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A study published in Diabetologia looks at timing of physical activity, liver fat content, and insulin resistance.
Dr Katarina Kos, Senior Lecturer in Diabetes and Obesity, University of Exeter, said:
“This study is novel in that it relates the timing of physical activity if performed in the morning, afternoon or evening to insulin resistance and fat content. This is from a cohort of middle-aged Dutch people between age 45 to 65 studied 10 years ago and based on self-reports of weight and eating behaviour and who were found to be generally overweight. Participants wore an activity monitor for four days which was analysed for timing and intensity of activity. Whilst it is not known how the participation in the study influenced uptake in physical activity in the four working days assessed, the results are of interest in that if the chosen timing was in the afternoon (63% of studied population) or evening (8% of the studied population), it seemed to relate with improved metabolism when compared to the morning exercising men and women (16% of population). Whether this was due to the (timing) of activity is yet to be shown. As this is not an intervention study, further research is needed to explain the cause of the observed association. Lower liver fat was found in those exercising most, unrelated to time of activity. For now, it is important to note that middle-aged people who exercise more have an observed health benefit.”
Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:
“This study raises some interesting and possibly important research questions about the timing of exercise throughout the day in relation to blood sugar control, but arguably it doesn’t really answer those research questions. The conclusion is that “the time of the day when physical activity is performed may be relevant for metabolic health” – note the ‘may’. But the study didn’t consider directly whether this actually relates to the risk of type 2 diabetes, and the researchers (rightly) don’t go further than proposing that more research should be carried out to investigate whether there is an association between the timing of exercise and diabetes risk.
“The research didn’t look at diabetes directly – the conclusions about timing of exercise relate to a measure of insulin resistance. The lower the insulin resistance, the better the body can control blood sugar. Arguably the most interesting new finding was that insulin resistance was lower, on average, in people who carried out most of their moderate to vigorous physical activity in the afternoon or the evening, compared to people who spread this activity evenly across the morning, afternoon and evening. People who took most of this level of exercise in the morning did not show evidence of a difference in average levels of insulin resistance, compared to those who spread this activity even throughout the day. It’s important to be aware that this does not refer just to deliberate taking of exercise, on health grounds or for enjoyment (like playing sport). The exercise was measured with accelerometers attached round the participants’ chests, and it would include exercise carried out, for instance, as part of what someone did in their work or their housework, provided that was at a moderate or vigorous level.
“This timing difference was in addition to an overall observation that people who took more moderate to vigorous exercise, regardless of the time of day, had (on average) lower levels of insulin resistance. So the findings are definitely not saying that carrying out moderate to vigorous exercise in the mornings, or spread evenly across the day, has no association with insulin resistance and hence, potentially, to blood sugar control. On average, people who exercise more have lower insulin resistance, whenever they do their moderate to vigorous exercise in the day. It’s just that this association between the exercise and the insulin resistance appears to be a bit stronger, on average, in people who do more of their exercise in the afternoon or the evening.
“But there are points to take into account in interpreting this finding. First, the study was observational. This means that there are several factors that differ, on average, between people who take their exercise at different times of day, apart from the time of day when they exercise. Some of these factors might influence when in the day people exercise, or the type of exercise they do, and, independently, might be associated with insulin resistance. In the jargon, they are potential confounding factors. For example, the exercise that some people take might come from their work. Perhaps people in certain manual jobs are more likely to exert themselves physically in the mornings, because of their jobs, and it could be that some feature of their work is associated with insulin resistance, in a way that has nothing to do with the amount of physical activity involved. Because of these potential confounding factors, we can’t be certain that the time-of-exercise association with insulin resistance is a matter of cause and effect. It could be that the difference in average insulin resistance is caused by the exercise timing, but instead it might be caused by one or more of the potential confounders.
“The researchers, naturally, are aware of this possibility, and they made statistical adjustments to allow for several potential confounding factors (age, sex, simple measures of ethnicity (White or not) and level of education (recorded as high or low), alcohol consumption, smoking, a measure of the healthiness of their diet, and a measure of their total body fat). But other potential confounders probably exist – for instance, perhaps the type of work people did might be important. So, as the researchers explain, confounding, that they could not adjust for, might still exist. So it does remain a possibility that the timing of exercise in the day affects insulin resistance in a cause-and-effect way, but the study can’t show that that is the case.
“Another issue that complicates the interpretation is that the comparisons of insulin resistance between groups that carried out exercise at different times were subject to statistical variability, and in fact they weren’t measured very precisely. Thus, for instance, the average insulin resistance in those who took most of their exercise in the evening might, plausibly, have been anywhere between just 4% less than the average for people whose exercise was spread evenly throughout the day, or as much as 49% less (so about half the level in the evenly spread group), though a difference somewhere nearer the researchers’ central estimate of 25% less (one quarter less) is more likely. This statistical uncertainty means that we can’t really be sure that those who took most of their moderate to vigorous exercise in the afternoon or the evening did actually have, on average, lower insulin resistance than those who did most of that exercise in the morning. Really, the study can’t measure those time-of-day differences very clearly, and so can’t give a very precise answer to the question about how much difference the time of day makes to insulin resistance – which is another reason why the researchers are calling for further study.”
‘Timing of physical activity in relation to liver fat content and insulin resistance’ by Jeroen H. P. M. van der Velde et al. was published in Diabetologia at 23:01 UK time on Tuesday 1 November 2022.
DOI: 10.1007/s00125-022-05813-3
Declared interests
Dr Katarina Kos: “I have no conflict of interest.”
Prof Kevin McConway: “I am a Trustee of the SMC and a member of its Advisory Committee. My quote above is in my capacity as an independent professional statistician.”