Exercise and T2DM—Move Muscles More Often!

From Nature Reviews Endocrinology

David Dunstan

Abstract

New guidelines from the American College of Sports Medicine and the American Diabetes Association provide specific exercise advice for individuals with type 2 diabetes mellitus, while new research emphasizes the importance of getting people off the couch and moving more often throughout the day.

Introduction

"If we could give every individual the right amount of nourishment and exercise, not too little and not too much, we would have found the safest way to health." Hippocrates's profound view of the importance of regular exercise has been brought to the fore in a joint position statement released in December 2010 by the American College of Sports Medicine and the American Diabetes Association, which provides new guidelines on exercise for people with type 2 diabetes mellitus (T2DM).

In the eyes of some, this position statement may conjure thoughts that we already know: that physical activity is a cornerstone in the prevention and management of T2DM.
However, these guidelines provide patients with T2DM, and those who care for them, with a timely update on new evidence generated from numerous high-quality studies conducted over the past decade on what works for people with or at risk of developing T2DM.

The recommendations call for at least 150 min a week of aerobic exercise of at least moderate intensity—brisk walking is a moderate-intensity aerobic exercise that is appropriate for most people with T2DM.
This amount of exercise should be spread over at least 3 days during the week, with no more than two consecutive days between bouts of aerobic activity.

The recommendations also acknowledge that aerobic activity alone cannot deliver the full benefits of exercise to individuals with T2DM.
Numerous recent studies have shown that resistance exercise (strength training) is a highly effective type of exercise in diabetes management and is the only type of exercise that offers some protection against the loss of muscle mass (sarcopenia) and muscle strength and physical function that occurs with advancing age.
As such, the recommendations state that, in addition to the aerobic activity requirements, resistance exercise of at least moderate or vigorous intensity should be undertaken at least twice weekly on nonconsecutive days, but more ideally three times a week.

Although these new guidelines recommend both aerobic and resistance training, few studies have directly examined the effects of combined training in individuals with T2DM.
Combined training offers the capacity to achieve the unique health benefits of both types of exercise and has the potential to be more time efficient, as the benefits of aerobic and resistance exercise can be derived from the one session rather than two separate sessions.
This consideration is important, given that lack of time is one of the most frequently cited reasons for nonparticipation in regular exercise.

A large Canadian study published in 2007 was the first to report that combined training may be of greater benefit for glycemic control than either aerobic or resistance exercise alone in patients with T2DM.
However, the additional benefit observed might have been due to the extra exercise time in the combination group (approximately double that in the single-exercise groups).
At a similar time to the release of the new exercise guidelines, Church et al. published a randomized controlled trial that sought to address this uncertainty by comparing aerobic training, resistance training and a combination of both on glycemic control in adults with T2DM; importantly, in this trial, duration of weekly training was similar across groups (approximately 150 min per week).

The main finding of the study by Church and co-investigators was that significantly improved glycemic control was observed only in the combination group, which supports the guideline recommendation that optimal physical activity programs consist of regular aerobic activity combined with resistance training. Intriguingly, combination training did not alter lean body mass, whilst the resistance training alone led to nearly a 1.0 kg increase after 9 months.
This result suggests that finding the optimal balance of resistance training and aerobic training is an area that still deserves research attention to optimize not only glycemic control but also the beneficial effects of preserving or halting the decline in lean muscle mass.

Finally, are we really addressing the full extent of the problem that exists in modern society?
Even if a person with T2DM is doing the right thing and engaging in at least 2.5 h of moderate-intensity aerobic activity and at least 1.5 h of moderate-vigorous intensity resistance activity per week, what about the remaining 164 h in the week?

To further illustrate this point, let's take a hypothetical person with T2DM who engages in the recommended levels of physical activity, who rises from their bed at 0700 h and takes a 30 min brisk walk and performs 45 min of resistance exercise prior to breakfast.
This person then sits for 15 min to eat breakfast followed by a car trip to work that takes 45 min. The next 8 h of the day is spent sitting at the office desk or sitting to eat during the lunch break. At the end of the work day, the person travels in the car for another 45 min before sitting down at the dining table to consume the evening meal. The day concludes with the individual watching their favorite television shows for the next 4 h while seated on the sofa.

If this person achieves at least 2.5 h of brisk walking and at least 1.5 h of resistance exercise during the week, according the new guidelines, this person is meeting the minimum recommended amount of exercise.
Yet, for this individual, up to 92% of waking hours may be spent in sitting activities.
The new joint guidelines highlight the emerging importance of sedentary behaviors in determining metabolic risk.
In the past decade, numerous epidemiological studies have reported inverse associations between sedentary behaviors (for example, television viewing, computer and console use, workplace sitting and time spent in automobiles) and mortality and chronic disease risk, with many showing these associations to be independent of reported exercise.
As acknowledged in the joint guidelines, these new insights on sedentary behavior suggest that consideration should also be given to decreasing sitting time and prolonged periods of sedentary behavior.
Feasible steps to break up prolonged sitting with light-intensity activity include walking down the hall to speak to coworkers instead of e-mailing, extending walking distance during trips to the break room or bathroom, and standing or even pacing when on the phone.

Since recent data from NHANES 2003–2004, obtained by objective activity monitoring (accelerometer), indicate that sedentary behaviors account for 55% of an American's typical day, reducing sedentary time (namely, avoiding prolonged periods of sitting) may provide an important adjunct health message, alongside the well-established recommendation for regular participation in exercise.