Depending on the duration of your fasting periods and the types of foods that you eat, you will go into fat burning phases of different levels. If you are following time-restricted, daily eating or intermittent fasting, a prolonged fasting period of 16 hours may be sufficient for fat burning to begin. By eating a lower-carb diet and following an intermittent fasting program, you may experience fat loss more quickly. Focusing on eating more protein and good fats is recommended for achieving fat loss while fasting.
The study also says that you can further increase your fat oxidation by eating lower-fat foods when you are able to eat in between fasts. The study also suggests intermittent fasting causes an increase in both fat-synthesis enzymes, suggesting both the subcutaneous and visceral fats start to accumulate more fat before an intermittent-day fasting period.
As to what your body burned first during an intermittent fast, it would be the oxidized visceral fat. Your body is burning fat far more easily during the fasted state, since insulin levels are lower. When you are in a fasted state, your body is able to burn fat that was not available when you were in the fed state. In short, when you are in the fed state, you do not have the appetite for burning fat; it is as if the fat stores doors are locked.
Fasting puts your body into a fed – state. Since we do not get into fasted status until about 12 hours after the last meal, our bodies are rarely in this fat burning state. Our bodies are storing energy (yes, this is exactly what fat is) to be used during these times, however, we never access that stored energy since we constantly refill our fuel tanks with food. Your body notices when you are going through long periods of eating very little to no food, and in order to make sure that you have a reliable energy source in those periods, your body stores extra food you are eating as fat.
The disruptions also trigger your body to enter ketosis, a metabolic state where your body uses fat as its main energy source (9). When the switch happens, and your human body starts burning stored fat for energy, this is called entering ketosis. In the fasting state, your blood glucose levels decrease, causing insulin to drop, which signals to your body to begin burning stored energy (carbs). However, we do not get any glucose from eating while fasting, so the body ends up turning to other sources of fuel.
Since you are not providing much, if any, glucose to the body while water fasting, you are more likely to burn off these reserves and move on to burning fat. Those water fasting over long periods of time consistently do not add any glucose to their reserves, allowing your body to immediately burn sugar for energy. If you are feeling up to exercising while on water fast, you can encourage your body to burn fat rather than protein.
As you are fasting, continue to keep up with exercise so that after glycogen is depleted, your body does not burn muscle protein in place of fat. If you enter the fasting phase with lower levels of glycogen in the muscles and liver, it is possible that you will begin burning fat immediately. Plus, if you exercise first thing in the morning before eating, you will double-dip fat burning, as your body will be using even more of your stored fat for energy.
Fasting for a set amount of hours every day, or eating only one meal for a couple days each week, may help your body burn fat. The authors of this study reported that doing an intense workout right before starting your fast helps your body go into fat-burning mode a lot quicker…three-and-a-half hours quicker, to be precise. The data notes that fasting for up to 14 hours at most, while exercising at least three days per week (and up to five days as well), can cause more fat loss than muscle loss. In addition, the Journal of the International Sports Nutrition Associations 2014 report said that you can improve your body composition by exercising four times per week, eating for up to eight hours per day, and fasting for 16 hours.
A 2014 Journal of the International Society of Sports Nutrition study found that eating just an eight-hour window per day, plus fasting the other 16 hours,A in addition to doing four endurance workouts a week,A led to improvements in body composition, but that intermittent fasting, or resistance training, per se, did not dramatically aid in improving body composition. In a study published in the American Journal of Clinical Nutrition in 2005, individuals who tried fasting every other day for 21 days lost approximately 2.5% of their body mass, some from muscles and some from fat. Both groups experienced similar losses of body fat and muscle, so intermittent fasting does not seem to work as well at maintaining muscle mass as losing fat.
The same study showing that intermittent fasting could lead to 3-8 percent weight loss also found that the study participants lost 4-6 percent waist size (visceral fat) after fasting for six to 24 weeks. In human studies reviewed, intermittent fastings effects on fat are less direct, as most paired caloric restriction with intermittent fasting. The science says it does; in one study, after eight weeks, participants following the intermittent fasting eating plan lost 3.5 pounds of fat, whereas those exercising equally and eating the same number of total calories did not. A recent review of IF literature found that fasting can boost weight loss rates of 3-8% over a period of only 3-24 weeks.
Experts who have studied intermittent fasting for years concluded that our bodies are capable of going several hours, and even days, on end, without eating. Going without food for too long may actually prompt your body to begin storing more fat as a reaction to hunger. Towards the end of early fasting states, your body will slowly exhaust its stores of liver glycogen and begin searching for an alternative source of energy. Researchers have theorized that exercising prior to a fast causes your body to burn a substantial amount of energy/glucose, eventually leading to a rapid shift into ketosis.
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