This study investigated changes in glycemic control following a small increase in time spent outdoors. Women participants with type 2 diabetes (N = 46) wore an iBUTTON temperature monitor and a pedometer for 1 week and recorded their morning fasting blood glucose (FBG) daily. They went about their normal activities for 2 days (baseline) and were asked to add 30 min of time outdoors during Days 3–7 (intervention). Linear mixed effects models were used to test whether morning FBG values were different on days following intervention versus baseline days, and whether steps and/or heat exposure changed. Results were stratified by indicators of good versus poor glycemic control prior to initiation of the study. On average, blood glucose was reduced by 6.1 mg/dL (95% CI − 11.5, − 0.6) on mornings after intervention days after adjusting for age, BMI, and ambient weather conditions. Participants in the poor glycemic control group (n = 16) experienced a 15.8 mg/dL decrease (95% CI − 27.1, − 4.5) in morning FBG on days following the intervention compared to a 1.6 mg/dL decrease (95%CI − 7.7, 4.5) for participants in the good glycemic control group (n = 30). Including daily steps or heat exposure did not attenuate the association between intervention and morning FBG. The present study suggests spending an additional 30 min outdoors may improve glycemic control; however, further examination with a larger sample over a longer duration and determination of mediators of this relationship is warranted.
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Special thanks to the participants and community partners Ethel Johnson, Sheryl-Threadgill Matthews, Sheila Tyson, Keisha Brown, Clarice Davis, and Emily Ingram. All authors certify that they have participated sufficiently in the work. Funding was provided by National Institute of Environmental Health Sciences (Grant No. R01ES023029).
Conflict of interest
Molly B. Richardson, Courtney Chmielewski, Connor Y. H. Wu, Mary B. Evans, Leslie A. McClure, Kathryn W. Hosig and Julia M. Gohlke declare that they have no conflict of interest.
Human and animal rights and Informed consent
All procedures followed were in accordance with ethical standards of the responsible committee on human experimentation (institutional and national) and with the Helsinki Declaration of 1975, as revised in 2000. Informed consent was obtained from all participants prior to inclusion in the study.
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1. Flow diagram (adapted from the CONSORT flow diagram) (DOC 46 kb)
2. Decision Tree for Inclusive and Restrictive Criteria for Pedometer Step Data. 1—“Day prior”–“day of” rule applied to all days (i.e. Day2–Day1); 2—Individually evaluate days identified with original daily log (322-48 = 14.9% of person-days modified in the Inclusive Dataset; n = 59, 18.3% in the Restrictive Dataset); 3—Baseline day 2 missing so repeated day 1 values; 4—Intervention days as last day or multiple intervention last days were missing so applied average of existing intervention days were imputed (i.e. Days 6 and 7 missing then used average of Days 3,4,5); 5—Days surrounding were reset so the average of intervention days was imputed; 6—Missing day was followed by the same type of day (i.e. intervention missing and intervention day following known) then the missing day was replaced with ½ of the following day and the day following was ½ as well (i.e. If Day 3 = Missing, Day 4 = 4818, then Day 3 = 2409, Day 4 = 2409) (PDF 49 kb)
1. Results of linear mixed effects models describing the relationship between the intervention and personal temperature (daily mean hourly) or steps (inclusive criteria) adjusting for weather variables (precipitation, weather station maximum and minimum temperatures). *0.047611 (DOCX 14 kb)
2. Results of linear mixed effects models testing to screen for partial mediation by steps day prior (inclusive and restrictive criteria) (DOCX 15 kb)
3. Results of linear mixed effects models testing to screen for partial mediation by personal temperature (daily mean average and daily max average) (DOCX 15 kb)
4. Model stratified by glycemic thresholds and adjusting for individual steps and/or personal temperature (Models 1-3 Poor Glycemic Control, Models 4-6 Good Glycemic Control). *0.05548 (DOCX 16 kb)
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Richardson, M.B., Chmielewski, C., Wu, C.Y.H. et al. The effect of time spent outdoors during summer on daily blood glucose and steps in women with type 2 diabetes. J Behav Med 43, 783–790 (2020). https://doi.org/10.1007/s10865-019-00113-5
- Fasting glucose
- Time spent outdoors
- Ambient temperature
- Physical activity