Monday July 14 2008
Could the rain be to blame for increased BMI?
“Why heavy weather makes you fat” is the headline in The Daily Telegraph, which goes on to suggest that the “grey British weather” may be one reason why dieters struggle to shed a few pounds. Scientists have found that people who are overweight have lower levels of vitamin D in their blood, and that this interferes with normal appetite control.
The report is based on a study which looked at the relationship between levels of vitamin D in the blood and bone health, sun exposure, dietary intake and being overweight in postmenopausal women. One of the study’s findings was an association between a raised body mass index and low vitamin D levels. However, the design of the study means it is impossible to conclude that one causes the other. For the moment, any justification for increasing vitamin D levels should be based on proven improvements in bone health, rather than “yet to be proven” benefits in reducing rates of obesity.
Where did the story come from?
Dr Helen Macdonald and colleagues from the University of Aberdeen and the Royal Liverpool University Hospital carried out this study. Funding was provided by Grampian Osteoporosis Trust and the UK Food Standards Agency. It was published in the peer-reviewed medical journal: Bone.
What kind of scientific study was this?
This cross-sectional study enrolled women from Aberdeen who were also participating in a larger prospective study – the Aberdeen Prospective Osteoporosis Screening Study. Some 3,113 postmenopausal women who provided blood samples were included in the analysis. As part of the larger study, the women completed dietary questionnaires, including one that specifically assessed the quantity of vitamin D being ingested. Other questionnaires were used to determine the amount of physical activity the women performed, their frequency of exposure to sunlight (classed as seldom, occasionally or often), which parts of the body were usually exposed, sunlight intensity (which depended on latitude in Aberdeen), and the time spent abroad.
Of the women, 2,402 completed the questions about sunlight. Other details about the women were available from their participation in the larger study, including bone mineral density and the concentration of vitamin D in the blood. Most of the women were assessed between 1998 and 1999.
The researchers determined the vitamin D intake of the women by assessing the dietary contribution of different foodstuffs. The researchers also compared the vitamin D concentrations between women who had taken holidays abroad or in the south of England with women who hadn’t. To analyse the link between exposure to sunlight and other characteristics, the researchers first determined whether the season had an effect on vitamin D levels, and whether there was a link between the amount of vitamin D in the blood and bone health. They took into account other factors that may have influenced this relationship, including age, height, weight, physical activity and socioeconomic status. The researchers also compared people who had the highest levels of vitamin D in the blood with those who had the lowest.
The participants were divided into five groups according to their body mass index (BMI), with each group containing 20% of the participants (quintiles). Researchers then compared those in the highest BMI quintile (those in the top 20%) with those in the bottom 20% to see if there was any difference in their levels of vitamin D.
What were the results of the study?
The researchers found that average vitamin D levels were highest in autumn and lowest in spring. Exposure to sunshine played a significant part in the difference in levels during summer and autumn, while in winter and spring vitamin D from food sources (excluding supplements) was more important. In all seasons, the link between total vitamin D intake (including supplements) and levels of vitamin D in the blood, was significant. In addition, those with high levels of vitamin D had better bone health.
When they compared people with the highest levels of vitamin D with those who had the lowest levels, the researchers found that people in the high concentration group were more likely to holiday abroad, have high sun exposure (including sunbed use), higher socioeconomic status and were less likely to smoke.
When the researchers looked at BMI, women in the highest BMI quintile had the lowest levels of vitamin D in the blood. This relationship was significant even after adjusting for age, physical activity, smoking, use of HRT and socioeconomic status.
What interpretations did the researchers draw from these results?
The researchers conclude that dietary intake of vitamin D contributes to vitamin D status throughout the year in postmenopausal women living at a high latitude (57°N). They say that more research is needed to establish what concentration of vitamin D in the blood is required for “optimal health”. The main conclusion from their study is that dietary intake appears to “attenuate the seasonal variation of vitamin D…in postmenopausal women at northerly latitudes”.
What does the NHS Knowledge Service make of this study?
- This was a large cross-sectional study that examined relationships between many different factors and vitamin D. One of the findings is that there is a relationship between BMI and the level of vitamin D in the blood; that people with the highest 20% BMI in this sample had lower circulating vitamin D than those with the lowest BMI. With this particular result, it is important to bear in mind that a study of this design, i.e. cross-sectional, cannot establish the direction of the relationship between BMI and vitamin D. Lower vitamin D could cause weight increase (as the newspapers imply is the case). Alternatively, being overweight could lead to a reduction in vitamin D levels, perhaps by reducing the time people spend outside, or there could be other factors that are related to both vitamin D levels and BMI.
- The researchers did not look at the relationship between exposure to sunlight and weight, though they had this data. It is not accurate to claim, as the Telegraph has, that this study shows that the “grey British weather” may be responsible for difficulty with weight loss.
- In their exploration of the relationship between BMI and vitamin D levels, the researchers attempt to adjust for some factors that may explain the relationship – namely age, physical activity, smoking, HRT use and socioeconomic status. They did not adjust for sunlight exposure or dietary intake, and there may have been other factors that more reasonably explain the association. As the researchers themselves say: “the lower serum concentrations could be due to decreased sunshine exposure in the obese”.
- As this study focused on women at a particular latitude (quite far north in Aberdeen), the applicability to women living in different regions of the UK (where sunlight exposure may be greater) or indeed in other parts of the world where exposure may be more or less, is unclear.
For the moment, any justification for increasing vitamin D levels (by supplementation or sunlight) should be based on proven improvements in bone health, rather than “yet to be proven” benefits in reducing rates of obesity.