Intermittent fasting may seem like the latest diet trend, but it's actually been around since the beginning of human history (and maybe even before).
Deep in our ancestral past, meals weren't nearly as scheduled as they are now.
Relying on hunting and gathering as the primary source of food, early humans didn't have the luxury of eating three consistent meals each day, and may have regularly gone an entire day without a meal.
While not willingly, early humans followed a food schedule that may have incorporated intermittent fasting by default.
Intermittent fasting is the practice of fasting periodically, which could be as short as 8 hours a day or an entire day weekly.
For most of human existence the food security and abundance we know in the modern world was non-existent. Because of this, it's believed that the human body evolved to handle an intermittent feeding schedule, meaning infrequent meals with a half-day to a day in between. Many people are now appreciating this eating schedule and paying attention to its potential health benefits.
On a similar note, our physical activity has also drastically changed in the past century or two.
Many people find themselves working a sedentary job rather than needing to exert themselves physically. While it may sound nice at face value, the consequences of inactivity have reared their head in weight gain and the development of health problems related to metabolism. Diabetes and obesity are considered epidemics in the U.S.
Below is a closer look at how fasting and exercise impact the body, and how they can potentially work synergistically to help and maintain a healthy weight and combat metabolic disorders.
Fasting 101
Unlike most diets that place restrictions on what you should and should not eat, fasting diets are more focused on when you eat.
Fasting involves avoiding food and relying mainly on your body’s energy stores as the primary source of energy.
Fasting diets were initially quite polarizing, but there is quite a bit of emerging evidence to support their ability to help individuals lose stubborn weight and achieve some metabolic improvements––when done correctly.
The ability to lose weight on a fasting diet is thanks to the body's ability to deplete short-term energy stores from food and begin using energy stored in adipose (fat) tissue.
Below is a closer look at some of the most common and best approaches to intermittent fasting.
Time-Restricted Fasting or Time-Restricted Feeding (TRF)
Time-restricted fasting has been around for quite some time and is even an integral part of the Islamic holiday Ramadan, where Muslims fast from sunrise to sunset.
Time-restricted fasting is a form of intermittent fasting that focuses on a feeding window and a period of fast during any given 24 hour period. When people think intermittent fasting, this is typically what comes to mind.
There are different thoughts on how much time you should fast in each 24 hour period, but the most common forms of time-restricted fasting include 16/8 and 14/10 methods.
The first number represents the number of hours you are to go without food while the second is the timeframe you are able to eat.
While it can seem intimidating at first glance, pushing your breakfast back by a few hours and only eating between 9 a.m. and 7 p.m. falls within the 14/10 method.
Calorie Restriction
Another form of intermittent fasting focuses on reducing calorie intake on certain days of the week.
On fasting days, you consume roughly a quarter of the calories that you would traditionally have in a normal day.
With a standard 2,000 calorie diet, fasting might consist of limiting your intake to 500 calories on one day. Because of the large decrease in intake, this method is typically used in an alternating fashion, where you are only fasting every other day at most.
Day Fast
The most extreme form of intermittent fasting is when an individual goes an entire 24 hours (or longer) without eating. Some people may find that it fits their schedule well, but the many other methods provide similar results without as much of the associated side effects such as fatigue, irritability, and headaches.
Exercise 101
When trying to lose weight, one of the first recommendations is to get more physical exercise.
While this may be reminiscent of your middle school health class, routine physical exercise brings serious weight loss potential.
Extra weight in the form of fat can be thought of as your body setting aside the extra energy from your food to be used at a later time. Exercise burns energy, and when done consistently, can help whittle away excess fat stores, lose weight, and improve how your body treats energy intake through improved metabolism.
Types of Exercise for Weight Loss
While any exercise is better than no exercise at all, there is one form of exercise that seems most well adapted to getting rid of fat: cardiovascular exercise.
Cardio expends significant energy and requires your body to constantly supply energy to high-consuming muscle groups for extended periods of time. Done consistently, this can burn fat and help you maintain a healthy weight.
Cardio workouts include running, biking, hiking, and any other exercise that gets your heart rate up and large muscle groups working. Strength exercises such as weight lifting also have a place in improved metabolism and energy expenditure, though strength training tends to be less effective for simply "losing weight."
Fasted Cardio (Fasting + Exercise)
Fasted cardio employs cardiovascular exercise while in a fasted state. The supposed benefit is that rather than spending time burning off the food you've eaten, you go straight into burning fat and using your body’s natural energy stores.
Health experts are divided on the safety and effectiveness of this approach.
One 2017 analysis found that in five studies of fasted exercise, "the effect of fasted and fed aerobic exercise revealed trivial to small effect sizes on body mass."
Other athletes and weight loss guides suggest the opposite.
While the jury is out, it's important to be cautious and take your time, as exercising with no natural "food energy" can be quite different from a regular workout.
While efficient at burning fat, studies have shown a decline in performance while fasting — i.e., a decrease in available energy that could cause you to perform to a lesser degree.
Not to mention, if not done properly, fasted exercise can be dangerous — after all, you’re starving your body of nutrients, then asking it to burn energy it doesn't necessarily have. Some people may just feel sick, or worse, pass out.
The Takeaway
Intermittent fasting is increasingly recognized as an effective means of losing weight and optimizing metabolic health (at least in the short-term) when done carefully, though health experts are not aligned on safety and precise execution.
Cardiovascular exercise is recognized for multiple health benefits, including maintaining proper weight and improving metabolism.
Fasted cardio combines these two methods. Research so far is lacking into how effective and safe it is for weight loss.
Whether you're losing weight for aesthetic reasons or to help improve an existing medical condition, talk to a doctor before making significant life changes like long periods of fasting or aggressive weight loss techniques.
Sources
Intermittent fasting: Surprising update | Harvard
Intermittent Fasting: 4 Different Types Explained | Cleveland Clinic
Intermittent Fasting and Its Effects on Athletic Performance: A Review | NCBI
