Evidence based 
Whether you’re new to intermittent fasting or if you’ve been doing it for a while, you’ve probably wondered how does intermittent fasting work or how does intermittent fasting work for weight loss? These are great questions and there are three main reasons to explain how this diet works.
The first reason is that you “teach” your body how to use your stored body fat for energy. Your body stores energy in the form of glycogen (glucose) and body fat.
When you fast, you aren’t consuming energy from food that your body can immediately use so it will instead turn to its stored energy. Your body will use some glycogen but it also wants to conserve some of it so it will also naturally start breaking down body fat for energy as well.
If this happens consistently, your body adapts and will use more stored body fat throughout the day, enabling you to lose unwanted body fat. Some scientists have called this ability “flipping the metabolic switch” and called individuals who use more fat for energy “metabolically flexible” (1).
Studies have demonstrated that individuals who are more metabolically flexible, i.e. they burn more fat than the average individual, have a decreased risk for being overweight and for having different metabolic diseases such as insulin resistance (2).
The second reason is that intermittent fasting helps to align our eating patterns with our circadian rhythm. Most of our metabolic pathways and processes adhere to a 24-hour rhythm based on the sun (3,4). To elaborate, digestive processes increase during the day when there is sunlight and downregulate during the night when there is no sunlight.
This is because our ancestors were likely not eating during the night and we evolved to conserve energy on processes we don’t utilize while we sleep to maximize our chances of survival.
Studies show that disrupting our circadian rhythm by eating late into the evening when these digestive pathways are downregulated may increase our risk to gain weight in the form of body fat as well as increase our risk for different metabolic disorders such as high blood pressure, high blood glucose levels, and high blood triglyceride levels (5–7).
Lastly, when you restrict how many hours in the day you are eating, you are likely to eat less calories, which results in weight loss. Just make sure you are still eating enough to fuel your body and give it the energy and nutrients it needs!
These three reasons explain why so many people are losing weight with intermittent fasting. However, if you are not losing weight with intermittent fasting, try paying attention to your serving sizes and try logging your food to determine how many calories you are eating each day. Intermittent fasting is an excellent tool but if you are overeating while fasting, you may still gain weight.
References:
1. Anton SD, Moehl K, Donahoo WT, Marosi K, Lee SA, Mainous AG, Leeuwenburgh C, Mattson MP. Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity. Blackwell Publishing Inc.; 2018. p. 254–68.
2. Goodpaster BH, Sparks LM. Metabolic Flexibility in Health and Disease. Cell Metabolism. Cell Press; 2017. p. 1027–36.
3. Ackermann K, Dehghani F, Bux R, Kauert G, Stehle JH. Day-night expression patterns of clock genes in the human pineal gland. J Pineal Res. J Pineal Res; 2007;43:185–94.
4. Kyriacou CP, Hastings MH. Circadian clocks: genes, sleep, and cognition. Trends in Cognitive Sciences. 2010. p. 259–67.
5. Mattson MP, Allison DB, Fontana L, Harvie M, Longo VD, Malaisse WJ, Mosley M, Notterpek L, Ravussin E, Scheer FAJL, et al. Meal frequency and timing in health and disease. Proc Natl Acad Sci U S A [Internet]. National Academy of Sciences; 2014 [cited 2021 May 6];111:16647–53. Available from: /pmc/articles/PMC4250148/
6. Jehan S, Zizi F, Pandi-Perumal SR, Myers AK, Auguste E, Jean-Louis G, Mcfarlane SI. Shift Work and Sleep: Medical Implications and Management.
7. Huang W, Ramsey KM, Marcheva B, Bass J. Circadian rhythms, sleep, and metabolism. Journal of Clinical Investigation. 2011. p. 2133–41.
Asked by: Katherine M.
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With a Ph.D. in nutritional biology, Dr. Richardson is an expert in the field of nutrition, particularly intermittent fasting. Her dissertation project was an intermittent fasting study in athletes, which is where she gained substantial knowledge on the topic. She has contributed to a number of peer-reviewed articles spanning a wide array of topics and works as a freelance writer trying to make scientific knowledge accessible to the public.
