Let’s talk insulin for a second. Insulin is a storage hormone, and it gets released by beta cells in the pancreases in response to rising blood glucose. As the blood glucose raises, the pancreases secrete insulin. Insulin binds to an insulin receptor on a cell, which will cause GLUT4 proteins to translocate from its storage vesicles to the cell membrane. Once inserted into the membrane, glucose can enter the muscle cell, thus lowering blood glucose. As blood glucose lowers, the secretion of insulin gets reduced. You can find insulin receptors on muscle, adipose, and some organ cells, including liver.
When insulin binds to an insulin receptor, the cell increases the uptake of amino acids, carbohydrates, and fats. When a receptor is bound by insulin, that cell cannot breakdown fatty acids to be used as energy. The lower amount of insulin in the blood will allow a greater number of cells to be able to burn fat. For cardio, avoiding times where insulin levels are high is a good idea.
Steady state cardio is effective at burning fat for energy. It burns a higher percentage of fat during the cardio session compared to other types of cardio. There are some ways you can make sure you are burning even a higher percentage of fat, by the timing of your cardio session. As stated earlier, fat cells have insulin receptors and when bound, that cell cannot breakdown fat to use as energy. If you do steady state cardio with a lower blood glucose, less cells are bound by the hormone insulin, and more cells can be broken down, the higher the fat loss will be.
Fasted cardio is very effective at burning fat. When you just wake up, you went through multiple hours without eating food. Blood glucose at this time will be low. Doing your cardio fasted will ensure that the cardio you are doing is even more efficient at burning fat.
- Post workout
Muscle contraction during exercise is a more potent physiological stimulus of skeletal muscle glucose uptake than even maximal insulin . During exercise, muscle contractions cause GLUT4 to translocate from its storage vesicles to the cell membrane to uptake glucose, which lowers blood glucose. After your workout, your blood glucose will be at one of the lowest points it’s going to get during the day due to this contraction stimulated glucose uptake. This is a good time to do cardio.
- Prior to your next meal.
If you are eating every 3 hours, your highest blood glucose will be shortly after you eat, and each hour that goes on, blood glucose will lower. If you have 20 minutes of cardio to do, doing it 20 minutes prior to your next meal would be the best-case scenario in terms of having more fat cells being available to be catabolized as energy.
- Low carb days
On days I do not train, I eat much lower carb diet compared to training days and my total calories are much lower, usually I am hypocaloric during these days. On these days, I am eating the least amount of carbs I eat all week, my calories are low, and I will be burning more energy that I am taking in, so my blood glucose should be lower this day. Choosing to do cardio on this day is a good idea, due to the low blood glucose.
- Multiple sessions
I looked through PubMed briefly to see if there were any differences between 1 long cardio bout or the same length split up into multiple sessions, and most studies that I have read suggest that both are similarly effective. This is cool, because it can be hard for some people to sketch out 30 minutes of your day to do cardio. But if you can go on 2, 15-minute walks throughout the day, or 3, 10-minute walks, in all scenarios you are still getting in your 30 minutes of cardio.
- James D.E., Kraegen E.W., Chisholm D.J. Muscle glucose metabolism in exercising rats: comparison with insulin stimulation. American Journal of Physiology. 1985;248:E575–E580.
- Murphy, M., Nevill, A., Neville, C., Biddle, S., & Hardman, A. (2002, September). Accumulating brisk walking for fitness, cardiovascular risk, and psychological health. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/12218740