"Short men more likely to die from dementia," The Daily Telegraph reports, though the results of the study it reports on are not as clear cut as the headline suggests.
Researchers combined the results of 18 surveys, which included more than 180,000 people. They aimed to see whether reported height was associated with deaths from dementia over 10 years of follow-up.
They found decreasing height was associated with higher rates of death from dementia. Each standard deviation decrease in height was associated with a 24% increase in risk of dementia death for men, and a 13% increase for women. This was after adjustment for factors such as age and smoking.
However, there are important limitations to consider. Despite the large cohort size, only 0.6% of the cohort died from dementia. These are small numbers on which to base any analysis.
Also, despite the trend, none of the smaller height categories were associated with a significantly increased risk of dementia death.
So, for both men and women, the smallest people in the study did not have a significant increased risk of dementia when compared with the tallest.
This means the association seen between height and dementia death isn't entirely convincing.
Where did the story come from?
The study was carried out by researchers from the University of Edinburgh, University College London, and the University of Sydney.
The Health Survey for England is part of a programme of surveys commissioned by the UK NHS Health and Social Care Information Centre.
Other surveys have been carried out since 1994 by the Joint Health Surveys Unit of the National Centre for Social Research, and the Department of Epidemiology and Public Health at University College London.
A number of other funding sources are also acknowledged. No conflicts of interest were reported.
The study was published in the peer-reviewed British Journal of Psychiatry.
The UK media took the reported results at face value without considering the limitations of this research. That said, all news sources that reported on the study took pains to emphasise that shortness in itself is very unlikely to cause dementia.
What kind of research was this?
This was a meta-analysis of data collected from participants as part of English and Scottish health surveys. It aimed to investigate the association between height and death as a result of dementia.
A meta-analysis aims to summarise the evidence on a particular question from multiple related studies.
The researchers say height is a marker of early-life illness, adversity, nutrition and psychosocial stress, and that these characteristics influence brain development, which may then affect dementia risk.
As this study is based on observational data, it cannot prove cause and effect. Its limitations include the inability to adjust for all possible confounders that may be influencing the results.
Also, deaths as a result of dementia were identified through the use of death certificates, which have not always clarified the type of dementia or whether it was directly involved in a person's cause of death. As it is primarily a disease of ageing, many people die with dementia rather than of dementia.
What did the research involve?
The researchers performed a meta-analysis of 181,800 participants from the Health Survey for England for the years 1994 to 2008, and the Scottish Health Survey for 1995, 1998 and 2003.
As part of the health surveys, participants were visited by a trained interviewer, who measured their height and weight. Participants were also asked about their:
- age on leaving full-time education
- ethnic group
- smoking status
- whether they suffered from a longstanding illness
They were subsequently visited by a nurse, who measured their blood pressure and took a blood sample to measure their cholesterol levels.
Each participant was linked to the UK NHS death register. Researchers reviewed death certificates to look for International Classification of Diseases (ICD) codes related to dementia.
In their analyses, they considered any mention of dementia on the death certificate (it may not always have been the direct cause of death).
The researchers looked at the association between height and death from dementia, controlling for age, gender and the other factors they had information about.
What were the basic results?
Increasing height was generally associated with a more favourable risk factor profile in both men and women.
Taller study members were younger, from higher socioeconomic backgrounds, had slightly lower body mass index, a lower prevalence of longstanding illness, and lower blood pressure and serum cholesterol levels. Taller men were also less likely to smoke, but the reverse was true of women.
During an average follow-up of 9.8 years, there were 17,533 deaths, of which 1,093 (0.6% of the cohort) were dementia related (426 men and 667 women).
Overall, there was a 27% increased risk of dementia death per standard deviation decrease in height in men (corresponding to 7.3cm; hazard ratio [HR] 1.24, 95% confidence interval [CI] 1.11-1.39) and a 13% increased risk of dementia death in women (corresponding to 6.8cm; HR 1.13, 95% CI 1.03-1.24).
These results show the association was stronger in men than women. This overall trend for increasing dementia risk with each standard deviation decrease in height was significant for both men and women.
However, when comparing the tallest height category with each of the three smaller height categories, none were associated with a significantly increased risk of dementia compared with the tallest – in other words, for both men and women, the smallest people in the study did not have an increased risk of dementia when compared with the tallest.
How did the researchers interpret the results?
The researchers concluded: "Early-life circumstances, indexed by adult height, may influence later dementia risk."
This study has combined the results of 18 health surveys for England and Scotland involving more than 180,000 people.
They found, overall, each standard deviation decrease in height was associated with an increased risk of death from dementia, with the trend slightly stronger for men than for women.
But this study has important limitations to consider. Despite the large cohort size, only 0.6% of the cohort (426 men and 667 women) died from dementia, as identified by documentation on their death certificate. These are small numbers on which to base analyses, particularly when further subdividing by gender and height category.
Though there was an overall trend for increasing risk with each standard deviation decrease in height, none of the smaller height categories were associated with a significantly increased risk of dementia death for either men or women, when compared with the tallest. Therefore, the association between height and dementia death isn't as clear cut as the media reporting implied.
The researchers considered dementia deaths to be any mention of dementia on the death certificate. We don't know from this study what the specific type of dementia was (Alzheimer's or vascular dementia, for example).
We also don't know that this was necessarily the direct cause of death. It could be the case that the person with dementia died from other causes. It is also possible the results are being influenced by confounding.
As the researchers consider, it is unlikely that height itself is a risk factor for dementia. It is more likely that decreased height could be a marker of other exposures, such as socioeconomic circumstances, nutrition, stress and illness during childhood.
This study did adjust for various factors, such as age, smoking, BMI, socioeconomic status and long-term illness, but the researchers would not have been able to take into account all the factors that could be influencing the relationship.
Overall, people with a shorter stature should not be too concerned by this study. The causes of dementia – in particular Alzheimer's, the most common type – are not clearly established.
Improving your cardiovascular health (keeping the flow of blood to your brain and heart well regulated) is probably the most effective step you can take to reduce your dementia risk.