Fasted High-Intensity Interval and Moderate-Intensity Exercise Do Not Lead to Detrimental 24-Hour Blood Glucose Profiles

Sam N. Scott; Matt Cocks; Rob C. Andrews; Parth Narendran; Tejpal S. Purewal; Daniel J. Cuthbertson; Anton J. M. Wagenmakers; Sam O. Shepherd


J Clin Endocrinol Metab. 2019;104(1):111-117. 

In This Article


This study examined the effects of a fasted bout of HIT and MICT on acute and 24-hour glucose levels in people with T1D. The most important findings are that[1] there was no difference in the effect of HIT on 24-hour glucose profiles compared with 30 minutes of MICT,[2] both HIT and MICT performed after an overnight fast do not increase the incidence of or time spent in hypoglycemia over the 24-hour or nocturnal period in comparison with a day of no exercise, and[3] blood glucose concentration remained stable during a bout of fasted HIT and during 30 minutes of MICT. This suggests that a single bout of HIT or 30 minutes of MICT can be carried out after an overnight fast in people with T1D with no increased risk of hypoglycemia or hyperglycemia. In addition, if the starting glucose is between 7 and 14 mmol/L, our data suggest that there is no need to ingest carbohydrate during and after HIT and after 30 minutes MICT in the fasted state.

The current exercise guidelines for people with T1D report that aerobic exercise decreases blood glucose levels if performed during the postprandial period with insulin administration.[1] This is supported by a systematic review and meta-analysis[3] that aggregated the results from 10 studies to estimate the rate of change in glucose concentration in response to different types of exercise in people with T1D. García-García et al.[3] reported that individuals typically experience a rapid decline in glycemia during continuous exercise (−4.43 mmol/L/h on average), whereas the response to intermittent high-intensity exercise is more varied and dependent on the protocol. In contrast, our results showed that blood glucose concentration remained stable during both HIT and MICT, and the CGMS data showed no increased risk of hypoglycemia over the 24-hour period. It is likely that our results do not agree with the findings of García-García et al.[3] because our study was performed in the morning after an overnight fast, whereas their analysis did not control for time of day or nutritional status. Indeed, the most marked drop in blood glucose concentration among the publications included in their meta-analysis was by Yamanouchi et al.,[13] who reported a mean drop of 4.3 mmol/L after a 30-minute walk at < 50% maximum oxygen consumption after breakfast.

The exercise guidelines published by Colberg et al.[1] recommend that exercising while avoiding food may produce a lesser decrease or even a small increase in blood glucose concentration. The evidence to support this recommendation, however, is based on only one study that investigated the effects of fasted resistance training on glycemia with no control day as a comparison.[14] We provide evidence that blood glucose concentrations are stable after both HIT and MICT when undertaken after an overnight fast and with the duration of MICT limited to 30 minutes. The findings of the current investigation should therefore be used to inform future exercise guidelines. Our observations are in line with the international consensus guidelines that no extra carbohydrates should be taken during 30 minutes of MICT under low insulin conditions.[12]

The use of CGMS allowed us to compare the complete 24-hour glucose profiles under dietary standardization but otherwise free-living conditions to assess whether there is a delayed response in the risk of hypoglycemia after HIT. Fear of hypoglycemia during and after exercise, as well as during the nocturnal period, is a major barrier to exercise for people with T1D, so it is essential to objectively establish whether exercise increases the risk of hypoglycemia. Here we found no differences in the time spent in level 1 (≤3.9 mmol/L) or level 2 (≤2.9 mmol/L) hypoglycemia in both the nocturnal and 24-hour period after either HIT or MICT compared with a day of no exercise. The food diaries that participants completed indicated that they consumed the correct food, and there was no difference in the amount of additional carbohydrate consumed to prevent hypoglycemia between the conditions.

