Energy Production in Our Bodies

First let's consider the fuels available for energy production and how they are regulated. The three possible fuels are carbohydrate (glucose), fat (fatty acids) and protein (amino acids). Since amino acids only make up 1-2% of our energy requirements in muscle tissue, whether at rest or exercising, we will not discuss them in any detail. With respect to glucose and fat use as a fuel, they will vary depending on the energy demands, whether at rest or exercising, exercise intensity and duration, the level of physical training, overall diet and of course meals taken before and/or during exercise which affect substrate availability.

At Rest In the body at rest in the fasting state the glucose concentration is kept relatively constant by the balance between 1. glucose utilization in all tissues and cells and 2. hepatic (liver) glucose production. In terms of a body budget for glucose utilization, approximately 50% is used by the brain, 30-35% by other tissues (blood cells, liver, kidneys, gonads etc) and only 15 to 20% by muscle tissues. In terms of potential energy sources (figure 1) you can see that metabolic fuels are made available by 1. breakdown of glycogen and triglycerides in the muscles 2. fatty acids released from adipose tissue (fat) and 3. glucose release from glycogen stores in the liver. The latter two are released into the blood circulation and delivered to where they may be needed, especially muscle during exercise.

If we consider just the muscles in the fasting state at rest only about 10% of the energy generated in skeletal muscle comes from glucose. That means that 85-90% is derived from the oxidation of fatty acids. This is an interesting fact and is often overlooked when discussing diet, weight loss and various forms of exercise. We tend to think only of glucose when we think of energy and foods while the potential energy reserves in fat tissue are not considered or even discussed. In terms of type 1 diabetes and exercise the reasons for this are the dangers of allowing the blood sugars to go too low (hypoglycemia) and hence the focus on blood glucose levels. In addition in type 1 diabetes there is also the inability of the bodies glucose/fat metabolism regulation to respond to the demands. The scenario for type 2 diabetics is different and will be addressed next issue.

During exercise. During exercise a number of cardiovascular, hormonal and neural responses occur in concert to ensure efficient delivery of fuels and oxygen to muscle tissue and removal of end products. The onset of exercise results in a rapid increase in glucose utilization but not as much as you might think. At 50% of our VO2 max (for most individuals this represents a brisk walk or what is called moderate intensity) our muscles are using about 50% fat and 50% glucose. Now you can see why walking, especially a moderate to brisk pace, is such a highly recommended form of exercise. It strikes a wonderful balance in terms of physical safety, cardiovascular fitness, burning fat and lowering blood sugars. And you can talk as much as you like while you're doing it.

As the intensity of exercise increases to 70-75% of VO2 max carbohydrate becomes the main metabolic muscle fuel. This intensity of exercise is comparable to a moderate to brisk jog depending on your level of fitness. As the intensity increases to 100% VO2 max nearly all energy is derived from carbohydrate oxidation with a very small (1-2%) being derived from fats and amino acids. Typically the muscles use up their glycogen stores first and then start to draw on circulating blood sugars. Blood sugars are replenished by glucose production from liver glycogen breakdown and liver gluconeogenesis (glucose synthesis) and as a result the blood sugars are kept relatively constant (refer to figure 1). At a moderate intensity these proportions of energy utilization are reliable estimates. After 2-3 hours, however, the glycogen stores can become depleted and there is a shift back towards a higher proportion of fatty acids for muscle energy.

Highly trained individuals perform the same work at a lower VO2 max than less conditioned individuals and as a result utilize less carbohydrate and more free fatty acids. Even if the conditioned individual performs at the same VO2 max they will consume less carbohydrate and a greater percentage of FFA. This means that their muscle and glycogen stores will be depleted less rapidly, they'll have greater endurance and burn more fat in the process.

Following exercise the body must replenish both muscle glycogen stores and liver glycogen stores. Therefore, even after exercise, the muscles continue to take up glucose and in this case convert it into glycogen. This will happen very slowly in the absence of food and more quickly with carbohydrate intake; normal glycogen levels being established within 12 to 24 hours. Muscle glycogen replenishment occurs more rapidly than liver glycogen replacement. This post-exercise demand for glucose is the cause of post-exercise hypoglycemia. The best solution is frequent BG testing and appropriate food intake. Some will recommend BG testing at 1-2 hour intervals following exercise in order to determine how your body responds to exercise. Accurate records of this information allows you to make the appropriate adjustments. A key piece of advice is to always err on the side of caution. That means taking in additional carbohydrate or reducing insulin and accepting a BG reading higher than target rather than going low. This is especially true for post-exercise in the evening and bedtime when a low BG can be more than just an inconvenience but can actually be lethal. Be particularly cautious if you are just beginning an exercise program or exercise irregularly since BG values and your body's response are even more difficult to predict.

Regulation of all these metabolic processes during and after exercise is highly integrated and beyond the scope of this article. Even though we will focus primarily on insulin many hormones are involved and ensure that fluid and electrolyte balance is maintained in addition to fuel balance.

The Role of Insulin. As mentioned earlier hormones play a major role in the regulation of these metabolic processes. Insulin in particular has several roles. First of all, in the non-diabetic insulin secretion is actually suppressed during exercise thus reducing the amount of glucose uptake by non-exercising tissue. Secondly, since insulin suppresses liver output of glucose and inhibits lipolysis in fat tissue, falling insulin levels result in an increase in both liver glucose production and release of fatty acids into the circulation providing more energy for working muscles. Since little or no insulin is needed in exercising muscle a lower plasma insulin level does not adversely affect glucose utilization while exercising. A note of interest here is that there are glucose transporters in muscle tissue that do not require insulin which are activated during exercise. This partially explains the ability to exercise with reduced levels of circulating insulin.