Long term effects of smoking on physical fitness and lung function: a longitudinal study of 1393 middle aged Norwegian men for seven years

Subjects and methods

All apparently healthy men aged 40-59 who were employed in five companies in Oslo, Norway, were invited to participate in a survey that entailed a cardiovascular examination (August 1972 to March 1975). “Apparently healthy” implied absence of any known or suspected heart diseases, diabetes, cancer, hypertension treated with drugs, and miscellaneous diseases described in detail elsewhere.1 13 Of 2341 eligible men, 2014 (882 smokers and 1132 nonsmokers), accepted the invitation. An average of 7.2 years later (January 1980 to December 1982), 1757 of the 1933 men who were still alive by 31 December 1982 participated in an identical re-examination. Of these 1757 men, 364 were excluded because they had started treatment with antihypertensive drugs, developed symptoms or signs of coronary heart disease, had a stroke, developed diabetes, developed cancer, or were unable to conduct the second exercise test. This left 1393 men who were healthy according to the initial inclusion criteria and were eligible for the present study. Of the 176 men (1933-1757) who did not respond, 47 (29 smokers, 18 non-smokers) were severely diseased, 47 (24 smokers, 23 non-smokers) found their travelling distance too long, and 82 (51 smokers, 31 non-smokers) did not answer two mailing requests (later data indicated that most of these were diseased at the time of the second survey).

Examination

All examinations took place in the morning at the National University Hospital of Oslo (Rikshospitalet), and the subjects had fasted for 12 hours and had not smoked for at least eight hours beforehand. None was taking drugs, and none had intercurrent illnesses when examined. Table I shows the components of the examination. Detailed data on smoking and physical activity were obtained during interviews. The men were grouped as smokers or non-smokers—baseline non-smokers were labelled as such regardless of previous smoking habits. Men were labelled as physically active if they exercised for at least 30 minutes twice weekly to the level of breath shortness and sweating.1 Combining baseline and follow up smoking data allowed subclassification of subjects as persistent non-smokers (n=791), persistent smokers (n=347), baseline smokers who had quit (n=199), and baseline non-smokers who had started (n=56).

The exercise test was conducted on an electrically braked Elema bicycle as described previously and was continued until exhaustion if it was not stopped earlier for specified safety reasons.1 The starting load was 5.87 kJ/min and the load was increased by 2.94 kJ/min at intervals of six minutes. Physical fitness was defined as working capacity divided by body weight, with working capacity defined as the cumulated work performed during the symptom limited exercise test. Forced expiratory volume in one second was measured with a Bernstein spirometer.14

STATISTICAL METHODS

Possible group differences were tested with two sided t tests at a 5% significance level. Multivariate regression analysis was applied for studying simultaneously the influence of several factors on change in physical fitness and forced expiratory volume in one second.

FITNESS

Table III shows the results of the exercise tests, and table IV shows that baseline fitness was significantly higher among non-smokers than among smokers (P<0.001) and that the seven year drop in fitness was significantly lower in non-smokers (9% v 22%, P<0.001). This second difference was greater in the men aged 40-49 than in those aged 50-59.

For the 1138 men with unchanged smoking habits, multivariate regression analysis was used to test possible influence of age, smoking, level of physical activity and initial fitness on the seven year change in fitness. The analysis showed that high initial fitness and high age were associated with a high reduction in fitness (P<0.001) and that high physical activity was associated with a low reduction in fitness (P=0.039). When these three confounders had been accounted for, the seven year reduction in fitness was 13.6% higher among smokers than among non-smokers (P<0.001).

FORCED EXPIRATORY VOLUME

Table V shows that data for forced expiratory volume in one second showed a similar pattern in relation to smoking habits as physical fitness; that is, non-smokers had higher baseline values and a lower seven year decline.

Multivariate regression analysis of forced expiratory volume in men with unchanged smoking habits showed that high initial expiratory volume and high age were associated with a high reduction in expiratory volume (P<0.001) but physical activity was not associated with change in volume. When the two confounding factors were adjusted for, the seven year reduction in forced expiratory volume was 6.0% higher among smokers than among non-smokers (P<0.001).

PHYSICAL FITNESS

Various studies—none of which provide follow up data—have indicated a lower physical performance in smokers than in non-smokers.5 6 7 8 9 10 11 12 One major cross sectional study, which addressed this aspect by means of symptom limited exercise testing,12 produced results virtually identical to our baseline findings—that is, smoking being associated with lower heart rate response on all exercise levels, lower maximal heart rate, and lower maximal working capacity. By extrapolating from similar findings of submaximal heart rate (using the Astrand-Ryhming nomogram21), one research group erroneously concluded that smokers seemed more fit than non-smokers.22 Our results show that healthy smokers showed a substantially larger decline in physical performance than non-smokers. Men who started smoking seemed to mimic persistent smokers whereas, with due caution for small numbers, men who stopped smoking seemed to show slightly differing patterns depending on their age: those aged 40-49 seemed to benefit from stopping smoking while those aged 50-59 did so only marginally if at all.

Although our spirographic results agree with other reports of the negative effects of smoking,4 15 19 20 these observed differences in lung function alone can hardly explain the differences in fitness. Thus cardiovascular factors seem to be the main limiting factors in maximal exercise in healthy subjects, although respiratory muscle fatigue may be the limiting factor to endurance exercise even in athletes.23 The explanation for our results from the exercise tests is therefore uncertain. One possible explanation might be that binding of carbon monoxide and cyanide to haemoglobin and cytochrome inhibited oxygen delivery and metabolism in smokers. We have no data to support this notion, but the half lives of bound carbon monoxide and cyanide are sufficiently long that the eight hours' abstention from smoking that preceded exercise testing would not have eliminated this possible effect even if all the smokers had abstained from smoking. Conceivably, smoking associated impairment of pulmonary oxygen exchange may also in part explain lower maximum oxygen consumption in smokers than in non-smokers.4 16 24 Alternative but speculative explanations are down regulation of adrenergic receptors among smokers25 and long term cardiac damage caused by stimulation of catecholamine by lifelong smoking.26 27 28 29 30 Such hypotheses could be tested in populations with a low prevalence of coronary heart disease and a high prevalence of smoking.