This longitudinal analysis of registry-based data documented between 2018 and 2022 reveals that the two pandemic years were not associated with systematic detrimental consequences on weight and glycemic control in our population with T1D. However, the individual responses of people with T1D to the COVID-19 pandemic, in terms of BMI or HbA1c-changes, differed widely.

Considering only the adjusted mean changes, BMI trends were not affected by the pandemic, except a slight increase in prepubertal children. Nevertheless, beyond the mean, it appears clearly that the first year of the pandemic has been accompanied by more individuals with BMI increases or decreases, and that this greater heterogeneity particularly affected children and adolescents < 16 years. With the second COVID year, the proportion of individuals with stable BMI increased again, but we observed a larger proportion of prepubertal children with weight gain, as well as a larger proportion of pubertal girls with weight loss. While increasing before COVID, mean HbA1c trends stabilized during the pandemic in all subgroups or even improved in women. At individual level, no significant change was observed in adult men, but in all other subgroups, the proportion of individuals with increasing HbA1c decreased.

Whereas some studies performed in the general population have found a greater BMI increase in all children during the pandemic compared to previous years [3, 5], other found that this was more pronounced or, like in our results, restricted to prepubertal children [8, 19]. In general, the accelerated increase in BMI in children due to the pandemic has often been explained as a consequence of reduced physical activity and altered eating habits due to the widespread closure of schools and leisure or sport facilities during the lockdown [3]. In line with these studies, we found a higher proportion of children with weight gain, but the average excess BMI-Z-score gain related to the pandemic in this age group was very small. It is possible that the impact of the COVID-lockdown on children’s weight development has been attenuated in the presence of diabetes. This disease being part of well-documented risk factors for worse COVID outcomes, parents of children with this chronic condition could have paid particularly attention to alimentation and/or to physical activities during the pandemic [20]. In addition, working from home, which was widespread during the lockdown, may have made it easier to prepare healthy meals, and improved parental supervision. Moreover, children with diabetes and their parents may have benefited from more frequent medical consultations and health advices than people without chronic disorder. Thus, T1D could have constituted a moderating factor against weight gain in this time period when compared to the general population.

Regarding older children and adults, longitudinal studies in the general population which have taken a pre-pandemic control period into consideration indicate that BMI trends were not modified by the pandemic [9, 10, 21]. Our findings in adolescents and adults with T1D confirm these results. In pubertal girls however, even if the proportion with weight gain was still greater that the proportion with weight loss after two years pandemic, about one third more have lost weight compared to the year before. This aspect is not apparent when only the adjusted mean values are evaluated. These results are in line with many other studies indicating more cases of eating disorders, in particular in young women, in conjunction with the pandemic [6, 7, 22].

Our findings indicate a stabilization or even an improvement of glycemic control in the second COVID-year in both children and adults with T1D. During the pandemic, the proportion of individuals with increasing HbA1c fell in nearly all subgroups. Few studies found no significant changes in children [11] or only very slight changes in adults [10], but a greater number of analyses [23, 24], including three systematic reviews [5, 25, 26], found better glycemic control, in line with our results. To explain the possible positive effect of the lockdown on glycemic control in T1D, several hypotheses have been formulated [5]. In particular, the stay-at-home orders may have been associated with more time, not only for diabetes management, but also for self-care, healthy meals and exercise. Moreover, the lockdown could have enabled a more predictable daily routine, with more regular meal and sleep times [27]. Last but not least, the increasing use of telemedicine during the pandemic [28] combined with a widespread use of diabetes technology (in particular, continuous glucose monitoring [CGM] and automated insulin delivery [AID]) in T1D seems to have contribute to an effective diabetes management despite lockdown [11].

A limitation of this study is that we did not take the use of diabetes technology into account, although diabetes technology and CGM in particular is associated with improved glycemic control [29]. For example, a more frequent use of CGM during the pandemic could have improved glycemic outcomes. Another bias could have resulted from potential differences in frequency of weight and HbA1c measurements during the pandemic compared to the period before. However, a recent study from Germany indicates that access to healthcare did not change considerably during the pandemic for children and adolescents with T1D [30]. For our study population, BMI and HbA1c were documented only slightly less frequently during the pandemic compared to the pre-pandemic period (in mean: 3.6 instead of 4.2 times per year for BMI, 3.1 instead of 3.4 times per year for HbA1c). Nevertheless, a possible selection bias could have affected the results, if individuals with worse glycemic control had fewer medical visits or less health care utilization during the pandemic than those with better glycemic control. Inversely, one could consider that especially individuals with worse glycemic control were more prone to visit their diabetes center during the pandemic than other individuals with better health outcomes. A strength of this longitudinal analysis is the use of repeated measurements over four years, which allows the comparison of trends between the COVID and the pre-COVID periods. In addition, real world evidence could have been provided by the use of a nationwide registry comprising about 90% of children and adolescents and about 30% of adults with T1D in Germany. Moreover, the report of the distribution of individual weight and HbA1c changes over the years provides useful information to understand the real impact of two years COVID in people living with T1D.

To summarize, this longitudinal analysis contributes to reduce the concerns over potential detrimental impact of the COVID pandemic on health outcomes of individuals with T1D. First, the pandemic seems to have been associated with perturbations or instability in terms of weight and glycemic control at the individual level, but eventually, in the second COVID year, these variations have been significantly reduced in all subgroups. It is possible that most individuals progressively adapted their lifestyle to the pandemic situation with its lockdown restrictions, and that the psychological impact of the pandemic consequently reduced over time. In addition, in our large population with T1D, the pandemic was not associated with clinically relevant changes in BMI trends, but with a slight improvement in the HbA1c trend. Stay-at-home orders combined with the fear of adverse COVID outcomes in individuals with diabetes may have encouraged a heathier lifestyle [5]. In addition, the development of telemedicine [28] associated with the increasing use of diabetes technology may have played a positive role during the pandemic.

In conclusion, even if the COVID pandemic did not affect weight and glycemic control negatively in individuals with T1D on average, our data indicates greater variation in some subgroups, in particular children and adolescents. Since weight and glycemic variability are associated with adverse diabetes-related outcomes [31, 32], it is important to pay attention to these changes to improve the care of vulnerable groups in the future.