Intermittent Fasting 101: The Basics
Fasting is not a new concept. In fact, humans have been fasting for the majority of our evolutionary history and many continue to fast as part of their religious beliefs (e.g. Lent, Ramadan, etc.). As a species we became adapted to periods where food was scarce which meant that our bodies regularly entered into states of ketosis, burning stored fat after depleting glucose stores. Over the past five decades fasting has been replaced with uninterrupted, continuous food availability and increased intake. This has led to a global obesity epidemic (more than 1.9 billion adults worldwide) with increases in chronic illness such as diabetes, cancer, cardiovascular and respiratory diseases. In fact, for the first time in history, life expectancy in certain western countries (USA, UK) is decreasing. Simply put, the “Western Diet” (high carbohydrates, sugar and protein) is not only making us less healthy while we are alive but is also shortening our lifespan.
So what is Intermittent Fasting (IF) and why should we be interested? IF is an eating pattern that involves interspersing normal daily caloric intake with periods of intense calorie restriction. This can take various forms (18:6, 5:2, alternate day fasting, etc.) and has become a popular topic in current media. Fasting enables our body to enter a state of ketosis (burning fat instead of glucose) which has been linked to multiple positive health outcomes including weight loss, improved brain connectivity and neuroplasticity, reduced inflammation, reduced risk and improved outcomes of certain chronic diseases (e.g. diabetes, asthma, hypertension), improved energy levels, increased antioxidant production, increased autophagy (clearance of debris proteins) and potentially improved outcomes in cancer.
Let’s review the details of what happens when we eat vs. when we fast. During the “fed” state we are typically eating different types of macronutrients, including carbohydrates, protein and fat. Once consumed, carbohydrates convert to glucose which increases our body’s blood glucose load. This increase stimulates the pancreas to release the hormone insulin which moves glucose out of our blood and into our cells where it can be used for energy. Excess glucose that is not needed for energy is stored in the liver (or to a lesser extent in muscles) as glycogen or as fat.
The “fasting” state occurs when we are not actively eating. During the first 12 hours of fasting our bodies typically use glucose stored in the liver (glycogen) as the primary energy source. Once glycogen stores have been used up our body then shifts into a state of “ketosis” which is when we burn stored body fat as a source of energy. An enzyme called hormone sensitive lipase plays a critical initial step needed to get in to ketosis. This usually occurs after around 14 hours of fasting. Ketones provide an alternative fuel source and also serve as important signaling molecules for our body which impact physiological pathways.
It is worth noting here that fasting and ketogenic diets are NOT one and the same. Very simply, ketogenic diets are high fat, moderate/low protein and very low carbohydrate (<20g/day) diets. 70% of calories should come from fats (ideally healthy) such as fatty fish, eggs, coconut oil, fatty meats, cheese, etc. The goal for ketogenic diets is to be fat dominant in order to be in a permanent state of ketosis (you can measure your ketone production via breath, blood and urine tests, with varying degrees of reliability). Conversely, intermittent fasting does not specify the type of food consumed but rather specifies the timing of food consumption. You can also choose to do intermittent fasting using a ketogenic diet.
If you have decided that you would like to try fasting then Intermittent Fasting (IF) is potentially the easiest and most flexible approach to start with. Keeping in mind that most benefits occur after around 14 hours of fasting then a 16:8 IF protocol (16 hours of fasting, 8 hour period during which meals are consumed) may be a good starting point. This means that you eat within an 8 hour window during the day. Cutting out snacks after dinner or having breakfast later (or removing it entirely) is a relatively easy way to achieve this. A cup of tea or coffee (ideally black with no sugar) is not considered by most practitioners to be a break in the fast so many choose to continue with their morning cuppa. Exercise has also been shown to increase catecholamines (which activate a hormone needed for ketosis) in the body and can be continued during fasting. When breaking any fast the first meal should ideally be one that is high in fibre and fat and low in sugar and starch. This limits the surge of insulin after fasting and allows stable blood sugar levels to be maintained.
It is worth noting that not everyone has the ability to function well in a fasting state. It depends to some degree on the efficiency of individual ketosis pathways. For example, individuals who regularly skip meals or burn off blood sugar through exercise may have more efficient ketosis pathways which may make it easier for them to fast without too many side effects. For other individuals, adopting a ketogenic diet for a few weeks prior to fasting may improve the outcomes of fasting. In this case a gradual approach may be recommended - for example, adopting a ketogenic diet for a few weeks and then transitioning to 12:12 fasting, 14:10 fasting and then finally to 16:8 and then 18:6 ratios.
Current research indicates that intermittent fasting is safe and well-tolerated in healthy adults. However, fasting may not be right for everyone and if you are currently under the care of a medical doctor or are hypoglycaemic then you should discuss any proposed fasting regime with your medical team or practitioner.
If you would like to read more here is a link to an excellent scientific article: https://www.nejm.org/doi/full/10.1056/NEJMra1905136