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  • Writer's pictureKim Smolders

Intermittent Fasting for Metabolic Flexibility.

There are many benefits to intermittent fasting, calorie restriction and eating within a shortened feeding window, such as allowing the body to focus on detoxification, autophagy (literally meaning "self-eating" or the cleaning up of old & damaged cells), giving you a physical & mental boost, and allowing the body to use body fat for fuel.

What is Intermittent Fasting?

Intermittent fasting (IF) is an eating style where you eat all your food within a specific time frame, and fast for the remainder of the time. There are many different ways in which you can implement an IF schedule, the most popular ones being 16:8 (16 hours of fasting & 8 hour feeding window), 20:4 (20 hours fasting & 4 hour feeding window), One Meal A Day (literally only eating one meal a day), 5:2 (severely restricted calories - under 500 - on 2 days of the week), alternate day fasting (fasting for 24 hours every two days) or longer fasts of 36 hours plus (not recommended without supervision of a medical professional).

Finding a fasting schedule that suits your lifestyle & goals is the most important factor in long-term success. Goals can be weight loss, metabolic health (becoming more metabolically flexible burning fat as well as carbs), blood sugar & insulin control, longevity, clarity of thinking, hormone balance ... In this blog article I will focus on metabolic flexibility as this will positively influence the aforementioned goals.

Metabolic Flexibility.

Metabolic flexibility is all about how flexible and adaptive your body is in terms of fuel for energy, being able to get it from food or stored sources in the body, as well as being able to burn carbohydrates and fat for fuel. Due to our high intake of carbohydrates on a daily basis, this is flexibility is not a given. In order for your body to tap into your fat stores or use fat from food to get you energised, certain hormones need to be balanced in the right way - the hormones insulin, glucagon, leptin & ghrelin are all part of this balancing act.


Insulin is released by the pancreas when the body senses glucose in the bloodstream after eating. Its main job is to shuttle this glucose into the cells for energy, but it also inhibits hormone sensitive lipase, an enzyme which is responsible for the mobilisation of stored fats. In other words, when insulin is present, no fat burning takes place as the body is signaling that there is plenty of glucose for energy in the bloodstream already. It can therefore be thought of as the fat-storage hormone. When you eat, carbohydrates in particular but also to some degree protein, the body is getting the message to turn excess sugar from the bloodstream that it does not need for energy into fat & glycogen (the storage form of sugar) in the liver and muscles to keep our blood sugar levels within normal range. Keeping both glucose & insulin levels within a healthy range is key in maintaining metabolic, cardiovascular, endocrine & neurological health as well as the right weight for you.


Glucagon is also a hormone made by the pancreas, but in opposition to insulin, it is secreted during periods of fasting and in between meals. It is released to maintain and raise blood sugar levels when glucose is not available from the diet. Glucagon is our fat-burning hormone, telling the body to burn stored glycogen & fat for fuel in periods of fasting to maintain blood sugar balance. Glycogen will first be tapped into for energy before stored forms of fat are used, as glycogen is more easily accessible. To tap into stored body fat, glycogen stores need to be depleted, which long enough intermittent fasting (or/and a strict keto diet) allows you to do. How fast these stores are depleted will depend on your last meal, how much glycogen is stored, your day to day diet and how metabolically flexible you are.


Leptin is the satiety hormone produced in our adipose tissue, aka our fat cells, that helps to regulate our longterm weight. Fat cells are not just inactive tissues; they are actually actively part of our hormonal system signalling our brain. Leptin specifically tells our brain to use our fat stores for energy, which is key to metabolic flexibility. Leptin resistance occurs when the hypothalamic cells in the brain stop recognizing leptin’s signals - this can happen due to too much fat or too much insulin, just to bring it full circle. As a result, the brain doesn’t perceive that enough food has come in, and it reads that as starvation. During this leptin resistance, your brain will turn on all the signals it can to make up for the falsely perceived food deficit, meaning everything you eat will go straight into fat storage without being used for energy. Staying leptin-sensitive, through IF for example, is essential to metabolic health and healthy eating behaviour.


Ghrelin, known as the "hunger hormone", is produced and released predominantly by the stomach. It is decreases during and after food intake. In addition to regulating appetite, ghrelin plays an important role in mediating whole-body glucose and energy homeostasis. Ghrelin stimulates the release of growth hormone from the pituitary gland, which, unlike ghrelin itself, breaks down fat tissue and causes the build-up of muscle. In the pancreas, ghrelin stimulates the release of glucagon, which increases production and release of glycogen from the liver & the fat tissue into the bloodstream. It is also suggested that ghrelin suppresses insulin secretion to reduce glucose storage in key organs, all leading to more optimal metabolic health and weight loss.

All in all, the combined effects of different factors during food-intake and fasting facilitate the organisms’ fuel availability and metabolic flexibility.

Intermittent Fasting Tips for Beginners.

Ease into IF.

If you are new to IF, do not attempt to go hard core straight away! Ease into fasting to allow your body to adjust by starting with a 12-hour eating window & a 12-hour fasting window, which hopefully most of us do naturally overnight. This really is the bare minimum of rest you should be giving your digestive system and body. Once you can easily fast for 12 hours, without feeling hungry (or hangry!), extend the fasting period and shorten the time frame in which food is consumed. To get used to eating less frequently, increase your feeding window slowly every couple of days so that you get used to the increasing fasting window. Start with an extra fifteen or thirty minutes every day and work your way up to the desired fasting time.

Earlier dinners & later break-fasts.

Be mindful when you are eating late at night - even the small snacks in front of the tv & before bed count! Try not to eat after your dinner, or at least two to three hours before bed. Not only will it work wonders for your body, you will notice that you will sleep better too. Furthermore, feel into your body in the morning as often we are not in fact hungry but eat out of habit, so try to extend the period between rising & breaking your fast.

Remove sugar & refined carbohydrates.

Sugar and refined carbohydrates like white bread, pastries, sweet desserts, white flour, pancakes, candy, fruit juice, white rice, oatmeal, ice cream, cookies ... have the most impact on stimulating insulin levels and getting you on a blood sugar rollercoaster. When insulin rises and consequently crashes, your body will crave more sugar to correct this and in general you will feel more hungry. Reducing your carbohydrate intake also means you deplete your glycogen stores faster, so when fasting you will tap into fat burning more quickly.

Eat more fiber, protein & fat.

Very complementary to the above point - adding fiber, protein & fat to your diet will make you feel more satiated and will nourish you body more, reducing hunger and cravings. Fiber from vegetables, fruits and from healthy, complex carbohydrates & starches such as sweet potatoes, squash, pumpkins, quinoa, tartary buckwheat, whole grain rice .. will slow down the absorption of sugar in the intestines, and the fat & protein will add to the satiety you will feel, allowing you to go longer without eating between meals and overnight.

Start the day with at least 250ml of water.

Starting your day with a large glass of water, potentially infused with lemon or lime, will not only help detox your body, stimulate the digestive system & liver pathways, and energise you, it will also dampen any hunger feelings you may be experiencing. Whilst fasting in the morning you are also allowed to enjoy coffee, tea or matcha which may satisfy a craving and give you an energy boost should you need it!

Do some exercise.

Exercising in a fasted state in the morning can get you set up well for the day, especially when you do this outside. It will take your mind off the hunger or cravings, and stimulate glycogen depletion and fat burning. Being outside has the added benefit of getting sun exposure earlier in the day and clearing your mind.


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