Physical activity is an integral part of the growth and development of children.¹ Exercise capacity may be limited in patients with cystic fibrosis (CF) with respiratory² or nutritional compromise.^3,4 Inactivity itself results in deconditioning, and a negative feedback loop is formed, making physical activity even more difficult.

Life expectancy among patients with cystic fibrosis (CF) has improved dramatically over the past twenty years.^5,6 However, there remains a significant gender difference in survival rates, with females with CF under the age of 20 years being significantly more likely to die than males of the same age.⁷ While nutritional status, pulmonary function, and airway microbiology were independent predictors of survival, none of these parameters account for the gender difference in survival rates.⁵

As aerobic capacity is an important predictor of survival,⁸ the fact that girls with CF have a lower peak aerobic capacity than boys with CF of the same age⁹ may be of relevance. Moreover, compared to reference values obtained in healthy girls, girls with moderate to severe CF related lung disease have a significantly lower peak aerobic capacity.⁵ This finding has been thought to be due to reduced daily physical activity levels and nutritional status in girls with CF.⁵

Maintaining high levels of physical activity is an important objective in the management of CF. Nixon and colleagues¹⁰ showed in 1992 that aerobic fitness is an independent predictor of survival in CF and that CF patients who had active lifestyles had more slowly progressive disease.

As osteoporosis has been recognised in patients with CF, it is of note that increased physical activity levels have been shown to improve bone mineral density.¹¹ Furthermore, aerobic fitness correlates well with quality of life in CF.¹² Two previous studies have shown that activity levels were similar in children with mild CF and healthy subjects.^13,14 Unfortunately, these studies had very limited sample sizes and this precluded analysis of gender differences in habitual activity. In addition, activity levels of patients with moderate to severe disease were not studied. As the principal factors that limit exercise capacity of patients with CF, such as respiratory and nutritional compromise, become evident in more severe disease, there is clearly a need to study the activity levels of this group of patients. In addition, previous studies have estimated habitual activity from self report questionnaires. The reliability of such questionnaires in children with chronic conditions such as CF is unknown.

The purpose of this study was twofold. First, we aimed to compare the activity levels of prepubescent and pubescent boys and girls with different grades of CF pulmonary disease severity and healthy controls. Second, we aimed to determine the relation between activity levels in children with CF with quality of life, aerobic capacity, anaerobic power, lung function, and nutritional status. In analysing the effect of nutrition, we sought to evaluate the impact of pancreatic function on habitual activity.

The hypotheses were: (1) that boys with CF are more active than girls with CF; (2) that children with more severe CF disease would be less active than healthy controls; and (3) that pancreatic sufficient children with CF would have higher habitual activity levels.

METHODS

RESULTS

DISCUSSION

In this study, we have shown that the habitual activities of prepubescent boys and girls with CF were similar to each other. However, after the onset of puberty, girls with CF were significantly less active than their male counterparts. This pattern of gender difference was similar in the healthy controls.

We have previously shown that the annual decline in lung function was greater in adolescent girls with CF than boys.³⁰ While girls with CF are following similar patterns as their healthy peers in being less active than boys, the consequences of reduced habitual activity may be greater. Without an interventional trial, it is not possible to tease out the cause and effect of low habitual activity. However, there appears to be a close association between gender differences in habitual activity and decline in lung function.

The best correlates with activity levels were anaerobic power, aerobic capacity, quality of wellbeing, and nutrition. The excellent correlation between anaerobic power and habitual activity is not surprising given that typically, children’s activity is performed in short bursts, and therefore a predominantly anaerobic pathway is employed by the muscles to obtain energy. This is the first study to show the clinically plausible relation between quality of life and habitual activity in children with CF.

