Intermittent fasting, otherwise known as IF, is an eating method that focuses on fasting and not eating food during a specific time frame. There are several kinds of intermittent fasting routines, such as the 16/8 and 20/4 method. One of the most popular types of IF is the 16/8 method, which is when people eat during an eight-hour timeframe and then fast for 16 hours. Many people like to combine IF with the ketogenic diet, a high fat, low carb diet. Since the body burns fat as opposed to glucose on the keto diet, it’s only natural that fasting expedites the process. IF may have incredible health benefits, and they’re not only losing weight. Combined with the keto diet (which we discuss in great detail in the Body Reboot book), IF can also reduce inflammation and help control blood sugar. 

Before we dive into how IF helps your body burn fat, let’s find out how the body burns fat without IF according to VeryWell Fit. Keep in mind that if you go on the keto diet, you will drastically cut down your carb intake. Cutting back on carbs results in your body burning fat and thus, losing more weight. 

If you're trying to lose weight, knowing how your body uses calories for fuel can make a difference in how you approach your weight loss program. You get your energy from fat, carbohydrates, and protein.

Which one your body draws from depends on the kind of activity you're doing. Most people want to use fat for energy, which makes sense. You figure that the more fat you can use as fuel, the less fat you will have in your body. But, using more fat doesn't automatically lead to losing more fat.

Understanding the best way to burn fat starts with some basic facts about how your body gets its energy:

The body primarily uses fat and carbohydrates for fuel. A small amount of protein is used during exercise, but it's mainly used to repair the muscles after exercise.

The ratio of these fuels will shift depending on the activity you're doing.

For higher-intensity exercises, such as fast-paced running, the body will rely more on carbs for fuel than fat. That's because the metabolic pathways available to break down carbs for energy are more efficient than the pathways available for fat breakdown.

For long, slower exercise, fat is used more for energy than carbs.

When it comes to weight loss, it doesn't matter what type of fuel you use. What matters is how many calories you burn as opposed to how many calories you take in.

If you want to get into ketosis, the fastest way to do that is by fasting. You may be wondering why it pushes your body into ketosis faster. The reason why is because your body is forced to burn fat when it runs out of glucose. Healthline explains how IF boosts your metabolism by explaining the process below: 

Combining the diet and the fast may help you burn more fat than the diet alone.

Because intermittent fasting boosts metabolism by promoting thermogenesis, or heat production, your body may start utilizing stubborn fat stores.

Several studies reveal that intermittent fasting can powerfully and safely drop excess body fat.

In an eight-week study in 34 resistance-trained men, those who practiced the 16/8 method of intermittent fasting lost nearly 14% more body fat than those following a normal eating pattern.

Similarly, a review of 28 studies noted that people who used intermittent fasting lost an average of 7.3 pounds (3.3 kg) more fat mass than those following very low-calorie diets.

Plus, intermittent fasting may preserve muscle mass during weight loss and improve energy levels, which may be helpful for keto dieters looking to improve athletic performance and drop body fat.

Additionally, studies underscore that intermittent fasting can reduce hunger and promote feelings of fullness, which may aid weight loss.

Not only can IF help you lose weight, but it can also prevent you from gaining more weight as well. That’s precisely what a study from 2017 by Cell Research discovered: 

Intermittent fasting (IF), a periodic energy restriction, has been shown to provide health benefits equivalent to prolonged fasting or caloric restriction. However, our understanding of the underlying mechanisms of IF-mediated metabolic benefits is limited. Here we show that isocaloric IF improves metabolic homeostasis against diet-induced obesity and metabolic dysfunction primarily through adipose thermogenesis in mice. IF-induced metabolic benefits require fasting-mediated increases of vascular endothelial growth factor (VEGF) expression in white adipose tissue (WAT). Furthermore, periodic adipose-VEGF overexpression could recapitulate the metabolic improvement of IF in non-fasted animals. Importantly, fasting and adipose-VEGF induce alternative activation of adipose macrophage, which is critical for thermogenesis. Human adipose gene analysis further revealed a positive correlation of adipose VEGF-M2 macrophage-WAT browning axis. The present study uncovers the molecular mechanism of IF-mediated metabolic benefit and suggests that isocaloric IF can be a preventive and therapeutic approach against obesity and metabolic disorders.

During an eight-week study in 2016, J Transl Med. found that 34 fit men who practiced the 16/8 fold of fasting least almost 14% more body fat than being on a regular food plan. 

Background

Intermittent fasting (IF) is an increasingly popular dietary approach used for weight loss and overall health. While there is an increasing body of evidence demonstrating beneficial effects of IF on blood lipids and other health outcomes in the overweight and obese, limited data are available about the effect of IF in athletes. Thus, the present study sought to investigate the effects of a modified IF protocol (i.e. time-restricted feeding) during resistance training in healthy resistance-trained males.

