Medical Blog

To understand the possible metabolic responses to exercise in diabetes, it is useful to first describe the mechanisms of glucose regulation in people without diabetes.

People without diabetes

The increased metabolic demand of exercise requires a dramatic increase in fuel mobilization from sites of storage and an increase in fuel oxidation within the working muscle. Normally, the increase in fuel mobilization for oxidation is under neuroendocrine control. During the transition from rest to exercise, the working muscles shift from using predominantly free fatty acids released from adipose tissue to a complex mixture of circulating fats, muscle triglycerides (TG), muscle glycogen and BG derived from liver glycogen.

During the initial stages of exercise, muscle glycogen is the main source of energy, but the reliance on this limited fuel source decreases as the duration of exercise increases. As a result, contributions from circulating free fatty acids and glucose in the blood stream increase to replace diminishing muscle glycogen stores. This greater reliance on liver glycogen can have dramatic effects on BG levels.

The mixture of fuel utilization differs markedly depending on the intensity of exercise. During low to moderate intensities, plasma-derived free fatty acids make up the majority of oxidized substrate. As the intensity increases, there is a greater reliance on carbohydrates. During heavy exercise, BG utilization may be as great as 1 to 1.5 g/min and this fuel source must be continuously replaced at an equal rate or hypoglycemia will ensue.The mix of fuel utilization during exercise in people with type 1 diabetes appears to be similar to that of people without diabetes, except that individuals with diabetes may have a slightly greater reliance on fat as an energy source and a slightly lower rate of carbohydrate oxidation.

To facilitate the changes in glucose delivery during exercise, pancreatic insulin secretion decreases and circulating levels of glucagon, growth hormone, cortisol and catecholamines increase. The primary role of these hormonal changes is to ensure an adequate supply of glucose for the exercising muscles. Usually, the magnitude of change in these hormones is greater with increasing exercise duration and intensity. That is, during prolonged heavy aerobic exercise (i.e. exercising for over 30 min at 60 to 80% of VO2max), the reduction in insulin secretion is more pronounced, while the release of the other glucose counterregulatory hormones is increased to a greater extent.

Even before the 19th century, it was known that BG concentrations typically decrease with endurance-type exercise in most individuals with diabetes (1). In the 1950s, the American physician E.P. Joslin emphasized the importance of regular physical activity to effectively manage his patients’ symptoms. His idea of ‘victory’ was the triad of nutrition, insulin and regular exercise to properly manage BG levels
and thus provide a life free from complications from diabetes.

For patients with either type 1 or type 2 diabetes, there are both benefits and risks of regular exercise. An individual’s unique characteristics (e.g. age, sex, psychosocial milieu), comorbid medical conditions and medications need to be considered by healthcare providers when prescribing a training program. In fact, target work rates, used to determine training intensity, require modification and defined limits in the presence of coronary heart disease, hypertension or microvascular complications. In particular, the type and intensity of exercise may need to be limited in some patients with retinopathy and neuropathy.

Given the demonstrated benefits of low- to moderateintensity exercise, with its minimal associated risks, the benefits of regular physical activity almost certainly outweigh the potential side effects in the majority of individuals with diabetes, even those with some complications from the disease.
Unfortunately, training and competition are frequently associated with either hypo- or hyperglycemia in active people with diabetes and very little is known about the effects of diabetes on athletic performance.

Aerobic vs. Anaerobic exercise

Exercise can be classified into 2 forms—anaerobic and aerobic—based on the dominant metabolic energy sources used during the activity. Anaerobic activities are characterized by higher intensities of muscular contraction. Contractions are sustained by the phosphagen and anaerobic glycolytic systems
to produce lactic acid and energy in the form of adenosine triphosphate. Anaerobic activities include sprinting, power lifting, hockey and some motions during basketball and racquet sports. Anaerobic fitness refers to the ability to work at a very high level during these activities for relatively short
periods (5 to 30 s). Aerobic activities are characterized by lower rates of muscular contraction. These contractions usually have more prolonged durations and use carbohydrates, fats and some protein for oxidation by mitochondria within the muscle. Aerobic metabolism is the primary method of energy production during endurance activities such as running, cycling, rowing, swimming, soccer and ultra-
endurance events. Aerobic fitness indicates the endurance capacity (VO2max) of the individual’s heart, lungs and muscles that allows him/her the ability to offset fatigue over the course of an activity (game, practise, competition, etc.). It is crucial to note that these and similar activities often include short bursts of anaerobic metabolism. The distinction between the 2 types of exercise is important because of their
distinct effects on BG concentration. For example, many individuals find that aerobic-type exercise causes BG to decrease both during and post-activity. On the other hand, anaerobic activities, which may only last for seconds, tend to cause dramatic increases in BG levels.