Ethos Ho, BSc Pharm Student
“If you take your foot off the gas one day a week, but then, six days a week, you have it pounded to the metal, you’re not necessarily doing yourself a whole lot of good.” – Dr. Stephen Freedland, associate professor of urology and pathology at the Duke University Medical Center in Durham, North Carolina.
What is Intermittent Fasting?
Intermittent fasting is a dietary strategy that involves alternating between periods of fasting and eating during the week.1-5 It’s one of the most recent weight loss fads to hit the health and medical field. However, many healthcare practitioners remain skeptical about its safety and effectiveness. This weight loss method gained significant media attention when a BBC Horizon documentary in the UK aired in August 2012. This documentary outlined an intermittent fasting approach called the 5:2 diet that involves 5 days of regular eating habits followed by 2 days of fasting (max 500kcal for women and 600kcal for men).4
Intermittent fasting, alternate-day fasting (ADF), routine-periodic fasting, and intermittent energy restriction (IER) are all different forms of something called total caloric desistance (TCD). TCD may offer an efficient and effective way to promote weight loss and improve overall health.2,4 As mentioned before, intermittent fasting typically involves 1-4 days where energy intake is severely restricted for short periods.5 During periods of regular eating (larger energy intake), there may or may not be restrictions placed on the types and amounts of foods and beverages consumed.5 This method is very different from most conventional diet strategies – also known as continuous energy restriction (CER). In CER, diets are restricted all-day long, seven days a week to continuously keep energy below what’s required to maintain weight for extended periods. Additionally, CER strategies often involve restrictions on the types of foods consumed (e.g. carbohydrates and trans fats).5
How Does Intermittent Fasting Produce Weight Loss?
It has been suggested that there are distinct pathways involved in intermittent fasting and continuous energy restriction that produce weight loss.2 Possible explanations for this include:2
- During intermittent fasting the body may switch to directly using stored fat as an energy source, reducing adipose (fatty) tissue and resulting in a small, long-term reduction after each fasting episode.
- Nutritional stress during intermittent fasting results in repairs at the cellular level, functional optimization, and metabolic rejuvenation. This may improve long-term cardiovascular health and glucose metabolism.
Additionally, intermittent fasting may reduce markers of inflammation and increase the production of ketones. As previously mentioned in one of our previous pieces, ketones are released into the blood when the body is burning fat rather than glucose for energy.1 In addition to the weight loss benefits of the ketogenic diet, ketones have been shown to preserve memory, learning functions and slow the progression of disease in the brain.1
Intermittent fasting can also produce similar effects as intensive exercise by increasing heart rate variability while reducing resting heart rate and blood pressure.1 These changes are also seen in trained athletes, providing potentially important clues about the underlying benefits of fasting in humans.1
What Does the Evidence Say?
Many of the benefits of intermittent fasting have only been observed in animal studies, making it difficult to draw conclusions that apply to people. Three well-controlled trials in humans found benefits for fasting weight and other risk-related outcomes (e.g. cardiovascular risk), while two lower-quality studies found an association with a lower prevalence of coronary artery disease and diabetes.2
A review that examined studies of intermittent fasting programs found that alternate-day fasting was effective in improving several cardiovascular risk factors, such as a reduction in total cholesterol, triglycerides, and low-density lipoprotein (LDL) cholesterol (bad cholesterol).3 Increases in high-density lipoprotein (HDL), or good cholesterol, have also been observed in some studies.3 Many of the studies in the review didn’t address the effect of fasting on insulin sensitivity.3 However, one study found no changes in insulin sensitivity in obese individuals after 8-weeks of alternate-day fasting.3 With regards to whole-day fasting, which involves 1-2 days of complete food abstinence (and the primary focus of this article), consistent reductions in body weight and body fat were observed in the studies.3 Interestingly, when the total caloric restriction was equal between subjects who were fasting (i.e. intermittent fasting) and those who were on continuous calorie restriction, the losses in body weight and body fat were not different between the groups.3 This may suggest that both methods are equally effective in terms of producing weight loss.