Methods

Thirty-four resistance-trained males were randomly assigned to time-restricted feeding (TRF) or normal diet group (ND). TRF subjects consumed 100 % of their energy needs in an 8-h period of time each day, with their caloric intake divided into three meals consumed at 1 p.m., 4 p.m., and 8 p.m. The remaining 16 h per 24-h period made up the fasting period. Subjects in the ND group consumed 100 % of their energy needs divided into three meals consumed at 8 a.m., 1 p.m., and 8 p.m. Groups were matched for kilocalories consumed and macronutrient distribution (TRF 2826 ± 412.3 kcal/day, carbohydrates 53.2 ± 1.4 %, fat 24.7 ± 3.1 %, protein 22.1 ± 2.6 %, ND 3007 ± 444.7 kcal/day, carbohydrates 54.7 ± 2.2 %, fat 23.9 ± 3.5 %, protein 21.4 ± 1.8). Subjects were tested before and after 8 weeks of the assigned diet and standardized resistance training program. Fat mass and fat-free mass were assessed by dual-energy x-ray absorptiometry and muscle area of the thigh and arm were measured using an anthropometric system. Total and free testosterone, insulin-like growth factor 1, blood glucose, insulin, adiponectin, leptin, triiodothyronine, thyroid stimulating hormone, interleukin-6, interleukin-1β, tumor necrosis factor α, total cholesterol, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, and triglycerides were measured. Bench press and leg press maximal strength, resting energy expenditure, and respiratory ratio were also tested.

Results

After 8 weeks, the 2 Way ANOVA (Time * Diet interaction) showed a decrease in fat mass in TRF compared to ND (p = 0.0448), while fat-free mass, muscle area of the arm and thigh, and maximal strength were maintained in both groups. Testosterone and insulin-like growth factor 1 decreased significantly in TRF, with no changes in ND (p = 0.0476; p = 0.0397). Adiponectin increased (p = 0.0000) in TRF while total leptin decreased (p = 0.0001), although not when adjusted for fat mass. Triiodothyronine decreased in TRF, but no significant changes were detected in thyroid-stimulating hormone, total cholesterol, high-density lipoprotein, low-density lipoprotein, or triglycerides. Resting energy expenditure was unchanged, but a significant decrease in respiratory ratio was observed in the TRF group.

Conclusions

Our results suggest that an intermittent fasting program in which all calories are consumed in an 8-h window each day, in conjunction with resistance training, could improve some health-related biomarkers, decrease fat mass, and maintain muscle mass in resistance-trained males.

That Helpful Dad offers a few tips on how to make IF work for you: 

Skipped Days = Lethary: you can skip a day or two if you like but I discovered that when I skipped an Intermittent Fasting day, I felt poor physically – specifically, I felt lethargic, unmotivated, and bloated. As a result I rarely skip a day of fasting and usually get in at least 14 hours. 

Fasting Cardio vs HITT: the combo of Intermittent Fasting and Fasted Cardio (AKA Cardio before Breakfast) did increase my energy level but I still think longer HITT later in the day is more effective because you can burn more calories overall. I think an alternating combo of the two is best.

Workouts Burn Even More Fat: CrossFit and other constant movement workouts worked particularly well from an energy standpoint. There is also the theory that working out larger muscle groups (legs, chest, back, etc) and building up those muscles will continue the post-workout fat-burning process longer.

Fluids are a Must: Drinking 1 glass of water immediately upon waking and then my Cinnabun Coffee Cup was more than enough to take me through to 10-11am for the 16-hour fast cycle to work its magic. I also like adding in some turmeric to my coffee as well. Additionally a nice cup of green or moringa tea is also good if you want something more to drink too

Eating Schedule is Flexible: When I am ready to break my fast (at 10-11am) I load up on protein – yogurt, milk, eggs, bacon, etc. I then eat lunch around 1.5-3 hours later, then have some fruit/snack around 330-4, and then dinner at 6ish. Many people say to avoid fruits because they are high in sugar and that impedes the fat loss – that may be true but I like fruits and believe their overall health benefits outweigh the negatives for long-term health.

Many people feel that going through IF is a positive experience. Fat Burning Man elaborates on why IF can improve a person’s lifestyle and explains other benefits of fasting: 

If you could put the mental, physical, and spiritual benefits of fasting in a pill, you would make billions. The many benefits of fasting include:

Promoting human growth hormone production, which helps your body burn fat, build muscle, and slow the aging process.

Normalizing insulin sensitivity, which prevents chronic diseases like diabetes, heart disease, and even cancer.

Regulating ghrelin levels, also known as “the hunger hormone.”

Decreasing triglyceride levels.

Reducing inflammation and free radical damage.

To learn more about the keto diet, check out the Body Reboot book. Combined with IF you could experience some incredible results! To get a free copy of our book, help us cover shipping. Visit this page to get your free copy right away. 

Sources: That Helpful Dad, Healthline, VeryWell Fit, Fat Burning Man, NCBI: Cell Res. 2017, NCBI: J Transl Med.