Smoking 'rots brain’ causing mental decline

Smoking “rots the brain” the BBC reported today, while the Daily Mail said ‘smoking doesn't just age your lungs, it damages your brain too’.

Both stories are based on a large study looking at the risk of heart disease (cardiovascular disease or CVD) and stroke, and any associated decline in mental functioning in adults aged 50 and older, over a four-year period.

Researchers looked at people’s overall risk for CVD, but also at individual CVD risk factors including smoking, blood pressure, obesity and cholesterol.

They found that people with the highest risk of stroke had lower scores in tests of mental function at four years, compared with those who had the lowest stroke risk.

In particular, as well as being bad for your heart, lungs, skin and blood vessels, smoking appears to be bad for your mind – as the headlines suggest.

This was a well-conducted, if complex, study which supports previous research suggesting that the combined risk factors for heart disease and stroke may also increase the risk of cognitive decline. In particular, it suggests that smoking raises the risk of cognitive decline.

The study only followed people for a relatively short period (four years), so it is uncertain how these factors may affect cognition in the longer term. However, it does add yet more evidence to the suggestion that one of the best things you can do in life is to quit smoking – or never start.

Where did the story come from?

The study was carried out by researchers from Guy's and St Thomas' NHS Foundation Trust and King's College London and was funded by the National Institute for Health Research (NIHR). The study was published in the peer-reviewed journal, Age and Ageing.

The BBC’s report was fair, although its claim that smoking “rots” the brain is perhaps a little colourful as the researchers did not assess whether any physical damage to the brain had occurred.

A number of media reports on this study made a link between its findings and risk of dementia, but this is inaccurate as the research only looked at cognitive function, not specific conditions.

What kind of research was this?

The researchers aimed to explore the association between cardiovascular risk and cognitive decline in adults aged 50 and over. They point out that cognitive decline becomes more common with ageing and can interfere with daily functioning and wellbeing.

Several cardiovascular risk factors, including high blood pressure, high cholesterol, smoking and obesity have been suggested as important risk factors for cognitive decline, although results from studies have been contradictory. Age also determines cardiovascular risk itself, so the researchers had to allow for this.

For their analysis, the researchers used data from an ongoing longitudinal study following a representative sample of English adults aged 50 and over (the English Longitudinal Study of Aging – ELSA).

To date, the study has completed five data collection surveys (1998-2001, 2002-03, 2004-5, 2006-7, 2008-9), of which this current analysis drew primarily on the data from 2004-5 and 2008-9 (for follow-up). On each of these surveys, both cognitive (relating to mental functioning) and physiological (relating to physical functioning) measurements were collected.

What did the research involve?

In each survey, participants were interviewed by a trained nurse who collected biomedical and physical measurements as well as lifestyle factors. Participants underwent three validated tests on their cognitive ability.

These tested memory (such as how well a participant could memorise 10 random words) and executive functioning (higher mental functions, such as verbal fluency, for example, naming as many animals as possible in the space of one minute) and attention (such as looking at some random text and then highlighting all the ‘H’s in the text).

The combined ‘score’ of the memory and executive functioning tests were then used to provide an estimate of global cognition (a measure of overall cognitive functioning).

Their individual cardiovascular risk factor was calculated using the following methods:

• blood pressure was recorded (the average of three measurements) and the measurements classified as normal, borderline-high and high.
• total cholesterol levels were measured and classified as optimal, mildly high and high
• body mass index (BMI) was measured and classified as normal, overweight or obese
• participants were classified as non-smokers (never smoked or ex-smokers) and smokers (current smokers)
• the study also calculated overall risk scores for heart disease and stroke, based on information from the 2004-05 survey

The risk scores were calculated using validated measures and were based on information about age, gender, blood pressure, blood lipids, diabetes and smoking. 

These risk scores were divided into quartiles, from lowest to highest risk.

The analysis looked at whether there were any associations between measurements of blood pressure, total cholesterol, smoking and BMI from the 2004-5 survey and measurements of cognition in the 2008-9 survey (after adjusting these for participants’ original cognitive scores in 2004-5).

They analysed the association between the (10 year) cardiovascular risk scores calculated in the 2004-5 survey and measurements of cognition at the four year follow up, adjusting for baseline (2004-5) data. Those in the lowest quartile of stroke and CVD risk were used as the reference category.

The researchers adjusted their results for other factors which might influence cognition, such as education, alcohol intake, physical exercise and depression.

What were the basic results?

The main results are outlined below:

• Participants in the highest quartile of stroke risk in 2004-5 had lower global cognition (baseline = −0.73, 95% confidence interval [CI] −1.37 to −0.10), memory (baseline = −0.56, 95% CI: −0.99 to −0.12) and executive (baseline = −0.37, 95% CI: −0.74 to −0.01) scores four years later, compared with those in the lowest quartile.
• High (systolic) blood pressure in 1998-2001 was associated with lower global cognition (baseline = −1.26, 95% CI: −2.52 to −0.01) and specific memory (baseline = −1.16, 95% CI, −1.94 to −0.37) scores at eight year follow-up.
• Smoking was consistently associated with lower performance on all three cognitive measurements.

How did the researchers interpret the results?

They say that high risk of cardiovascular disease may be associated with accelerated decline in mental function in older people, with smoking emerging as the most significant of the individual risk factors. High blood pressure may also be a risk factor for cognitive decline, over a longer period.

Conclusion

This was a large, well-conducted study, that seems to support previous suggestions that a high risk of heart disease and stroke is also associated with mental decline. In particular, that smoking (and possibly high blood pressure) are important risk factors.

Other individual risk factors, such as high cholesterol and BMI, did not have any significant association with cognition.

One limitation is the short duration of follow up, which means it is unclear how the risk of stroke and heart disease might influence cognition over a longer period. 

It is also possible that other factors, called confounders affected the results, although researchers tried to adjust their findings for many of these.