“People who give up smoking begin to improve their health almost immediately”, The Guardian reported today. It goes on to report that a study looking at more than 100,000 women between 1980 and 2004 found those who gave up smoking reduced their risk of death by 13% in the first five years after quitting, and had no extra risk of death 20 years after quitting.
The newspaper also reports that women who started smoking at 17 were 22% more likely to die within the study period than those who started at 26 or older. One of the study’s authors, Stacey Kenfield, said, “effectively communicating risks to smokers and helping them quit successfully should be an integral part of public health programmes”.
This report is based on a well-conducted study, the results of which reliably demonstrates that risk of death reduces after women stop smoking. This study should give reassurance to women who have stopped smoking, and encouragement to current smokers to stop. It should also be pointed out that, regardless of sex or age, quitting smoking increases the chances of a longer life.
Where did the story come from?
Dr Stacey A. Kenfield and colleagues from Harvard School of Public Health, Harvard Medical School and Washington University School of Medicine, carried out the research. The study was funded by the National Institutes of Health, the Association of Schools of Public Health and the Legacy Foundation. The study was published in the peer-reviewed Journal of the American Medical Association.
What kind of scientific study was this?
In this prospective cohort study – the Nurses’ Health Study - 121,700 female nurses were tracked to ascertain the effects their lifestyle had on their health. The study began in 1976 when women aged between 30 and 55 were recruited by questionnaire. Follow-up questionnaires asking about their health were then sent out every two years. The current study looked at data collected between 1980 and 2004.
At the beginning of the study, the participants answered questionnaires about their medical history, and risk factors for diseases such as heart disease and cancer. They were also asked if they currently smoked or had ever smoked and if so, when they started smoking. Current smokers were asked how many cigarettes they smoked a day and those who had quit were asked when they had quit and how many cigarettes they smoked per day before quitting. The questionnaire that was sent out every two years after that asked similar questions to determine whether women had started or stopped smoking, and how many cigarettes they were smoking.
For analyses, the researchers classified current smokers according to how much they smoked, and when they started smoking. Past smokers were categorised by how long it had been since they stopped smoking.
The researchers identified women who had died between 1980 and 2004 either by being told by the women’s families or by looking for women who did not return their questionnaires in the National Death Index. Causes of death were established through medical records and death certificates. They were grouped into six broad categories: vascular disease (including coronary heart disease and cerebrovascular disease); respiratory disease; lung cancer; smoking-related cancers (based on the 2004 US surgeon general’s report, and including lung, lip, mouth, oesophagus, pharynx, larynx, trachea, kidney, stomach, cervical and bladder cancers and acute myeloid leukaemia); other cancers; and other causes.
The researchers then compared the risk of death between current or past smokers in the different categories with those women who had never smoked. In their analyses, they took into account factors that might affect death rates. These included high blood pressure, diabetes, high cholesterol, BMI, change in weight between age 18 and start of the study, alcohol intake, physical activity, use of oral contraceptives, HRT use, menopausal status, parental history of death from heart attack, red and processed meat consumption, duration of aspirin use, and calcium and folate intake.
The researchers excluded all women who had a history of cancer, vascular disease, or respiratory disease at the beginning of the study, and also those who smoked but did not report when they had started smoking. This left 104,519 women for analysis.
What were the results of the study?
During the period looked at (1980-2004) there were 12,483 deaths (about 12% of the participants). About 64% of deaths in current smokers and 28% of deaths in past smokers were attributed to cigarette smoking. Current smokers were almost three times more likely to die from any cause, and over seven times more likely to die from smoking-related cancers than women who had never smoked.
The more cigarettes a woman smoked a day the greater her risk of death; women who smoked 35 or more cigarettes a day were four times more likely to die from any cause than women who had never smoked. The earlier in life that a woman started smoking, the greater her increase in risk of death during the time of the study, particularly death from respiratory disease or smoking related cancers.
Within five years of quitting, women’s risk of death from any cause reduced by 13% compared with continuing to smoke, and within 20 years their risk returned to that of a non-smoker. The risk of deaths from different diseases reduced at different rates after stopping smoking. The risk of death from vascular disease reduced the most rapidly, with a 31% reduction in risk in the first five years. However, the risk of death from respiratory causes only reduced by 18% five to 10 years after quitting.
What interpretations did the researchers draw from these results?
The researchers concluded that stopping smoking rapidly reduces women’s risk of death from vascular diseases, and their risk of death from lung diseases returns to the level of non-smokers within 20 years. Delaying starting smoking reduces risk of respiratory diseases and related cancers.
What does the NHS Knowledge Service make of this study?
This was a well-conducted and large study that increases understanding about the effects of smoking, and particularly the effects of stopping smoking. There are some points to consider when interpreting this study:
- Women were studied and the results may not be representative of men.
- Because researchers cannot randomly choose if people smoke or not, or decide if they should continue to smoke or to quit, the groups that are compared are always likely to differ in ways other than smoking status, and these may also affect the risk of death. For example, current smokers had less increase in weight since age 18, less high blood pressure, and lower BMI, but did less vigorous exercise, and drank more alcohol than past smokers or those who had never smoked. To reduce the effects these factors may have had, the researchers assessed a wide range of risk factors both at the start of the study and every two years, and made adjustments for these. Although this may not completely remove the effects of these factors, or account for unknown or unmeasured factors, it does increase the reliability of the results.
- Information about smoking, other lifestyle factors, and medical history were all reported by the participants themselves in a postal questionnaire. This could have led to some inaccuracies, particularly on items that required remembering events in the past, for example, change in weight since age 18.
This study should give reassurance to women who have stopped smoking, and encourage current smokers that quitting does substantially improve their chances of a longer life.
Sir Muir Gray adds...
Links to the headlines
The Guardian, 7 May 2008
Links to the science
Cochrane Database Syst Rev 2006, Issue 3
Cochrane Database Syst Rev 2004, Issue 4
Cochrane Database Syst Rev 2005, Issue 2
Cochrane Database Syst Rev 2003, Issue 4
JAMA 2008; 299:2037-2047