Another study found that intermittent fasting appears to produce similar effects to continuous energy restriction in reducing body weight, fat mass, fat-free mass and improving glucose balance, and may reduce appetite.5 In this particular study, this led to the conclusion that intermittent fasting is a valid strategy that is not superior to continuous energy restriction for weight loss.5
What Are the Risks of Intermittent Fasting?
Although it appears that intermittent fasting is a viable option for weight loss, there are safety concerns with the diet that should be addressed. For those who are already on a diet that’s lacking vitamins and protein, eating less food could lead to vitamin deficiency and muscle loss.1 However, this can be offset with multivitamins, proper protein intake and engaging in strength training.1 Also, if fasting is taken to the extreme, it can lead to malnourishment and semi-starvation.1,3 There is a risk that semi-starvation can result in an abnormal increase in appetite, leading to an increase in fat mass beyond initial levels.3
In very low-calorie diets (<800 kcal/d), there is an increased risk of nutrient deficiencies and electrolyte abnormalities.3 It’s important to note that very low-calorie diets don’t produce greater long-term weight loss than diets with less-severe restriction.3 Additionally, a reduction in energy intake can trigger a variety of biological adaptations that can promote weight regain after weight loss.3
The well-known risk of protein deficiency and suboptimal protein intake is a concern with fasting.3 Therefore, appropriate medical supervision is recommended in individuals that are implementing intermittent fasting.3
The Bottom Line
Intermittent fasting is a broad dietary strategy that involves alternating periods of fasting and eating in a variety of patterns. It’s an emerging trend that appears to be an efficient and effective method for weight loss, though many healthcare practitioners remain uncertain about its long-term benefits and safety. Most studies examining the potential benefits of intermittent fasting have only been performed on animals; however, emerging evidence demonstrates consistent benefits in reducing body weight and body fat, as well as reducing cardiovascular risk factors. Some studies have even shown that intermittent fasting is equally effective as continuous energy restriction, making it a valid option for those who are trying to lose weight. Of course, there are risks associated with fasting, such as poor nutrient and protein intake, as well as malnourishment. Therefore, appropriate supervision is recommended before this type of diet is initiated.
As always, we hope you took away something valuable from this piece. If you have any questions or concerns regarding this article or others, feel free to reach out to us on Instagram, Facebook, or at firstname.lastname@example.org with your feedback. We’d love to hear from you.
- Collier, R. (2013). Intermittent fasting: the next big weight loss fad. Canadian Medical Association Journal, 185(8), E321-E322. doi: 10.1503/cmaj.109-4437
- Horne, B., Muhlestein, J., & Anderson, J. (2015). Health effects of intermittent fasting: hormesis or harm? A systematic review. The American Journal Of Clinical Nutrition, 102(2), 464-470. doi: 10.3945/ajcn.115.109553
- Tinsley, G., & La Bounty, P. (2015). Effects of intermittent fasting on body composition and clinical health markers in humans. Nutrition Reviews, 73(10), 661-674. doi: 10.1093/nutrit/nuv041
- Jane, L., Atkinson, G., Jaime, V., Hamilton, S., Waller, G., & Harrison, S. (2015). Intermittent fasting interventions for the treatment of overweight and obesity in adults aged 18 years and over: a systematic review protocol. JBI Database Of Systematic Reviews And Implementation Reports, 13(10), 60-68. doi: 10.11124/jbisrir-2015-2363
- Seimon, R., Roekenes, J., Zibellini, J., Zhu, B., Gibson, A., & Hills, A. et al. (2015). Do intermittent diets provide physiological benefits over continuous diets for weight loss? A systematic review of clinical trials. Molecular And Cellular Endocrinology, 418, 153-172. doi: 10.1016/j.mce.2015.09.014