Wednesday December 23 2009
Several factors, including weight, are involved in the ageing of skin
The Daily Mail warns that “too much sun can pile on the years”. It reported results from a new study that apparently identifies “sunburn, smoking and being overweight” as key factors in making people look older than they are.
The study was based on a survey in twins that assessed skin ageing and various other factors. Greater weight, smoking and a history of skin cancer were associated with higher skin damage scores. Alcohol consumption and the use of sunscreen were associated with less skin ageing.
This is a small cross-sectional study and therefore cannot suggest causation. However, it does confirm some associations that are already known, such as the benefits of sunscreen. It also highlights some associations for further research, such as how weight is linked with skin age.
Where did the story come from?
The study was carried out by Dr Kathryn Martires and colleagues from the Case Western Reserve School of Medicine and the Cleveland Clinic Foundation in Ohio. The study was published in the peer-reviewed medical journal Archives of Dermatology. No details of funding are given.
The Daily Mail’s coverage of this research is generally balanced, although it does not mention the shortcomings of cross-sectional studies in establishing causation. The newspaper also reported that ‘sunburn’ makes people look older than they are. This is a slight distortion of the actual finding that sunscreen is associated with reduced photoageing and that fairer skin, which burns more easily, is associated with more photoageing.
What kind of research was this?
This was a cross-sectional study in 130 identical (monozygotic) and non-identical (dizygotic) twins aged between 18 and 77 years. The participants volunteered to have their skin examined for signs of ageing and to be questioned about their skin, behaviour and health while they attended the annual Twins Days Festival in Twinsburg, Ohio in 2002.
The researchers were investigating whether particular environmental factors are associated with skin ageing (as established through evidence of photoageing). Twin studies are useful because they can compare the effects of the environment on people who share 50% or 100% of their genes with another person, providing information on what is caused by the environment, and what is genetic. However, this is still a cross-sectional study, a design that cannot establish causation.
What did the research involve?
Researchers asked both twins in 65 twin pairs about their skin types, history of skin cancer, smoking and drinking habits and weight. Each twin was also examined by dermatologists who scored their skin type and photodamage.
The researchers then assessed the link between skin damage and various factors, including age, and used a statistical method called regression analysis to estimate how strongly each factor was associated with scores on the skin damage scale. The similarity in skin damage between twins was determined by calculating how similar their scores were. This was then used to adjust for the contribution of genetics.
What were the basic results?
There was a high correlation between the skin damage scores of the twins, i.e. they were similar between identical and non-identical twin pairs. The correlation was slightly greater in the identical twin pairs, but not significantly so. Several factors were significantly associated with higher photodamage scores, including a history of skin cancer, the type of twin (whether identical or non-identical), weight and cigarette smoking. Alcohol consumption and the use of sunscreen were associated with lower photodamage scores.
How did the researchers interpret the results?
The researchers say that the relationships found between smoking, weight, sunscreen use, skin cancer and photodamage in these twin pairs may “help to motivate the reduction of risky behaviours”.
This small cross-sectional study has highlighted the association of particular environmental factors with skin ageing. There are several important points to consider when interpreting the results:
- Cross-sectional studies such as this cannot establish causation as they don’t determine the temporal relationship (which came first) between exposure (various factors) and outcome (skin damage, in this case).
- The twins in this study were assessed for photoageing (the damage done to the skin by prolonged sun exposure) through a skin examination. This is not a diagnosis of skin cancer, so claims that any of these factors are related to skin cancer are an extrapolation and are not supported by the findings from this particular study.
- The researchers acknowledge that relying on self-report of behaviours, such as alcohol and smoking behaviours and of the participants' weight, means there may be inaccuracies in the data.
- The similarity in skin damage scores within the pairs of identical and non-identical twins suggests that environmental factors are important in relation to genetics. Pairs of identical twins tended to have more similar scores, but not significantly more so, than the non-identical twins. This suggests that there may be some contribution of genetics to skin damage.
This is a small cross-sectional study and therefore cannot suggest causation, but it confirms some associations that are already known, such as the benefits of sunscreen. It also highlights some associations that may be considered in further research, such as the association with weight. Larger, prospective studies are needed.