24 may 2009--The Daily Telegraph has claimed that “sunbathing could help older people reduce the risk of developing heart disease and diabetes”, while the Daily Express says that “sunshine can add years to your life”. The Express bases its claims on a study that found that “exposure to sunlight stimulates vitamin D in the skin”.
It is already well established that the skin produces vitamin D through exposure to sunlight, as well as through diet. This Chinese study reported in the newspapers did not assess exposure to sunlight, but observed low vitamin D levels in a middle-aged to elderly population and found they were associated with a combination of conditions that increase the risk of diabetes and heart disease. However, this study did not actually assess reasons for these low vitamin D levels and many features of the study limit its interpretation.
Based on this research, it is a great jump to conclude that increasing your exposure to sunlight will reduce your risk of developing these diseases or even prolong your life. The dangers of extensive exposure to sunlight are well known, and this study should not be taken as a reason to start sunbathing.
Where did the story come from?
This study was conducted by Oscar Franco and colleagues of Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences and Graduate School of the Chinese Academy of Sciences, Shanghai. The research was funded by Chief Scientist Program of Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences and other Chinese development funds. The study was published in the peer-reviewed medical journal Diabetes Care.
What kind of scientific study was this?
This was a cross-sectional study that examined the link between levels of a form of vitamin D and metabolic syndrome in the middle-aged to elderly Chinese population. Metabolic syndrome (MetS) is a group of conditions that increase the risks of developing cardiovascular disease, diabetes and liver disease. The conditions making up metabolic syndrome include high blood pressure, being obese or overweight, glucose intolerance, raised cholesterol and fatty liver.
The conditions comprising MetS, which is increasingly becoming a global health problem, have previously been noted to be associated with lower vitamin D levels. However, the reasons for this association are unclear, and to date there has been little evidence from Asian populations.
The research was part of the Nutrition and Health of Aging Population in China project, which involves a cross section of the Chinese population aged 50 to 70 years. The researchers recruited 3,289 participants (1,458 men and 1,831 women) and excluded those without adequate blood samples for vitamin D measurement, leaving 3,262 individuals.
Participants were interviewed on demographic details, education, smoking status, alcohol use and physical activity. They then received a physical examination calculating weight, height and blood pressure. Participants also self-reported diabetes, high blood pressure, lipid disorders, heart disease, stroke and medication use.
Based on the blood sample, vitamin D levels were classified as sufficient (above 75nmol/L), insufficient (50 to 75nmol/L) or deficient (less than 50nmol/L). MetS was determined according to threshold measurements of waist circumference, triglyceride and cholesterol levels in the blood, blood pressure (or use of blood pressure medications) and fasting glucose level (or use of diabetic medications or confirmed diagnosis of diabetes).
What were the results of the study?
In the population sample, 69.2% were deficient for vitamin D, 24.4% insufficient and 6.4% sufficient. The average vitamin D level of the whole sample was only 40.4nmol/L. The odds of having MetS were increased by 52% for the group with the lowest levels of vitamin D (below 28.7nmol/L) compared to in the group with the highest levels (above 57.7nmol/L) (odds ratio 1.52, 95% confidence interval 1.17 to 1.98).
The researchers also noted individual associations between certain components of metabolic syndrome and vitamin D level.
What interpretations did the researchers draw from these results?
The researchers conclude that vitamin D deficiency is common among middle-aged and elderly people of the Chinese population. They note that a low vitamin D level was associated with having metabolic syndrome.
What does the NHS Knowledge Service make of this study?
This large population study has observed an association between having a low vitamin D level and metabolic syndrome, which increases the risk of heart disease and diabetes.
It is a great leap to conclude from the findings of this study that exposure to sunlight will help to reduce the risk of heart disease and diabetes, or even prolong your life, as some newspapers have claimed. Indeed, the study did not explore the reasons why the population sample actually had low vitamin D, or look into whether increasing vitamin D level would have any effect on improving risk of heart disease or diabetes or duration of life.
While the Daily Express has made a front-page claim about the benefits of sun exposure, it is particularly worth noting that this factor was not assessed in any part of this study. The dangers of extensive exposure of the skin to sunlight are well known, and this study does not support sunbathing.
There are several important points to note when interpreting this study:
The sample population had low vitamin D levels overall, with an average level of only 40.4nmol/L compared to a desirable level of 75nmol/L. The levels found are generally considered to be inadequate for overall health, and 94% of subjects would be considered to have either vitamin D deficiency or insufficiency.
The reasons for the low vitamin D levels found in this research remain unclear, and might result from low dietary intake, low sun exposure or some other reason. As this study only took a single blood sample, it is possible that these results may not reflect the vitamin status of the participants over time.
Being a cross-sectional study, it cannot prove causation, as it cannot establish that the vitamin deficiency was present before the onset of MetS. It may be, conversely, that the bodily changes associated with the conditions of MetS (for example, obesity, raised blood pressure and glucose) have contributed to the body becoming deficient in vitamin D for various unexplored reasons, or that the two observations are related to some other factor (for example, poor diet).
Although the laboratory measures and clinical examinations performed at the time of assessment will have increased the reliability of diagnoses, the self-reported measures of health (including self-reported diagnoses of previous heart disease, stroke or diabetes) may have led to misclassification of whether or not people had MetS.
As the study involves a Chinese population, cultural, ethnic and lifestyle differences mean that findings may not be easily extrapolated to other populations and ethnic groups.
Although there may be an association between vitamin D levels and metabolic syndrome, it is worth remembering that diabetes and heart disease are complex conditions determined by a number of medical, genetic and lifestyle-related factors. Sun exposure is not a single solution to these problems.
As the researchers themselves conclude, further study is needed to examine the physiological reason for the observed association between vitamin D and metabolic syndrome.