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Intermittent Fasting (commonly referred to as IF) is a popular diet protocol used to help people lose weight by restricting when they eat instead of restricting what they eat. The term “intermittent fasting” is slightly misleading because “intermittent” literally means at irregular intervals, and “fasting” refers to not eating food for a period of time, which is what most people do every day. The intermittent fasting diet is better described as time restricted feeding, where an individual will pick a specific time frame to eat and not eat. Typically, the fasting window is between 14-20 hours, and the feeding window is between 4-10 hours.
A meta analysis of the scientific literature of both human and animal studies found that time-restricted feeding led to a reduction in body weight, cholesterol, fat concentration, and insulin insensitivity, as well as mitigating metabolic diseases (1). In an 8 week scientific study, 34 resistance-trained males were split into a normal diet (ND) group and a time-restricted feeding (TRD) Group. The TRD group ate all of their daily calories within an 8 hour window with feeding times at 1pm, 4pm, and 8pm, while the ND group ate the same number of calories and macronutrients but could eat throughout the day. The results showed that the time restricted group had decreased fat mass, maintained muscle mass, and improved biomarkers in metabolic rate and thyroid function (2). Many studies have also indicated several health benefits of intermittent fasting other than weight loss, such as improvements in glucose regulation, blood pressure, heart rate, inflammation, endurance training, and abdominal fat loss (3). While what we eat is important, when we eat is also important.
The central idea behind the physiological benefits of time restricted feedings is that eating causes mild oxidative stress on your body, while fasting allows your body to repair DNA, synthesize protein, initiate autophagy, and down-regulate inflammation. Generally, when you eat carbohydrates, your blood glucose rises. Your body will produce insulin to either use the energy or store it as fat. When you eat sporadically throughout the day, your organs are forced to continuously work, which leads to higher glucose levels, higher insulin output, and higher inflammation (4). When you eat on a consistent time restricted schedule, your organs start to synchronize through peripheral clock genes, allowing your body to optimize digestive function, increase insulin sensitivity, accelerate fat metabolism, reduce inflammation, and increase your overall metabolism (5).
There are also proven psychological benefits to time restricted eating. In a meta analysis of nearly 500 articles, the overall adherence rate to time restricted eating was around 80%, which is much higher than average diet compliance which is below 50% (6) (7). Many individuals find it difficult to stick to a calorie restrictive diet because of the difficulty of portion control, underestimation of calories consumed, overestimation of calories burned, and overall mental strain of constantly making the choice to not eat something. Time restricted eating relieves much of the mental strain by removing food choices with a clear and simple rule to limit when you eat. In the same meta analysis, the study found that 20% of its participants in time restricting eating unintentionally reduced their calories and experienced an overall weight loss of 3% without consciously attempting to restrict calories (6).
There are two main methods of fasting: alternate-day restricted eating and daily time restricted feedings. An alternate day fasting protocol incorporates at least one day of low calorie intake around 500-700 total calories for a 24 hour period. The most well studied alternate day protocol is the 5:2 diet, which consists of five normal days and two fasting days. For five days of the week, you can continue to eat your normal diet of about 2000 to 2500 calories. However, on the fasting days, you can only eat 500-700 calories for the entire day. This should reduce your normal ~15,000 calorie week to ~11,000 calories, which can equate to close to a pound of weight loss per week. The benefit of this diet is that you do not have to restrict the types of food you eat but rather focus on drastically reducing your calories on two out of the seven days. This type of behavior can help you switch from burning glucose to burning ketones and fatty acids (8). However, there are several drawbacks to this diet. A 500 calorie day can leave you feeling exhausted, unfocused, lightheaded, and unable to workout. One study found that adherence to this diet program declined significantly over time, with 74% completion at 6 weeks, 31% at 6 months, and 22% at one year (9). However, those who stuck to the program experienced significant fat and weight loss.
Most people will find it easier to follow the second method of restricting their eating within a 24 hour period, because it is less disruptive and eliminates low calorie days. Daily restrictive eating protocol requires you to select recurring time periods to eat and not eat. Since most people are awake between 16-18 hours and sleep for 6-8 hours, you should try to limit your eating window as small as possible while maintaining a normal healthy lifestyle.
How long should I fast for? This is a highly individualized question that will require you to do some experimenting. On the most restrictive side, some individuals will follow the “warrior diet,” which is fasting for 23 hours and only eating one large meal within one hour each day. On the least restrictive side, people eating within a 12 hour window have seen positive results. Studies have shown that the optimal feeding window is usually between 6 to 10 hours, because eating longer than 10 hours in days will result in minimal benefits, and eating less than 6 hours tends to cause people to overeat (10) (11). Research has also found that the longer the fast, the greater the weight and fat loss benefits (12). However, there is no magic bullet to finding the ideal protocol since there are so many variables. You should consider your fitness goals: weight loss, muscle growth, or athletic performance as well as biological factors, such as gender, age, body fat, and muscle mass. Then consider behavioral patterns, including work, fitness routine, and sleep. It is also important to take into account your mental capacity of adherence, performance, and happiness. From there, you should try to slowly restrict your eating window until you find your sweet spot.
Creating A Time Restricting Feeding Schedule
Here are seven easy tips for creating a Time Restricting Feeding Schedule.
Tip #1: Wait at least one hour before eating in the morning, and stop eating 2-3 hours before going to bed.
Tip #2: Take time transitioning into this new diet by shortening your eating window by one hour per day.
Tip #3: Try drinking a glass of water with lemon and salt to temporarily curb your hunger during your fasting window. People often mistake dehydration or low salt for hunger.
Tip #4: Keep a consistent eating window, such as 12pm to 8pm. This eating window is great because you can eat lunch and dinner at a normal time, so you can enjoy meals with friends, family, or business partners.
Tip #5: Keep yourself in a fasted state by eating less processed carbohydrates and refined sugars, and eat more high protein and fibrous foods.
Tip #6: Try lowering your glucose after a big meal by going for a walk, stretching, or taking a supplement like Berberine.
For more information on intermittent fasting, check out the Huberman Podcast (13).
A note of caution: Time restricted eating may not be right for you. Those only concerned with building muscle mass and size could be limiting their muscle growth potential. Time restricted eating can also cause changes in your hormones, which can lower testosterone or create fertility issues. Prolonged fasting can lower your overall athletic performance. Lastly, time restricting eating may make you feel unhappy and unsatiated. Therefore, please make any nutritional changes carefully, and seek professional advice when necessary.
Intermittent fasting can improve multiple indicators of cardiovascular health in humans, including blood pressure, resting heart rate, levels of high-density and low-density lipoprotein (HDL and LDL) cholesterol, triglycerides, glucose, insulin, and insulin resistance. Because of this, intermittent fasting can lead to longer life spans. There is also strong preclinical evidence that alternate-day fasting can delay the onset and progression of the disease processes in animal models of Alzheimer’s disease and Parkinson’s disease (8). Whatever your reason, choosing to practice fasting in any form can serve as a great way to optimize your health and fitness lifestyle.