If reaching ketosis while fasting is a proxy for fat-burning, we totally understand why you might think you need to be in ketosis to lose weight. The good news is: you don’t! Ketosis is certainly a product of accelerated fat burning, however, we still burn fat in the absence of ketones. Reaching ketosis is not necessary for fat loss.
It would be easier if we could say with authority that in any given moment you are burning either fat or sugar or glycogen, but the truth is humans are never burning one fuel exclusively. At rest, around half of our energy is generated from burning fat, the other half, mostly carbohydrates, but these contributions are influenced by the fed- and fasted-state. During a fast, we rely more heavily on body fat to support our energy needs. As a fast progresses, fat is burned at an accelerated rate, and the breakdown products of fat are used to make ketones. This is because our brain can’t use fat directly as fuel like most of our other tissues can and relies on ketones as an alternative to glucose during prolonged periods of fasting.
So, increasing ketones does correlate with increased fat burning while fasting, but, it is not required to burn fat, and therefore isn’t required to lose weight. Maintaining a Caloric deficit over time is the primary driver of weight loss, and intermittent fasting is an effective strategy for achieving this!
Parsons, S.A., Jones, K.P., Yi, F. et al. 2016. A novel clinical approach to evaluating changes in fat oxidation in healthy, overnight-fasted subjects. Transl Med Commun 1, 6.
Melzer K. 2011. Carbohydrate and fat utilization during rest and physical activity. E Spen Eur E J Clin Nutr Metab. 6(2):e45-e52.
About the Author: Kristi Storoschuk
Kristi Storoschuk is a science communicator who focuses her research on ketogenic diets, metabolic therapies, and fasting for health optimization. She currently works alongside the world’s leading ketogenic researchers providing scientific education for the mainstream audience. She completed her Bachelor of Science (Honours) at the University of Guelph in Ontario, Canada. Outside of her research, she spends her time doing CrossFit and optimizing her health with an ancestral approach.