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Low levels of physical activity — like the 5,000 steps per day that Americans typically achieve, which is only about half of the recommended amount — have been implicated in the development of insulin resistance and its progression to full blown type 2 diabetes. However, the studies that suggested this connection were performed under laboratory conditions; they did not use people eating real meals, and they did not assess glycemic variability, so the direct impact of physical activity on glycemic control is not completely clear.
Elevated postprandial glucose (PPG) often precedes the development of type 2 diabetes, and is a risk factor for adverse cardiovascular events, independently of diabetes status. Thus, Dr. Catherine Mikus and her colleagues at the University of Missouri set out to precisely define the impact of reducing physical activity on PPG and glycemic variability in healthy, active volunteers as they went about their daily lives. The results of their study were recently published in the journal Medicine and Science in Sports and Exercise .
Twelve volunteers between the ages of twenty and thirty-five, eight men and four women, were recruited for participation. They were given pedometers to wear to count the number of steps they took each day, and glucose sensors were implanted under their skin and connected to continuous glucose monitors. They had to keep detailed diet and physical activity records for three days, and were instructed to maintain their regular physical activity patterns. Their glucose levels were taken 30 minutes and 60 minutes after meals. They then rested for a week, and then repeated the study protocol but this time they had to restrict their physical activity for three days. They significantly reduced the amount of time they spent walking, climbing stairs, and running — they pretty much just sat around. Four of the study participants did the protocols in reverse order — first the inactive period and then the active period — to reduce any effects of the sequence.
Only three days of reduced physical activity led to significant increases in postprandial glucose. The swings between the highest glucose levels and the lowest also increased, as did the length of time the postprandial blood glucose concentration stayed above target levels. Fasting plasma insulin was increased as well, and the researchers speculate that this may be because inactive people need more insulin to dispose of a given amount of glucose than an active person would require.
The scientists conclude that even short-term reductions in physical activity can result in robust changes in postprandial blood glucose concentrations and glycemic variability, and thus that regular physical activity plays an important role in the everyday maintenance of glycemic control. They note that although physical activity is generally assumed to improve health by promoting cardiorespiratory fitness and reducing fat, these factors are not impacted by a short-term change in physical activity. The role of physical activity in maintain glycemic control may thus be just as important as these other effects, as large fluctuations in blood glucose levels have been shown to have a number of deleterious effects that can help insulin resistance blossom into full blown type 2 diabetes. Interestingly, another study showed that reducing energy intake during reduced physical activity in order to maintain energy balance does not completely abrogate the effects of reduce physical activity on insulin sensitivity, so an energy imbalance cannot fully explain this phenomenon.
An obvious limitation of this study is its small sample size and the homogeneity of the participants. Further studies with larger cohorts are required to see if these findings hold up in a diverse population, and to see if factors like age, sex, body composition, ethnicity, type of physical activity, and glucose tolerance status impact the degree to which physical activity affects glycemic variability. But in this group, it is clear that physical activity is vital in maintaining stable glucose levels in the blood, and that reductions in physical activity can very quickly exert negative effects on glycemic control and insulin sensitivity.
Mikus et al. Lowering Physical Activity Impairs Glycemic Control in Healthy Volunteers. Med Sci Sports Exerc. 2011 Jun 28. [Epub ahead of print]