Intermittent fasting is a proven method of weight loss. It reduces insulin levels, caloric intake and hunger. However, many people wonder when weight loss and fat loss, in particular, becomes active.
Reminder about intermittent fasting
Intermittent fasting is the voluntary deprivation of all solid food and drink other than water for part of the day, or certain days of the week. There are several types of fasting, the most common is the 16:8 fast (lean gains method) where you don’t eat for 16 hours out of 24. If you want to go deeper into the subject, the guide to intermittent fasting gives you all the details to get started and choose the method that suits you.
Fasting begins when you stop eating. Digestion varies depending on the person and what they eat, but we can say that after 6 hours the digestion is almost complete. If you have eaten at 8 pm, around 2 am the digestion and absorption of food is almost complete. What many people don’t know is that almost everything you eat is stored. Day and night, our bodies are programmed to store and then un-store what we eat, only a small part of the food we eat is used on the fly. Three factors will determine when you will start losing fat. Your glycogen stores, the composition of your meals, your insulin sensitivity.
The composition and size of the meal
The composition of the meal is important because it affects the secretion of insulin and other hormones that regulate the metabolism. If the meal contains a lot of processed carbohydrates, such as bread, the hormonal response will be important. Your body will produce a lot of insulin. When insulin is high, glucagon levels drop and without glucagon it is hard to burn stored fat. Your body will therefore use the carbohydrates you have just eaten, some medium chain fats such as MCTs (Medium Chain Triglycerides) and will store the rest of the carbohydrates and fats. If you eat processed carbohydrates frequently, it will take longer for your insulin level to drop and you will find yourself in a phase of destocking.
If you are giving your body a wide range of insulin changes with meals that are too high in sugars, you will feel lethargic and weak afterwards. The famous 2 p.m. bust, because your body stores what you’ve just eaten well thanks to the insulin, but it has trouble burning your stocks.
Conversely, if you get used to your body, after a few weeks of intermittent fasting, and thanks to a diet low in processed carbohydrates, it will learn to quickly alternate between a storage phase when you break your fast and a destocking phase when a few hours have passed.
When you wake up after a night’s sleep, your body is usually already in the destocking phase. This can be measured with the ketone level. At this time your body is destocking two things, fats and glycogen.
The glycogen stock
Your body stores the sugar you eat in the form of glycogen, in your muscles and in your liver if there is room available. When you enter the destocking phase your body simultaneously destocks fat and glycogen to turn it into sugar.
Fats are generally used by certain organs such as the heart, but also by the muscles during simple efforts such as walking. Glycogen is transformed into sugar and used by the muscles during intense and/or sudden efforts, but also by the brain. The stock of glycogen depends on the size of your muscles and the amount of sugars you eat (bread, pasta, rice, sugars …). Usually the more you use your muscles, the bigger the size increases proportionally, so the amount of sugars you eat again determines how much glycogen you store. On average we have enough stored sugars to last 24 hours. So theoretically after 24 hours we burn almost only fat, and some sugar produced by the liver, from proteins and fats. It is impossible to burn only fat, part of the brain and some tissues like the retina and the kidneys need sugar, so the liver starts to produce sugar.
The loss of fat during intermittent fasting therefore starts just a few hours after the last meal and reaches its maximum between 8pm and 24 hours after the meal. However, if you do a physical activity that will empty your glycogen stores, the loss of fat will occur much earlier. This use of fat also becomes more effective after a few weeks of fasting adaptation. Our body adapts after about three weeks of intermittent fasting. Indeed, if the body is used to burning mostly sugar, it needs a period of adaptation to develop the enzymatic machinery necessary to use fat as a primary source of energy.