©2020 by Dr. Mashal Khan

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Fat Burning Zone in Intermittent Fasting

It's true that it is impossible to target fat burning in one spot of the body, but how do you target it at all? Today, we're going to talk about how you can specifically train your body to burn stored fat for energy using intermittent fasting.


Let's start from the top. When food is consumed it is taken through the digestive tract and broken down into its component nutrients for absorption into the bloodstream. We discussed those components in our last post but they were glucose, amino acids, and fatty acids.


Once these component levels are increased in the bloodstream, the beta cells in your pancreas get triggered to release insulin. All these components trigger insulin release a little bit but by far glucose has the largest effect on insulin secretion.


Insulin is responsible for keeping blood sugar levels stable, meaning it helps shuttle some of the glucose out of the bloodstream and into mainly the liver, muscle and adipose tissue or fat for use. More specifically, insulin molecules bind to receptors on the surface of these target cells and trigger the opening of specific glucose channels to accept the entry of glucose molecules. Glucose is used for immediate energy in these cells or converted to fat for long term energy storage. An NIH study used fluorescent imaging to look at how insulin stimulates fat cells to take in glucose.


The researchers looked at fat cells suspended in a neutral liquid, and then soaked the cells in an insulin solution to determine the activity of glucose, or specifically the GLUT4 molecule in the absence and then the presence of insulin. In the neutral solution, the GLUT4 molecules were present individually and in clusters throughout the outside of the cell, slowly fusing onto the cell membrane and releasing into the cell. The rate was infrequent. When exposed to insulin, however, the rate of entry drastically increased, quadrupling within 3 minutes and overall saw 60x greater fusion and release of the GLUT4 molecules when compared to the pre-insulin state. Within 10-15 minutes this process had slowed back down to neutral liquid levels.


Extrapolating this information we can explain the fed state vs fasted state phenomenon. In a fed state, our glucose is high, so our insulin is high, therefore we are burning off that glucose in various energy forms, and storing the excess as fat. In the fasted state, however, we have very very low glucose in our bodies, so our insulin remains low, which means that storage of glucose has to be liberated for energy, so we’re liberating or burning fat.


Humans under normal circumstances are always eating so we never get much time in the fasted state. But we can become fat-adapted - in other words, we can train our bodies into fueling themselves with stored body fat rather than free glucose. But this takes time and discipline and the utilization of different methods that up-regulate our fat burning pathways. For example - we need to improve our insulin sensitivity. Meaning we need our body’s to become so responsive to the smallest release of insulin that our insulin levels stay kind of low and easily depleted and therefore we shuttle into the fasted state sooner, where our stored fat is mobilized into free fatty acids and fat-burning pathways are upregulated


There are several ways to improve fat-adaptation or the ability to successfully burn stored body fat for energy.

  1. Low carb diets - eating a low carb/high fat diet improves the body’s ability to utilize fat for energy rather than glucose, as there is more fat and less glucose available at all times, even in the fed state. At the same time, carbohydrates cause a greater rise in insulin than the other macronutrients. Your body will not choose to burn stored energies to power itself when it has a readily available supply of carbs in the bloodstream that take far less effort to use. This is the truest if you have just eaten carbohydrates because these are rapidly converted to glucose and your body prefers to burn sugar as energy before any other source. This is because high amounts of glucose in the blood, or hyperglycemia, is toxic, and your body burns off extra glucose preferentially to get rid of it fast. Another toxin one might consume is alcohol, and in that case, the body will burn alcohol for energy before all other calories. In this way, alcohol completely sabotages fat loss.

  2. Exercise - high-intensity exercise depletes glucose and glycogen rapidly, forcing the body to switch over and utilize more fat or fuel. A meal directly after a workout will send energy intake to muscle stores of glycogen which have just been depleted, burned as energy as an aftereffect of the workout, leaving minimal amounts left to be stored as fat.

  3. Caloric restriction - eating in a deficit means less glucose is available for fuel, so the body is more frequently forced to rely on stored body fat for fuel. You will always naturally eat the lowest calories when you are maximizing nutrient density by eating whole, natural, unprocessed, real foods found in nature.

  4. Intermittent fasting - the body’s ability to burn fat is exercisable just like training a muscle - spending more time in the fasted states gives the body more practice at burning fat, so when you do eat, the body is so well trained in what to do that glycogen gets taken up faster over time. But just like the absence of muscle training - lack of use of the intermittent fasting technique will also lead to a slowing of this metabolic effect.


It takes on average 6-12 hours for your body to digest its food. Now of course everybody is different and there is no one size fits all value I can give you, nor do I claim this to be the case, but the higher end of that range we mentioned is reserved for people with digestive issues like gastroparesis where your digestive muscles aren’t contracting and moving food down your GI tract efficiently, neuropathy from diabetes, conditions such as Parkinson’s, MS, hypothyroidism, various autoimmune diseases and even medications such as antidepressants or narcotics. In an average, healthy individual, insulin levels drop to normal within 1-2 hours, but this depends on insulin sensitivity, the size, and content of the meal you’ve just eaten, and of course your overall health.


Say your average diet was a 12:12 split - and you practiced intermittent fasting with a 16:8 split. And say it took you the full 12 hours to digest food after your 8-hour eating window. 12 hours into your 16 hours fast your body would enter a fully fasted state and start burning stored body fat specifically. That's 4 hours of pure dedicated body fat burning even if you're doing nothing but sitting on your couch. It could be even more if you followed the suggestions we discussed earlier like eating at a slight deficit, eating LCHF, working out in the eating window so the glycogen from the meal was taken up quickly and directed towards replenishing your muscles - a double win. And say you practiced IF for 6 days of the week and took 1 day off - that's minimum 4 hours x 6 days = 24 hours of PURE fat burning per week! On TOP of the other IF benefits like promoting cellular regeneration and repair, starving bad gut bacteria, improving allergies, inflammation, the immune system, brain function, and overall wellness.


I for one am off to put all these tweaks to use, and I hope you will too!

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