While it remains difficult to study how effective diet regimens are, and how they compare to each other, one stands out for being unique. While most diets recommend some foods over others, intermittent fasting does not require a person to give up any particular foods but rather dictates when they eat. So, does it work?
The idea is that our ancestors (who were generally slimmer than we are) went long periods without eating before their meals. Our bodies, therefore, evolved to have these long fasting periods. Without them, we throw off the natural equilibrium. By feeding ourselves all of the time, we are challenging the body to do something it has not evolved to do.
In a recent review of intermittent fasting, it’s benefits and evidence, the New England Journal of Medicine (NEJM) reviewed the evidence behind intermittent fasting.
The goal of intermittent fasting is to achieve periods of ketosis, when the body exhausts itself of glucose and begins converting fat into ketones. Ketones are a backup energy source used when the body does not have access to glucose. These ketones signal to the body that the body is in fasting mode. This ‘fasting mode’ is what results in a variety of downstream physiologic benefits and weight loss.
Intermittent fasting has demonstrated better results than calorie restriction (ie simply eating less) in head to head comparisons for losing weight. Additionally, people utilizing intermittent fasting have an increase in insulin sensitivity, staving off diabetes, and reduction in waist circumference. Intermittent fasting regimens decrease cholesterol levels, blood pressure, and also markers of inflammation in the body. Fasting periods lead to cell signaling that increase antioxidant defenses and repair the energy producing components in the cells. In both human and animal studies, intermittent fasting regimens have demonstrated a positive impact on memory and cognitive performance. In other words, when the body is feeding, it is so busy metabolizing food that it neglects other essential function which return during the fasting state.
Multiple animal trials and some human trials have even shown a positive impact of intermittent fasting regimens on decreasing tumor growth in cancer patients, though this requires further investigation.
Another area that requires more evidence is autoimmune disease as there is some evidence that intermittent fasting can help autoimmune conditions such as multiple sclerosis and arthritis.
How to implement it
The NEJM article describes two basic regimens for intermittent fasting. Both regimens are broken into two basic periods; the feeding period, when a person can eat, and the fasting period, when a person refrains from eating any calories.
First, there is Time Restricted Fasting with a 6- 8 hour daily feeding period:
Using this regimen, a person can have a 6-8 hour feeding period at any part of the day that is convenient for them. Most importantly, the feeding period should stay the same, and a person should not eat any calories during the fasting period.
5:2 Intermittent Fasting- 2 days of fasting (1 meal, <500 calories total) otherwise the person eats a normal diet:
During the fasting period, a person can have water, black coffee, or tea. Adding cream or sugar would disrupt the fasting state and kick a person out of ketosis, so they are not allowed. During the feeding period, a person can eat as they normally would. A person should continue to eat responsibly but can indulge themselves and have their favorite foods during the feeding period.
Examples of 500 Calories foods to be eaten in one sitting:
- Bagel with cream cheese
- Omelet with veggies and cheese
- 2 peanut butter jelly sandwiches
- Chicken breast with veggies and rice
- 2 pieces of fried chicken
These are only a few examples; there are plenty of meals that can fit into this 500-calorie limit. This 500 calories meal should occur at once, with the person fasting the rest of the day. While the regimens described above are ideal, it may take a few weeks to work yourself up to this point.
The hardest part of any diet regimen is maintenance. Ultimately the goal is to find a diet you can maintain indefinitely. Restricting when you eat rather than the foods themselves may be a formula for sustained success. Either regimen can be adapted to suit your social life, and can generally adapt to any career or lifestyle.
Should you do it?
Intermittent fasting is an effective, sustainable diet regimen with both weight loss and physiologic benefits. With close adherence to the regimen, a person can find many benefits.
de Cabo R, Mattson MP. Effects of Intermittent Fasting on Health, Aging, and Disease [published correction appears in N Engl J Med. 2020 Jan 16;382(3):298] [published correction appears in N Engl J Med. 2020 Mar 5;382(10):978]. N Engl J Med. 2019;381(26):2541-2551. doi:10.1056/NEJMra1905136
Adafer R, Messaadi W, Meddahi M, et al. Food Timing, Circadian Rhythm and Chrononutrition: A Systematic Review of Time-Restricted Eating’s Effects on Human Health. Nutrients. 2020;12(12):3770. Published 2020 Dec 8. doi:10.3390/nu12123770
Internal Medicine-Pediatrics resident at the University of Chicago and aspiring Gastroenterologist.