamin D supplementation under the microscope. The meta-analysis examined vitamin D’s potential in the prevention of colds and appeared in British Medical Journal. The cohort study focuses on the relationship between vitamin D levels and headaches in men. It appeared in Scientific Reports, Nature Research.
Both studies have their weaknesses, but they make it clear as to who benefits from regular vitamin D supplementation and in which situation it is advisable to abstain from it. Whether these meta-studies and their very contradictory individual component studies serve as a suitable basis for assessing vitamin D in diverse settings remains however doubtful.
Earlier observational studies associated Vitamin D deficiency with an increased risk of respiratory diseases such as colds and flu. Clinical studies that have examined the protective effect of vitamin D came to conflicting conclusions. The recent study1 is a systematic review, in which the researchers examined the data from the participants in randomised controlled trials on vitamin D supplementation. Taking the perspective of looking at the individual study participants – in contrast to the comparison of pure trial results – the scientists hoped for more clarity with respect to the discrepancies in previous results.
The researchers analysed data from 10,933 study participants aged from 0 to 95 years obtained from 25 randomised controlled trials.
Overall, the meta-study showed that supplementation with vitamin D correlated with a 12 percent reduction in the proportion of patients with (at minimum) an acute respiratory infection – when compared to non-supplementation.
Subgroup analyses were to provide insight into the contradictory study results. This analysis showed a protective effect of vitamin D with participants who received their vitamin D doses daily or on a weekly basis and not in the form of a single high dose. Anybody who received one or more high dosages did not benefit from the supplementation.
The protective effect was greater for participants with distinct vitamin D deficiency (<25 nmol/ L) than for subjects with serum 25-hydroxyvitamin D levels of at least 25 nmol/ L.
The meta-analysis also showed that the administration of vitamin D is safe. The incidence of adverse events was in the verum group no higher than in the vitamin D group.
The scientists conclude from their results that a vitamin D deficiency should be corrected for the prevention of respiratory infections.
- Basis of this meta-analysis was observational studies, in which data had been collected on the health and life of subjects and was correlated retrospectively. Causal relationships cannot be detected in observational studies.
- Universally administered supplementation with vitamin D is considered by critics not to be sensible, probably more applicable to people who have a serum level of below 25 nmol/ L.
The male population in Finland in the 1980s was suffering by worldwide measures most frequently from heart disease. The data from the cohort study2 (Kuopio Ischaemic Heart Disease (Kihd) RiskFactor Study) was actually collected in order to make these men’s heart disease risk factors discernible. Scientists have now used this data cohort in order to consider a possible correlation between headache and vitamin D concentration in blood.
Data from this study is derived from 2,601 Finnish middle-aged men (42 to 60 years of age at start of study) collected from 1984-1989.
Depending on the serum concentration of 25-hydroxyvitamin D (25 [OH] D), the men were divided into 4 groups each with 650 men:
Group 1: D (25[OH]D) < 28.9 nmol/L
Group 2: D (25[OH]D) = 28.9 to 40.1 nmol/L
Group 3: D (25[OH]D) = 40.2 to 55.0 nmol/L
Group 4: D (25[OH]D) > 55.0 nmol/L
Men with low serum levels of (25[OH]D) in group 1 suffered twice as often from chronic headaches as did the men in the group with the highest vitamin D levels in the blood.
The average serum concentration of 25(OH)D among the Finnish men was 43.4 nmol/ L, ie under the Vitamin D deficiency limit of 50 nmol/ L. 67.9 percent of the cohort had 25(OH)D levels below 50 nmol/ L. Only 6.9 percent of men reported 25(OH)D levels of 75 nmol/ L or greater.
The increased headache risk was observed only in group 1, but here the correlation was strong. After adjustment by age and by date of blood collection (summer or winter), the average 25(OH)D concentrations in those subjects who reported frequent headaches was significantly lower than among participants who had no regular headaches (38.3 vs 43.9 nmol/ L; P <.001).
Of the men whose blood was taken during the UV-B range summer months, 7.2 percent reported frequent headaches; in the group of men whose blood was taken in the less sunny months the figure was 10.4 percent.
- The study is a retrospective observational study. Naturally only associations are able to be pointed out, but no causal relationship.
- Only middle-aged men were included, although three times more women suffer migraine-like headaches than do men.
- The associations of headache and vitamin D are already known from other studies.
- The information on the frequency of the headaches came from the participants. It had not been reviewed in the course of the study.
- Headaches can be very different in nature. No information exists on the type of headache (such as migraine, tension headaches …).
The currently ongoing Finnish vitamin D study (FIND) is supposed to settle some open questions concerning the relationship between vitamin D deficiency and headaches. The study runs over 5 years and studies 2,500 participants as to how the intake of 40 or 80 micrograms of vitamin D has impacts, in contrast to placebo, on various conditions, including pain. The first results should exist in early summer 2018.
Supplementation: yes, but not for all
Every year in winter much of the media revive the subject of vitamin D supplementation. In parallel to this the manufacturers of the “sun vitamin” impose themselves on the market. A distinct supplementation recommendation cannot be derived from the current study situation. Those who have levels far below the prescribed vitamin D deficiency-limit of 50 nmol/L could potentially benefit from supplementation.
But beware: although the intake of vitamin D – at least in moderate doses – is considered safe, the vitamin also has documented downsides. Thus, a recent published study3 states that the “bone vitamin” can even lead to increased falls and bone fractures among the elderly, instead of protecting them. Vitamin D supplementation during pregnancy4 also increases the risk of the child developing a food allergy in later life. Therefore, it should be accurately weighed up from previously tested serum levels as to when administration of vitamin D is really useful.
The human body can make vitamin D under UVB radiation from sunlight itself. However, the conditions for this are not always optimal. Thus, the angle of the sun, the elevation above sea level, the nature of the earth’s surface, cloud cover, smog or ozone in the air affect the production of vitamin D in the skin. If environmental conditions are optimal, the skin of a young adult man can create 10,000-20,000 IU (250-500 micrograms) of vitamin D daily. A few foods, such as fatty fish, organ meats, eggs and dairy products, can to a limited extent however contain vitamin D in comparable amounts.
2 Low serum 25-hydroxyvitamin D is associated with higher risk of frequent headache in middle-aged and older men. Sci Rep. 2017 Jan 3;7:39697.
3 Monthly High-Dose Vitamin D Treatment for the Prevention of Functional DeclineA Randomized Clinical Trial. JAMA Intern Med. 2016;176(2):175-183.