Fasted Workouts: The Key to Faster Weight Loss
Fasted exercise is beginning to gain a lot of attention in the fitness community as a method for weight loss. The idea behind this phenomena is that by having less glucose in the blood to be used for energy, the body must burn fat in order to fuel exercise. The increase in fat oxidation can help with weight loss by using up stored fatty acids. With continuous fasted exercise, there are some changes that the body makes in order to make this fat oxidation easier.
Here are a few key things that happen in the body in response to fasted exercise:
Increase in expression of genes related to fat break down.
Pyruvate dehydrogenase kinase 4 (PDK4) is a gene that encodes an enzyme responsible for inhibiting pyruvate dehydrogenase activity. Pyruvate dehydrogenase is a key component of glucose metabolism in which glucose from food is transformed into cellular energy. A decrease in pyruvate dehydrogenase inhibits glucose metabolism which results in greater usage of stored energy (fat; Dohm, Beeker, Israel & Tapscott, 1986). Hormone-sensitive lipase (HSL) is an enzyme that plays a role in the hydrolysis of fats. HSL is found in adipose tissue where it turns triglycerides into fatty acids which can move into the bloodstream as free fatty acids and be used for energy. The increase in the expression of these genes indicates that stored fat is being used to fuel metabolism instead of carbohydrates (Chen et al., 2017).
Maintenance of blood glucose levels preserves capacity for exercise.
Generally speaking, our body relies on glucose from food as the main source of energy. During exercise, we required more energy and thus more plasma glucose must be shuttled into cells to be used to fuel our workout. This decrease in plasma glucose levels can result in symptoms related to low blood sugar, even in people who are not diabetic. Keeping this in mind, one might expect that fasted exercise might lead to hypoglycemia faster than the fed state due to the already lower levels of plasma glucose. This is, however, not the case. One study found that plasma glucose levels remained more stable throughout a workout in people who had fasted beforehand versus people who had eaten. This is probably the result of an increase in the use of stored glycogen and fat for energy (Dohm, Beeker, Israel & Tapscott, 1986).
Increase in metabolic rate.
Adrenaline (also known as epinephrine) is part of our body’s fight or flight response. This hormone works to increase metabolic rate and promote fat breakdown in times of stress (Peters, Dyck, Bonen & Spriet, 1998). In the case of fasting, it is believe that the increase in adrenaline may be in order to increase energy levels to find food. The increase in adrenaline stimulates adipose tissue breakdown and the burning of fat for energy (Palmblad et al., 1977).
Increased use of fat in muscle tissue.
Intramyocellular lipids are droplets of fat that are stored as triglycerides in our muscles during rest. Normally our muscles rely on glycogen to power us through our workout however, when the glycogen stores are depleted (as they are in the fasted state), muscles rely more heavily on intramyocellular lipids as a source of energy. With continuous fasted exercise, the oxidative capacity of our muscles improves which makes the breakdown of these fat droplet much easier (Loon, Koopman, Stegen, Wagenmakers, Keizer & Saris, 2003).
Easier recovery and muscle growth.
Human growth hormone is secreted by the pituitary gland and promotes the synthesis of lean muscle and the storage of glycogen and fat. Secretion of this hormone is increased during fasted exercise relative to exercise alone. The effects of growth hormone are far reaching as it is broken down into many growth factors by the liver. Growth hormone can increase muscle strength by stimulating collagen synthesis, enhance weight lose by stimulating lipolysis, and promote bone growth. Changes promoted by growth factors can make recovery following a workout easier and more efficient (Ho et al., 1988).
Greater capacity to use stored fat.
Insulin is an enzyme that is secreted by the pancreas following a meal. The function of this enzyme is to lower plasma glucose levels by promoting the storage of glucose in cells. Insulin also works to block fat loss; when insulin levels are high, fat is unable to leave adipose tissue and is thus not able to be used as a source of energy. During fasted exercise, insulin levels are decreased due to the relative lack of glucose entering the body. This results in the ability to utilize fat stores for energy and also helps with insulin sensitivity which can have a protective effect from insulin resistance which leads to diabetes (Van Proeyen, Szlufcik, Nielens, Ramaekers & Hespel, 2010).
In general, exercising in the fasted state promotes the release of stored fat into the blood which is then used as a source of energy. With continuous fasted exercise, our body makes changes to become better equipped to breaking down fat and using it for energy. This increase in fat oxidation can promote weight loss, protect against type 2 diabetes and help with post-workout recovery. Although the research is still contradictory, there does not seem to be any effect on performance. For these reasons, fasted exercise may be a viable manner to increase fat loss from a given workout and promote weight loss without sacrificing the ability to train at high intensities.
Related Article: Does the Keto Diet Affect High-Intensity Exercise Performance?
You Might Like:
Chen, Y. C., Travers, R. L., Walhin, J. P., Gonzalez, J. T., Koumanov, F., Betts, J. A., & Thompson, D. (2017). Feeding influences adipose tissue responses to exercise in overweight men. American Journal of Physiology-Endocrinology and Metabolism, 313(1), E84-E93.
Dohm, G. L., Beeker, R. T., Israel, R. G., & Tapscott, E. B. (1986). Metabolic responses to exercise after fasting. Journal of Applied Physiology, 61(4), 1363-1368.
Ho, K. Y., Veldhuis, J. D., Johnson, M. L., Furlanetto, R., Evans, W. S., Alberti, K. G., & Thorner, M. O. (1988). Fasting enhances growth hormone secretion and amplifies the complex rhythms of growth hormone secretion in man. The Journal of clinical investigation, 81(4), 968-975.
Palmblad, J., Levi, L., Burger, A., Melander, A., Westgren, U., Schenck, H. V., & Skude, G. (1977). Effects of total energy withdrawal (fasting) on the levels of growth hormone, thyrotropin, cortisol, adrenaline, noradrenaline, T4, T3, and rT3 in healthy males. Journal of Internal Medicine, 201(1‐6), 15-22.
Peters, S. J., Dyck, D. J., Bonen, A., & Spriet, L. L. (1998). Effects of epinephrine on lipid metabolism in resting skeletal muscle. American Journal of Physiology-Endocrinology And Metabolism, 275(2), E300-E309.
Loon, L. J., Koopman, R., Stegen, J. H., Wagenmakers, A. J., Keizer, H. A., & Saris, W. H. (2003). Intramyocellular lipids form an important substrate source during moderate intensity exercise in endurance‐trained males in a fasted state. The Journal of physiology, 553(2), 611-625.
Van Proeyen, K., Szlufcik, K., Nielens, H., Ramaekers, M., & Hespel, P. (2010). Beneficial metabolic adaptations due to endurance exercise training in the fasted state. Journal of Applied Physiology, 110(1), 236-245.