By: Erin Kukura, MS, RD
UCSD Recreation Dietitian
What is Intermittent Fasting?
Intermittent fasting (IF) has grown in popularity in recent years, but fasting has been practiced for generations in various cultures and religions. There are different variations of IF including 16:8, alternate day fasting and a few others that have restrictions around when food can be eaten and some also limit the total number of calories consumed.
The most common version of IF is the 16:8, or fasting for 16 hours and eating in a time-restricted window for 8 hours. The window can be adjusted to your schedule and typically includes when you are sleeping. For example, you would eat your last meal or snack at 8pm and then wait to eat until 12pm the following day (typically skipping breakfast).
What does the research say?
Intermittent fasting has been touted for weight loss and preliminary research has indicated IF might be linked to positive health outcomes such as improved brain health, decreased inflammation, improvements in cardiovascular risk factors, and possible links to cellular aging and longevity. However, these findings have been mostly limited to animal models and the same findings have been inconsistent in humans.
Additionally, the current research to date in humans has mostly been small sample sizes for only a short duration often without control groups. There is also a large degree of variability due to differing versions of fasting being compared making these findings difficult to make any sort of generalized recommendation.
There are also a few negative effects documented including acute increases in cortisol (a stress hormone) and variances in glucose regulation and insulin sensitivity. Energy restriction in a small group of women reported increased feelings of hunger, worsened mood and increased fatigue (2). Additionally, prolonged energy restriction in women can disrupt hormones, linked to issues with reproductive cycles and potentially, fertility (4,5).
At this time there is not enough research to support recommending IF as a health benefit. Additionally, there are potential adverse outcomes that could have larger implications for individuals that choose to partake in this eating pattern. Also, it is advised that individuals with diabetes, eating disorders, and pregnant and breastfeeding women should avoid intermittent fasting.
This way of eating also disregards the quality of your diet (ie what foods you are actually consuming) which can have a much larger impact on overall health.
Perhaps even more important is that this method of eating is another set of rules and regulations around when and how to eat. For the majority of individuals this leads to increased distress around food, worsening self-esteem, and is linked to disordered eating behaviors (alternating between periods of restriction and overeating or binging).
Keep in mind that our health is impacted by a multitude of behaviors. I encourage you to look at areas you can make long-term sustainable changes, which will make a much bigger impact on health. Likewise, leaning in to your body’s cues can be helpful to determine what way of eating works best for you and your lifestyle.
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- Horne B.D, Muhlestein J.B, Anderson J.L; Health effects of intermittent fasting: hormesis or harm? A systematic review, The American Journal of Clinical Nutrition. 2015;102:464–470.
- Harvie M, Howell A. Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behav Sci 2017;7(1):4.
- Nakamura Y, Walker B, Ikuta T. Systemic Review and Meta-Analysis Reveals Acutely Elevated Plasma Cortisol Following Fasting but Not Less Severe Calorie Restriction. Stress. 2016;19(2): 151-157.
- Shufelt C, Torbat T, Dutra E. Hypothalamic Amenorrhea and the Long-Term Health Consequences. Semin Reprod Med.2017;25(3): 256-262.
- Williams N, Leidy H, et al. Magnitude of daily energy deficit predicts frequency but not severity of menstural disturbances associated with exercise and caloric restriction. Am J Physiol Endocrinol Metab. 2015;308(1): E29-E39.