Updated: Jul 25
One of the hottest new nutrition trends in weight loss these days ironically doesn’t actually involve food, focusing instead on the timing of your meals.
Wait, isn’t fasting just another word for starving?
Fasting is a traditional practice across multiple cultures that dates back for centuries, often done for religious or health reasons. There are many different forms of fasting: water fast, juice fast, bone broth fast, to name a few. Fasting is not the same as starving. Starving is when you don’t know when or even if you will have your next meal. Fasting is when you are voluntarily cutting back on eating by controlling the timing of your intake.
Intermittent fasting (IF) as a health practice in modern times helps us to reset our insulin and metabolism, as well as detoxify from habits/dependencies (e.g. carbs) that develop over time. Research has shown that intermittent fasting can improve outcomes for a number of health conditions ranging from diabetes and prediabetes to cancer.
“Starving is when you don’t know when or even if you will have your next meal; fasting is when you are voluntarily cutting back on eating by controlling the timing of your intake.”
The principle is simple: You stop stimulating your insulin response for a specified time frame in order to give your body a chance to use up some of the stored up fat. Some popular fasting regimens include the 5-and-2 (i.e. 5 days normal/2 days extreme restriction), and the 16:8 (i.e. 16 hours of fasting and 8 hours eating window). When your body burns fat for fuel instead of carbohydrates it produces ketones, another way of saying this is that it goes into ketosis. Even a 13 hour window of fasting has been shown to allow the body to transition to ketosis.
How does it work?
IF works by reducing insulin levels in the bloodstream and pushing the body to burn fat. Insulin is the key hormone produced by your body when digesting food. It normally works to store excess fuel in the body, which is usually fine when we are consuming natural whole foods. The problem arises with our modern day diet that tends to have a lot of quick and convenient options, or processed foods that have had flavor enhancers (like sugar) added. For example, eating a whole orange may cause the insulin level to rise as your body digests the carbohydrate along with the fiber in the fruit, but drinking orange juice spikes up the insulin a lot faster, without the compensatory fiber.
Snacking between meals causes insulin levels to stay elevated all day long, and excess weight can cause insulin resistance, which further elevates insulin levels in the bloodstream. When we can reduce levels of insulin through fasting, we allow the body to consume more of the excess stored fuel and encourage the body to flip over to burning fat. Most of our bodies have become very efficient at consuming carbs to produce sugar as energy. When we fast, we are retraining our bodies to consume fat to produce ketones as energy instead.
But what if I get hungry?
Turns out there are many different levels of hungry, just as there are many different levels of full. It’s a good idea to become attuned to the signals that your body is giving you and to learn how to assign a number value on a scale to that physical sensation. Like there’s “I’m too full to eat any more and it would feel good to go for walk” (let’s say 80% full) and then there’s “I feel uncomfortably full and need to have a rest” (more like 120% full).
Hunger can some represent actual physical hunger (like when you haven’t eaten in >5 hours), thirst (dehydration from inadequate fluid intake), emotional hunger (stress), or withdrawal. If you have ever felt voraciously hungry a few hours after a big pasta or sandwich meal, you are probably experiencing a carb withdrawal/craving rather than actual hunger.
Think about it logically - there is no possible way you could actually be starving, but it makes sense that a steep drop in insulin level could be signaling your brain that something is wrong and worth creating drama around. Insulin levels spike in response to high carbohydrate intake, but when those levels fall off we end up craving more carbs which in turn leads to a vicious cycle and eventually insulin resistance.
While there are many approaches to intermittent fasting (IF), I have found that “fat fasting” is a good way to ease into it. The more that you can teach your body to use fat as a fuel, the more “fat-adapted” (aka “in ketosis) you will be, and the less ravenously hungry you are likely to be even several hours after the last meal. And if you do get a little hungry, you can have a “fat snack”.
How does “fat fasting” actually look?
When you are hungry during a fast, reach for a “fat snack”, like 1-2oz of nuts (Brazil nuts are my favorite because they are filling, and you won’t eat too many), half an avocado, one whole hard-boiled egg, 1-2 oz cheese, or healthy peanut/almond butter. Give yourself about 30-60 minutes after consuming the “fat snack”, to gauge your hunger level. If you are still very hungry, you may want to have a meal like a salad or soup. If you are only a little hungry, you can add another “fat snack”, or try a “fiber/fat snack”, like celery stick with cream cheese, or chia pudding with coconut milk.
Our bodies tend to do best with gradual adaptation to fasting, slowly extending the fasting time from 12 hours, to 15 hours, and so on. Dehydration is a common early pitfall, so be sure to track your fluid intake: make sure you are consuming at least 8oz of water every 2 hours. Our bodies may also be craving salt during fasting, so consider adding ¼ tsp of salt to a glass of water, have a handful of salted nuts or sprinkle some salt on your avocado.
Okay – I’ll admit it
The urge to use the word “fast” as a marketing pun for this piece was irresistible. Intermittent Fasting is definitely NOT intended to be a quick fix and I do NOT believe that short term “diets” are a permanent solution to weight loss. IF is an approach to eating that is intended to be part of a long term food protocol that is customized to your body.
Please note that our bodies are all different and will respond differently to various eating plans. Do NOT do Intermittent Fasting without explicit guidance from a licensed dietician or doctor, if you have diabetes, are on any medications, are training for an athletic event, have experienced an eating disorder or have any other health issues. Ketosis can be dangerous in certain medical conditions and should not be taken lightly.
The Obesity Code by Dr Jason Fung is an excellent summary of the science behind intermittent fasting as an evidence based approach to weight loss and blood sugar management.