Intermittent Fasting Basics
Over the last few years, intermittent fasting or time restricted fasting has gained popularity. This has led hosts of people diving through the depths of the internet trying to figure out if they should be doing it, what is the best way to do it, and what is it good for. So we decided we are going to distill out the basics and tease out some of the details. about it.
So to start off, is the ever boring but necessary lexicon portion. When reading articles, there are a few terms that are used but are not interchangeable. First is time restricted fasting. This is where a person eats in a calorie deficit and only for a specified time period of a day. Then there is intermittent fasting which is typically defined as someone who fasts for a longer period of time, typically 24 hours, oncer per week or per designated time frame. Then there is time restricted eating. This is where an individual is allowed to eat as much and in some studies, whatever they want but only during a specified time frame. The difference between the two is that with any type of fasting total calories are lower than our resting rate. This means that they will have different effects on your body and you must be clear when you are reading or listening to advice from an expert, you know what one they are talking about. The last form is called time restricted feeding, this is used for animal studies where they are fed only during a specific time. While animal studies are very beneficial, human trials carry more significant weight. In these animal trials, it is also important to look for if the animals were fed in a free choice manner during the feeding window or were in a calorie deficit.
One of the first questions is typically when should my window be? The body runs on what is called a circadian rhythm which is a 24 hour cycle of the body. The body’s internal clock can be altered by external stimuli with the primary one being light. This is why people often recommend getting out in the sun when traveling to help set your internal clock to the local time. Food can also stimulate and alter our internal clocks. An identical meal consumed at different times of day has different blood sugar effects with the lowest being in the morning and the highest in the evening. (1) If your blood sugar response is an important part of your diet and you are aiming for the lowest possible response, you would want to have your feeding window earlier. If it is something that you aren’t worried about, do whatever time of day works best for your life.
The next question is often how long should my eating window be. Most research uses an 8 hour feed with a 16 hour fast. Some studies have done 6 hour feeds and 18 hour fasts and some have gone to 9 hour feeds with 15 hour fasts. Unfortunately, no study that we could find has done a head to head comparison to prove more effectiveness of one versus another. They do however almost always use a 12 hour eating window as their comparison so we know that there are some benefits to a time restricted eating pattern for some individuals. The current recommendations are to try different time frames, and pick what makes you feel the best.
All of this can be boiled down into a few simple points; if blood sugar is important to you, an eating window earlier in the day could be beneficial, if weight loss is your goal, it appears that using time restricted eating can be an effective way to reduce calorie intake. However, it does seem like the calorie is still the king so to effectively lose weight, you still need to be in a caloric deficit, but for some individuals using a reduced eating window can help them achieve it.
Van Cauter, Eve; Polonsky, Kenneth S.; Scheen, André J. (1997). Roles Of Circadian Rhythmicity And Sleep In Human Glucose Regulation* Endocrine Reviews 18, 5.
Jamshed H, Steger FL, Bryan DR, Richman JS, Warriner AH, Hanick CJ, Martin CK, Salvy SJ, Peterson CM. Effectiveness of Early Time-Restricted Eating for Weight Loss, Fat Loss, and Cardiometabolic Health in Adults With Obesity: A Randomized Clinical Trial. JAMA Intern Med. 2022 Sep 1;182(9):953-962. doi:
