In life, timing is everything. This holds true when it comes to eating: Research shows that by carefully planning when and what you eat, it’s possible to lose fat, improve body composition, and encourage better overall health.
Scientists came to study fasting protocols from studies done on animals: Research shows periods of fasting and calorie-restriction have an anti-aging effect and extend lifespan. Studies of religious fasting showed notable health benefits in humans including less body fat, improved cholesterol levels, better metabolic health, and less inflammation. Fasting has also been shown to improve hormone balance, support cellular health via autophagy, lower blood pressure, and increase stress resistance.
Unfortunately, fasting in humans is problematic—they don’t like it much and are loath to follow the long-term fasting protocols that extend lifespan in animals. This led to the development of various eating protocols that mimic fasting, giving you the benefits without the hunger.
This article will review the research on fasting and provide suggestions for adopting a fasting protocol that is right for you.
What Is Fasting?
Fasting involves periods without food or calories, which allows your body to burn stored fat, carbs, and proteins for energy. After an overnight fast, your body relies on liver glycogen to give you energy, but if you skip breakfast and keep fasting, those stores will be depleted within about 48 hours. As glycogen stores fall, fat burning increases and the body burns ketones and glycerol for energy. It also uses byproducts of carbohydrate metabolism such as pyruvate and lactate, and burns amino acids to provide energy as well.
Why Is Fasting Popular Now?
Prior to the 21st century, fasting was relegated to religious practitioners and hunger strikers. With the surge in obesity and metabolic problems that have hit western countries, scientists are searching for solutions. When researchers looked at how most people are eating nowadays, they found that the average person is eating frequently and erratically throughout their waking hours, typically consuming calories over the course of 15 hours every day. The only food-free fasting is overnight when they are in bed. Additionally, there is a bias toward eating later in the day, with more than 35 percent of calories being eaten after 6 pm and less than 25 percent of calories consumed prior to noon—a profile that is associated with obesity (1).
What’s wrong with eating like this?
- The more time spent eating, the more calories you are likely to eat, predisposing you to gain fat if you eat more energy than you are expending.
- Continuous eating never gives the digestive system a chance to “rest.” In addition to breaking down nutrients, the GI tract has several other duties that require a break from strict digestion, including motility, which occurs when the intestines contract to move food through the GI tract.
- Frequent, all-day long eating keeps the body burning glucose, never allowing it to shift into fat burning mode.
- In response to the constant surge of glucose into the bloodstream from prolonged eating, insulin becomes elevated. Over time, cells become resistant to insulin, raising diabetes risk.
- Constant eating disrupts the natural circadian rhythm of organs like the pancreas and liver that regulate metabolic function. Maintaining this circadian function is important for our genes, which evolved over thousands of years from ancestors who had limited and infrequent access to food.
- Eating a large number of calories late in the day close to bedtime may disrupt sleep by altering release of the sleep hormone melatonin. Decreased sleep quality and quantity are linked to increased appetite, fat gain, and metabolic disorders.
With this research, scientists and fitness coaches have developed a range of “intermittent” fasting protocols to reverse the negative health effects and improve weight loss.
Summary of Fasting Protocols:
Whether you’re new to fasting or are looking to switch up your protocol, let’s make sure we are all on the same page with some definitions:
Intermittent Fasting (IF) is when you extend the overnight fast so that there is a shorter eating period daily, ranging from 4 to 12 hours. IF is also called time-restricted feeding or TRF.
Modified Alternate Day Fasting (MADF) is when you eat only one meal on 1 to 3 days per week. People generally eat about 25 percent of calories on fasting days, having one 400- to 500-caloire meal around midday, and then eat normally on non-fasting days. The most popular version is 5-2 in which people eat a single meal twice a week on non-consecutive days and then eat normally the other five days of the week.
Alternate Day Fasting (ADF) is when you completely abstain from eating on 1 to 3 fast days and then eat normally the other days of the week.
Naturally, nothing is set in stone and it’s fine to experiment to find a fasting protocol that meets your goals.
