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Why Intermittent Fasting Works

Obesity shortens lives by increasing risk for diabetes, heart attacks, strokes and many cancers, yet the dozens of widely-promoted weight loss diets have failed to stop the epidemic of obesity that has spread for the last 50 years. Daily restriction of calories, whether done by carbohydrate restriction, fat restriction, calorie counting or any other method, does result in weight loss and other benefits. However, several decades of studies have shown that people will not stay on any type of constant calorie restriction program, will almost always gain back the lost weight and be left with a slower metabolism that makes them gain even more. Recent research shows why intermittent fasting may be an effective way to help people lose weight, keep it off, and slow down or reverse the diseases and disabilities that being overweight can cause (Obesity, Feb 2018;26(2):254-268). Intermittent fasting means that you avoid or strictly limit eating just for a set number of hours per day, or days per week, or days per month.

This new study from the University of Florida proposes that intermittent fasting works because it causes repeated "flipping of the metabolic switch." After you have fasted for about 12 hours, you start to lose body fat because your body is forced to change temporarily from its main energy source, glucose (sugar) to fat from the fat stored in your body, and using these fatty acids that are converted to energy to produce ketones that are also used for energy. The "metabolic switch" is defined as the shift from using glucose to using fatty acids and fatty-acid-derived ketones for energy, and then shifting back to glucose when eating is resumed. This leads to weight loss and a variety of other metabolic benefits including lowered blood pressure, resting heart rate, cholesterol, triglycerides and blood sugar levels. Most of the studies reviewed in this new article show that intermittent fasting caused weight loss primarily through loss of body fat, not loss of muscle.

Types of Intermittent Fasting
Several different patterns of intermittent fasting have been proposed and studied, including:
• Time-restricted fasting (every day), such as not eating after 6PM until the next morning, or eating twice a day at noon and at 5PM.
• Alternate day fasting: a 24-hour fast followed by a 24-hour non-fasting period.
• Weekly variations such as fasting two days a week and five days with the person's normal meals. Fasting does not have to be on consecutive days.
• Monthly variations such as fasting five days a month.
The fast periods may involve complete fasting (no calories) or modified "fasting," taking in no more than 25 percent of energy needs. So far, research has not proven any one method of intermittent fasting to be superior to the other methods. This means that if you want to try intermittent fasting, you can pick the type of schedule that seems to suit you best. It is not easy to restrict food when your refrigerator is full and you are hungry, so you may find it easier to fast overnight from 6:00 PM to the next morning.

Metabolic Switch Demonstrated in Rats and Mice
The University of Florida team demonstrated the activity of the "metabolic switch" in rodent studies that simulated three types of intermittent fasting (IF): alternate days, three consecutive days, or daily time-restricted eating. The authors said that, "All three IF patterns produce elevations in circulating ketone levels . . . indicating the metabolic switch is turned on intermittently."

They then analyzed the impact of IF on different organ systems (liver, muscle, cardiovascular system, central nervous system). For example, in mice, it takes only a single cycle of fasting every third day and eating unrestricted high-fat foods on the other two days:
• to make their cells more insulin sensitive, which lowers high blood sugar and insulin levels
• to lose body fat and reduce harmful inflammation by increasing vascular endothelial growth factor (VEGF). VEGF helps to form new blood vessels and activates anti-inflammatory macrophages that stimulate fat cells to burn stored fats by increasing fat cell heat production (Cell Research, October 17, 2017; 27:1309–1326).

The researchers then provide a review of studies on intermittent fasting and its benefits in humans, and conclude that, "intermittent fasting regimens may be an effective approach to help older adults lose unhealthy weight while retaining larger amounts of lean mass . . . [and] can be effective in preventing type 2 diabetes" (Obesity, Feb 2018;26(2):260).
More on Weight Loss with Intermittent Fasting
Why We Use Intermittent Fasting

Intermittent Fasting May Not Be for Everyone
Certain conditions can be worsened by fasting. Check with your doctor, particularly if you are diabetic, have low blood pressure, take medications, are underweight, have eating disorders, or are pregnant, trying to become pregnant or breast-feeding an infant.

My Recommendations
If you decide that you want to try intermittent fasting but are not sure which program to follow, here is what we have done for the last four years:
• Keep tempting but unhealthful foods out of the house (sugared drinks, sugar-added foods, red meat, processed meats, fried foods and most foods made from flour).
• Stock a variety of vegetables, fruits, whole grains, beans, nuts and other seeds.
• On "fast" days (2-3 days a week), eat a healthful breakfast such as oatmeal and then, for the rest of the day, have occasional snacks of vegetables, fruit or nuts.
• On non-fasting days, eat your main meal between noon and 6PM. Try to include lots of the healthful foods listed above.
• After 7PM, you can drink water, but avoid foods and drinks that contain calories.

May 13th, 2018
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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