What is Intermittent Fasting?
Intermittent fasting is regulating when you eat food as opposed to what you eat. Intermittent fasting involves periods of eating and periods of fasting within a given time frame. There are many different ways that an individual can choose to intermittently fast.
For every 5 days of the week, there are 2 days where you would either restrict your calories or fast.
Alternate Day Fasting
Every other day, you will restrict your calories or fast.
You are able to eat during certain hours of the day.
You can eat during 8 hours of the day, however will fast for the other 16 hours.
You can eat during 6 hours of the day, however will fast for the other 18 hours.
Limiting calories to 750-1100 kcal within a 5 day period.
What Does the Science Say?
Studies have been done that show positive benefits of intermittent fasting, periodic fasting, and time-restricted feeding. Altering diet in this way has been shown to result in weight loss due to metabolic effects, improve insulin resistance, improve cardiovascular risk factors, improve mitochondrial health, improve stem cell regeneration, and improve DNA repair,1 cancers, and neurological disorders.2
Intermittent fasting results in a phenomenon known as metabolic switching. Normally, the body uses glucose as its primary source of energy. With fasting, the body switches to ketones which are stored in fat for its primary source of energy. With the increased amount of ketones in the bloodstream, the body is signaled to remove or repair damaged molecules and defend itself against stress. Researchers recognize these processes improve mental and physical performance as well as resistance to disease.2
A stricter 18/6 method has been studied in human populations with positive results. Researchers compared overweight individuals when they were allowed to eat from 8am to 2pm (6 hours), vs 8am-8pm (12 hours). There was no change in amount of calories burned during the day, however there was a decrease in hunger, and an improvement in fat burning during the night when the individuals were under time-restricted feeding. A shift to eating earlier in the day has also shown improvements in metabolic flexibility.3
The 16/8 method has been shown to decrease body weight by about 3% and result in an unintentional 300 calorie reduction per day on average.4
Other forms of intermittent fasting have been shown to provide greater weight loss. This is believed to be a result of greater restriction in calories with alternate day fasting compared to time-restricted feeding. Alternate day fasting for 12 weeks has been shown to decrease body weight by approximately 4-6%.4 One study done, restricted calories to 25% of an individuals baseline requirements on fasting day. This study compared exercise + alternate day fasting to alternate day fasting alone. The group that performed exercise and fasted on alternate days had a decrease in body weight of 6 +/– 4 kg. The group that performed alternate fasting only had a reduction of body weight of 3+/–1kg.
It is important to recognize that these studies have been done on adults with excess weight. The impact of intermittent fasting has not been studied in younger individuals such as teenagers or individuals within a healthy weight range. These dietary methods may also be contraindicated in individuals with certain medical conditions. Please speak to your physician before beginning any new weight loss program.
- Matson MP, Longo VD, Harvie M. Impact of intermittent fasting on health and disease processes. Ageing Res. Rev. 2017;(39)46-58. https://doi.org/10.1016/j.arr.2016.10.005. Accessed July 20, 2020.
- De Cabo R and Mattson MP. Effects of intermittent fasting on health, aging, and disease. New England Journal of Medicine. 2019;381(26):2541-2551. doi: 10.1056/NEJMra1905136. Accessed July 20, 2020.
- Peterson CM. Effect of Time-Restricted Feeding on Fat Loss and Cardiometabolic Risk Factors in Overweight Adults. US National Library of Medicine. University of Alabama at Birmingham. 2018. Updated 2020. Accessed July 20, 2020.
- Gabel K, Hoddy KK, Haggerty N, et al. Effects of 8-hour time restricted feeding on body weight and metabolic disease risk factors in obese adults: A pilot study. Nutr Healthy Aging. 2018;4(4):345-353. Published 2018 Jun 15. doi:10.3233/NHA-170036. Accessed July 20, 2020.