Based on our findings, it appears that exercising after an overnight fast before using short-acting insulin helps to maintain blood glucose stability irrespective of the mode or intensity of the exercise, which means that patients do not need to consume carbohydrate to avoid hypoglycemia during exercise. However, the effects of longer duration MICT sessions will have to be tested. Future research should also investigate whether exercising regularly in the fasted state improves long-term glycemic control. Indeed, Kennedy et al.[15] suggested that previous research may have failed to show glycemic benefits of exercise because calorie intake and insulin dose around the time of exercise was not controlled. Future research examining how exercise of different type, intensity, and duration carried out in the fasted state effects 24-hour glucose control is needed to help produce more flexible exercise guidelines for people with T1D.

Previous research has investigated the effects of intermittent interval exercise in people with T1D.[7,16] However, this study investigated the effects of a time efficient form of HIT in the fasted state on 24-hour glucose profile in people with T1D. The results suggest that HIT in the fasted state offers a time-efficient exercise mode that does not increase the risk of hypoglycemia, thereby potentially overcoming two major barriers to exercise. Because typical low-volume HIT protocols require a 47% to 60% lower time commitment to MICT sessions, this may make HIT a more attractive training strategy to potentially increase exercise uptake and adherence in people with T1D. The efficacy of long-term HIT programs will have to be explored in people with T1D to determine whether this is an effective and time-efficient strategy to improve health. Furthermore, the effects of HIT in the nonfasted state have not been investigated.

The major strength of this investigation lies in the strict dietary standardization during the CGMS period and the fact that the exercise sessions were performed at the same time of day and in the same nutritional state. Another strength is that by using CGMS we were able to study the individuals under free-living conditions and thereby take an ecologically valid approach to investigate glucose levels after exercise. We also acknowledge that there are some limitations that will need to be addressed in further research. First, we did not record insulin dose during the CGMS period and participants were not blinded to the CGMS, so they may have corrected their insulin dosage or may have taken carbohydrate to prevent lows if they felt it was necessary. The fact that there were no differences in food intake between the days lessens the chances that change in intake could be the cause. Second, the small sample size makes it difficult to draw conclusions that can be applied to the wider T1D community. However, the sample size is in line with previous investigations that have compared the glycemic effects of different exercise intensities in people with T1D.[17–19] Because many people with T1D lead a sedentary lifestyle,[20–22] the development of exercise programs that increase physical activity levels in these individuals is warranted. The intention of this investigation was to target a sedentary population to first demonstrate the safety of HIT. This would provide evidence to support the development of a time-efficient HIT program, of which the efficacy could be tested in a future study. Findings from this study may not be generalizable to endurance-trained individuals with T1D, and this should therefore be explored in future research. There were three occasions on which participants arrived at the laboratory with a blood glucose concentration of <7 mmol/L on one trial in which, for safety reasons, participants ingested ~20 g carbohydrates before the start of exercise. This highlights the difficulty of testing people with T1D. However, excluding these participants from the statistical analyses made no difference to the results, so their data were kept in the final analysis. Finally, the MICT was only 30 minutes in duration, so we cannot exclude the possibility that prolonged (>30 minutes) MICT sessions may lead to falls in glycemia and increase the risk of hypoglycemia. The 30-minute MICT sessions are in line with the current exercise recommendations of 30 minutes on 5 days of the week.[1]

In summary, this study demonstrated that there is no increased risk of hypoglycemia over the 24-hour period or nocturnal period after a single bout of HIT or 30 minutes of MICT in the fasted state compared with a day of no exercise in individuals with T1D. Furthermore, blood glucose concentration was unchanged across HIT and MICT when undertaken after an overnight fast and after having omitted short-acting insulin. Therefore, we recommend that, in the fasted state, provided blood glucose concentration is between 7 and 14 mmol/L, carbohydrate ingestion is not needed during HIT or 30-minute MICT sessions. Because HIT may offer a time-efficient and safe alternative for people with T1D, future research should explore the efficacy of longer-term training programs.