New research suggests that late evening training sessions can alter gastrointestinal function. Sports Performance Bulletin investigates
In recent years, scientists have established that the day/night diurnal rhythm and sport/exercise performance have a strong relationship, mutually influencing each other, both positively or negatively. We know for example that daily biorhythms have a profound impact on various aspects of performance (see
this article for a detailed discussion). But it also works the other way round; the timing of exercise can also affect sleep quality.
For example, one study on athletes found that the highest scores for sleep quality were for those who had exercised vigorously in the morning compared to later in the day
(1), while other researchers found that both the exercise intensity and time of exercising were important in determining sleep quality, and that evening exercise became a problem when intensity was high
(2). Given that sleep patterns are profoundly influenced by hormone levels in the body, and also that the gut plays a major role in mediating levels of certain hormones in the body, some researchers have wondered if the timing of exercise not only affects sleep, but can also affect gut function?
The research
To try and answer this question, Australian researchers at Monash University, Victoria have just published a new study on exercise timing and digestive system function in the journal
‘Medicine and Science in Sports and Exercise’(3). In this study, the impact of exercising during the day (9am in bright daylight) and during the evening (9pm in artificial lighting) was assessed in 16 runners. On two separate occasions, the runners completed a 3-hour treadmill run at a low-moderate intensity (60% VO2max). Blood samples were collected from the subjects pre and post-exercise, and during the recovery period in order to determine plasma concentrations of cortisol, and catecholamines (both stress hormones). They also looked at various other markers of physiological stress including inflammatory proteins known as cytokines. In addition, the ‘orocecal transit times’ (OCTT – the time taken for food to transit from the mouth to the start of the large intestine where digestion is completed) was determined by lactulose challenge test administered at 150 minutes into the run, along with any gastrointestinal symptoms that followed.
The findings
The key finding was that levels of cortisol increased substantially (by 182%) pre to post-exercise when the runners completed the run in the evening. During the morning however, there was a barely perceptible increase in cortisol (just 4%). Inflammatory markers however were seemingly unaffected by the run timing. Another key finding was that the OCTT was very significantly increased (by over 62%) in the evening trial compared to the morning trial. A knock on effect of this was that the evening run produced significantly higher scores for gastrointestinal symptoms compared to the morning run.
Practical implications
The researchers concluded that late evening exercise DOES appear to produce greater gastrointestinal functional perturbations and symptoms compared to the same duration and intensity of exercise in the day – even though there are no known (yet) daily rhythms in gastrointestinal function. In practical terms, these findings suggest that athletes with sensitive tummies and who are prone gastrointestinal symptoms should try to schedule longer training sessions earlier in the day. If that’s not possible, food taken after training should be light on the tummy and easily digestible to reduce the risk of any GI symptoms.
References
- Sleep Medicine 2014. 15(7), 755-761
- Zhonghua Yu Fang Yi Xue Za Zhi. 2013 Jun;47(6):542-6
- Medicine & Science in Sports & Exercise: October 2020 - doi: 10.1249/MSS.0000000000002546