Benefits of Fasting Protocols
For many people, some variation of TRF will be most useful because it plays to the power of habit since you will get best results if you eat at the same times during a set window daily. Benefits of TRF include:
- Improved fat burning and greater use of ketones.
- Healthier profile of gut bacteria that is associated with leanness.
- Protection against fat gain when your diet isn’t ideal.
- Support for circadian function and organ health.
- Improved sleep and cognitive health, including better memory and learning.
One 10-week study found that limiting food intake to 8 hours by delaying breakfast and moving dinner forward allowed participants to lose 1.9 percent body fat. Calorie intake decreased due to less time to eat over the course of the day and especially less chance to snack at night (2).
A second 12-week study found that compared to a control group that lost no body fat, a TRF group that limited their eating to an 8-hour window lost 2 kg of fat while maintaining lean mass.. Calorie intake naturally decreased by 341 and they lost 2.6 percent body fat while also improving blood pressure readings (3).
For people who don’t do well with the restriction of TRF, ADF and MADF is a great choice. Also known as calorie cycling, ADF variations only require you to limit yourself a few days a week, whereas the rest of the time you can eat normally. This can produce significant benefits:
Studies show ADF protocols improve glucose tolerance for lower risk of diabetes, and lower inflammation and heart disease risk markers. They also lead to greater mitochondrial biogenesis for greater longevity and better health as you age.
Strict ADF leads to a 37 percent overall reduction in calorie intake on average, resulting in reductions in fat mass, especially visceral fat. One 12-week ADF study found that subjects lost an average 3.2 kg of body fat more than a control group (4).
Prolonged fasting that lasts longer than two days also has many benefits but should be done with the guidance of an experienced healthcare practitioner. Studies show long-term fasting, either used intermittently or as a one-off protocol, can improve body weight, metabolism, liver enzymes, cardiovascular parameters, inflammation (40 to 50 percent reduction in pro-inflammatory cytokines), and immune status with better white blood cells and lymphocyte. Fasting for more than 3 days leads to a 30 percent of more reduction in glucose, insulin, and IGF-1 (1).
Intermittent Fasting Recommendations
Macronutrient proportions should be individualized but general recommendations with TRF are for a balanced approach that gets 40 percent of calories from carbs, 30 percent from fat, and 30 percent from protein. Higher carb and higher fat approaches can be used but not at the expense of protein, which is crucial for maintaining lean mass. An intake of 1.6 g/kg of body weight a day is recommended.
For alternate-day fasting, scientists recommend higher fat, keto-style diets to improve fat burning. You should also be sure to consume high-quality protein to sustain lean muscle and plenty of phytonutrient-rich fruits and vegetables for fiber and anti-inflammation.
For people with irregular eating habits, try establishing structured eating with high-quality foods and appropriate balance of macronutrients prior to starting IF. A 10- to 12-hour TRF approach with set mealtimes is recommended.
For more severe fasting protocols beyond TRF, it is recommended that intense physical activity be avoided on fasting days in favor of recovery-focused modalities such as stretching or yoga.
Drinking water with electrolytes is recommended during fasting. Maintaining hydration is important to regulate the circulatory and lymphatic system, both of which are involved in the body’s waste removal system.
Although some animal studies have shown fasting can prevent weight gain on unhealthy processed food diets, avoiding these foods is recommended. Soda, alcohol, caloric beverages, refined foods, and added sugar should be avoided.
Regarding caffeine, there are two theories—one that supports the use of caffeine at a dose of about 200 mg a day, equaling about 3 cups a day, and a purist approach that doesn’t allow caffeine during fasting. The pro-caffeine group points to the fact that coffee can modulate appetite, slowing how quickly food leaves the stomach, and has cognitive and metabolic benefits that may enhance the benefits of fasting.
Who Should Not Fast?
Fasting is not recommended for children, adolescents, elderly people over age 75, or those with eating disorders. Following a heart attack fasting should be avoided. For people with health conditions, including cancer and diabetes, you should work with a health practitioner experienced in using fasting or TRF for therapeutic benefits.