Want to know whether you’re in energy storage mode or not? Let glucose levels be your guide.
When exercising, we are breaking down muscles and using our fuel stores. This is the catabolic process. When recovering (via the 4R’s) we are doing the opposite: repairing and replenishing. This is known as the anabolic process.
When we store energy, the body converts more simple smaller molecules (like glucose) into more complex molecules (like glycogen). This process of assembling these excess energy-rich substrates into complex energy storage molecules is called anabolism.
The opposite occurs when we need to use the stored energy. This process of converting substrates into energy-poor end-products to mobilize biologically usable energy is known as catabolism (Figure 1).
Your overall glucose level might help signal a general anabolic vs catabolic state with your body switching from production (catabolism) to storage (anabolism) mode and vice versa.
Let’s have a look at how you can use your glucose level to achieve this. We’ll also dig a bit into human physiology for clarity.
Imagine two very common situations: a recovery day and an active day.
Metabolism On A Recovery Day
This is plenty of food and not much exercise. The focus is on resting (Figure 2).
Breakfast is substantial with carbohydrates, protein and fat. Bacon and eggs, waffles and sweets.
In such a postprandial phase, glucose enters the circulation (traversing the liver) within about 15-30 minutes. After such a meal, glucose might remain elevated for another 3 to 4 hours. As the glucose leaving the intestine rises, glucose uptake in the liver increases as the liver is buffering the rest of the body from this large influx of glucose. The pancreas responds rapidly, secreting insulin. Glucagon (the antagonistic hormone to insulin) secretion is decreased.
Under such an influence (increasing glucose concentration and higher insulin:glucagon ratio) glycogen metabolism in the liver switches from breakdown to synthesis and storage.
At the same time, the increasing glucose and insulin levels act on adipose (fat) tissue to inhibit lipolysis (breakdown of fat).
At the level of the muscle, glucose utilization is stimulated by the increasing glucose and insulin concentrations: glucose uptake and utilization increases.
Muscle glycogen synthesis begins (also stimulated by insulin) given that you are not using it for immediate energy purposes.
Thus, the metabolic picture within the first one to four hours after a meal reflects an intense switch to glucose storage as glycogen, fat conservation and protein synthesis.
The body is in “storage and conservation mode” → the anabolic state. Easily reflected by a higher glucose level followed by a meal rich in carbohydrates.
But this is a recovery day and we are not training: It’s time for lunch or a snack.
Given the previous post-absorptive phase, insulin stimulated processes become primed by previous insulin stimulation. Therefore, the effect of a second meal following a carbohydrate rich breakfast will be to reinforce the pattern of substrate storage. The effect described for the first meal will occur again, but to a greater extent.
Now consider dinner in this same scenario. Another meal, more food; the body’s energy stores will be conserved and, at the same time, we are setting the scene for continued storage.
Metabolism On An Active Day
What happens if the above scenario happens in a day with more activity, requiring a substantial use of metabolic fuels?
Let’s make it a carbohydrate-based breakfast with cereals, nuts, and milk, but no bacon and eggs. Less fat and protein than before to aim for a workout in the morning.
The absorption of carbohydrates and the disposition of glucose will be pretty similar to what was described earlier, although there might be a sharper rise in glucose (and hence insulin) given the lower fat content.
An hour after breakfast, the plan is an easy swim session. Nothing intense, just a constant aerobic pace. In order to produce the extra energy required–compared to our recovery day requirements–the muscles need more fuel. This higher glucose demand blunts the rise in glucose concentration following the meal or the decline from the peak glucose concentration is quicker. The corresponding lower absolute glucose concentrations are associated by lower insulin concentration, and therefore less conservation of stores. Depending on the relative intensity of the exercise, sympathetic nervous system activity and increased adrenaline in the plasma increase substrate mobilization (glucose and fat) as well as blood flow.
The general metabolic tone is switched from one of intense substrate storage to one where substrate is diverted to the working muscle.
The next meal might be superimposed on a much less storage-primed mode. An afternoon session or even a long walk will direct more substrates into utilization rather than storage. That’s the catabolic state.
- During recovery, especially immediately after exercise, seeking a glucose rush driven by carbohydrate intake might be a strategic step to ensure proper glycogen storage and protein synthesis (given the favorable metabolic state in this moment). This has clear implications for ensuring glycogen stores are optimally loaded before key events.
- During active days, off-hours average glucose can be lower due to higher energy turnover and glucose metabolism (given the more catabolic state that we are exposing our body to). Keep an eye on this and ensure you are getting enough fuel in to ensure you are recovering appropriately, under fueling may be visible by a sustained low glucose. For more on this read about how Lisa Norden noted similar and our article on the dangers of under fueling.
- Frayn K., 2019 - Human Metabolism: A Regulatory Perspective.
- Feher J., 2016 - Quantitative Human Physiology: An Introduction