Nutritional status correlated well with activity levels in children with moderate to severe disease. This is consistent with previously shown effects of malnutrition in severe disease, resulting in reduced muscle mass and impaired functional ability.^31–,³³ Our study has shown for the first time that there were significantly lower levels of activity, anaerobic power, aerobic capacity, and quality of life in girls with established puberty with PI compared with those who were PS. These differences were not demonstrable in boys in either of the two age groups. While it may be argued that the sample size may have been too small to detect a difference in the boys, the sample size was similar in girls where the difference was detected. It has previously been shown that boys with PI are better able to maintain nutrition than girls who have PI.^34,35 It may be that with the onset of puberty, the increased nutritional demands for muscle and bone growth result in relative deficiencies of micronutrients required for activity. These deficiencies may be exacerbated in girls by poorer oral intake and/or hormonal changes.

Compared to other habitual activity studies in CF,^13,14 this study is unique in that a large sample of children with CF and matched healthy controls were studied. Unlike in previous studies, children with moderate to severe disease were included. In addition, by using accelerometers, our study was not totally reliant on self report measures but also objective measures of habitual activity. Our study is the first to assess the relations between habitual activity and anaerobic power and quality of life.

In our study the modified Bouchard activity diary, the only activity diary that has been validated using doubly labelled water,²⁷ was used to estimate activity levels. Habitual activity data were collected for two non-consecutive weeks. The assigned energy costs for the activity diary were based on published data.^28,29 If the energy cost of a given activity for a patient with CF is greater than that of healthy controls,³⁶ this may have been a confounding variable. However, the increased energy cost of activity has only been shown in patients with severe CF disease and not in those with mild disease.^37,38 Previous studies on habitual activity in children with CF^8,9 utilised activity questionnaires to report the intensity of the activity undertaken. However, the energy cost of a given activity in patients with CF is estimated from healthy subjects. This assumption of equity in energy cost may be erroneous. In our study, data on habitual activity were obtained using both a rigorous activity diary and an accelerometer.^13,14 As the prediction equations that convert raw activity counts to energy expenditure have not been validated in CF, only the raw activity counts were presented in this study. The data from the accelerometer is invaluable in that it provides proof of activity reported in the activity diary.

Using accepted definitions,¹⁶ we studied the prepubescent and pubescent groups separately. This was done because it is clear from the work by Livingstone and colleagues²⁹ that total energy expenditure after the onset of puberty is very different to that pre-puberty. In addition, there is a clinical need to identify risk factors that may lead to the increased morbidity and mortality still seen in girls late in their teen years.^5,6

By monitoring heart rate, a previous study estimated total daily energy expenditure (not just habitual physical activity) and postulated that children with CF compensated for their increased resting energy expenditure by being less physically active during the day.³⁹ Our study and other studies^8,9 have failed to support this hypothesis. The differences in results may be due to previously shown inaccuracies in converting heart rate to energy expenditure.²⁷ Eliakim and colleagues⁴⁰ measured the effect of training on total energy expenditure in non-obese prepubescent American girls using doubly labelled water. As we only measured habitual activity and not total energy expenditure, we are unable to make meaningful comparisons. Higher levels of aerobic fitness were noted in our subjects with mild disease than studies that were performed in North America.⁹ While one may postulate that the mild climate in Australia is more conducive than North America to year round exercise/activity, there is no comparative data to confirm this.

Children with CF, who had high activity levels (top quartile for the respective gender and age group), had significantly better anaerobic power, aerobic capacity, quality of life, and nutrition, and significantly lower disease severity than children with low activity levels (lowest quartile). However, consistent with the study by Boucher and colleagues,¹³ our study found that lung function was not related to activity levels. This would suggest that high activity levels do not necessarily reflect the level of lung function and may independently improve outcome. Although our study shows a relation between activity levels and several variables, causality is not inferred.

In conclusion, gender differences in habitual activity were evident after the onset of puberty and were similar in patients with CF and controls. The impact of pancreatic insufficiency was greatest in females after the onset of puberty. The reason for this gender difference is unclear and is the focus of a longitudinal study currently in progress.