This meta-analysis does not reveal a glycaemic benefit of exercise in people with type 1 diabetes. However, sub-analyses suggests that exercise may confer a glycaemic benefit in the young, and when undertaken for longer periods. Whilst adverse event reporting was poor, these studies demonstrate that exercise can be undertaken by people with type 1 diabetes without significant hypoglycaemia. This is important because qualitative studies have previously reported hypoglycaemia to be a barrier to exercise in type 1 diabetes 
We used predetermined inclusion criteria designed to identify as many relevant studies as possible, and conducted this analysis in accordance with PRISMA guidelines. 
However, this study is limited by the availability and quality of the obtained data. We were unable to obtain the necessary data from all identified trials, either because the data was not presented in a way that enabled extraction for analysis or because we were unable to contact the authors of the study. There is evidence from our analysis that studies showing a negative effect of exercise on glycaemia may not have been published (or identified during our search of unpublished trials), and so that the effect size shown in this meta-analysis is possibly overestimated.
There was considerable variation in the intervention (form and intensity of exercise, duration of intervention), which may have contributed to the heterogeneity of study results. However, ADA guidance on exercise for people with type 1 diabetes is limited only to duration and intensity 
, therefore, inclusion and comparison of studies with different exercise interventions is, in our opinion valid.
There are a number of potential explanations for the findings of this meta-analysis. Firstly, the programmes of exercise may not have been of sufficient duration. This is supported by our sub-analysis which shows a trend for HbA1c reduction with longer duration of intervention. Based on pooled data from these studies, and assuming that the rate of glycaemic benefit persists in a linear fashion, we estimate that studies of greater than 25 weeks duration would be needed to obtain an HbA1c reduction in the region of 0.5%. This has implications for the design of future trials. Secondly, as has been demonstrated in some studies of type 2 diabetes, 
the intensity of the exercise program may be important. Inadequate reporting of exercise intensity in the current type 1 diabetes studies makes this difficult to analyse. Increased calorie intake, either as a source of fuel to manage hypoglycaemia or as a reward, is another possible reason why our analysis failed to detect a glycaemic benefit of exercise. The interventions in the current studies were associated with additional carbohydrate intake, and this is in line with ADA guidance as a means of avoiding hypoglycaemia. However diet in general was poorly recorded in these studies. Laaksonen et al 
did attempt to record dietary intake. In those participants where this was achieved, dietary intake appeared similar between training and control groups. There is therefore a need for studies in which dietary intake is controlled for, or calorie intake clearly recorded. There have been studies of combined diet and exercise interventions 
which were excluded from our analysis as the dietary advice was given to the training arm alone. This study did show an HbA1c reduction with combined dietary and physical activity (8.9±2.6 to 8.6±2.1% vs 8.7±2.0 to 8.8±2.3% in the control arm), although it did not reach significance. Insulin dose adjustment is another approach to avoiding hypoglycaemia around exercise, and a reduction in insulin dosage may account for the absence of a reduction in HbA1c. Of the 11 studies that reported insulin dosage pre- and post-training, five reported a decrease in insulin requirement. 
Those studies with two intervention arms 
both reported greater insulin dose reductions in the higher intensity arms. However, six studies reported no significant change in insulin dose with their training intervention and only two of these reported a reduction in HbA1c. 
These studies therefore fail to clarify whether the lack of glycaemic benefit of exercise can be attributed to changes in diet or insulin dose.
HbA1c has been used as a measure of glycaemic control in our analysis but this may not be the most appropriate measure of glycaemic control. To illustrate, glycaemic variability has been suggested to contribute to the development of microvascular complications in type 2 diabetes. 
Unfortunately, none of the exercise studies in our analysis have examined glycaemic variability. There is recent evidence from patients with insulin treated type 2 diabetes that the time spent in hyperglycaemia is reduced in the 24 hrs following exercise (without an increase in hypoglycaemia), 
. Conversely in type 1 diabetes, wide blood glucose variability has been reported around exercise in the few studies that have been conducted 
In contrast to our results, a recent meta-analysis by Tonoli et al 
reported a significant but small HbA1c lowering effect of exercise in type 1 diabetes (Cohen’s d −0.27;95% CI −0.06 to −0.47). This paper however used different criteria for study selection. It included studies with no control group, or control subjects without diabetes. We purposefully excluded these trials to control for the effect of participation in a clinical trial. Our meta-analysis also included studies that were excluded in the Tonoli analysis 
. We believe these differences accounts for the differing conclusions of the two meta-analyses. Tonoli et al 
did however agree that studies of strength training exercises showed no overall improvement in glycaemic control.
Overall there is a lack of large well-conducted studies on glycaemic benefits of exercise in type 1 diabetes. Our systematic review identified thirteen studies, from which data on 452 patients has been used for analysis. In contrast, a recent meta-analysis of exercise intervention in type 2 diabetes (which did detect an HbA1c lowering effect) analysed 47 RCTs including more than 8500 patients. 
Further research is therefore required to demonstrate a glycaemic benefit of exercise in type 1 diabetes. We would suggest the following areas are worth considering when designing this research.
- designing larger trials lasting at least six months
- trial design that is randomised and well controlled (with matched type 1 diabetic subjects and recording of dietary intake)
- examining the effect of exercise on glycaemic variability
- examining the effect of exercise intensity, and the incorporation of a dietary programme on glycaemic benefit
- examining the effect of age and duration of diabetes on glycaemic benefit
Whilst this meta-analysis did not detect a glycaemic benefit to exercise, there are other well defined benefits in type 1 diabetes. These include reduction in macrovascular risk, mortality, and improvement in wellbeing. 
Therefore, we suggest exercise should continue to play an important role in the management of type 1 diabetes, whilst its glycaemic benefits are more thoroughly investigated.