One of the most popular dietary strategies today is the concept of fasting. To “fast” means to go without food for a period of time. Everyone is already doing this daily – we eat during the day and we don’t eat while we are sleeping. The average person consumes their meals and snacks over about a 12-hour period, and then they stop eating, or fast, for around 12 hours. Fasting diets increase the number of hours we don’t eat in order to stimulate various health benefits, including weight loss.
According to the research, there are different theories on why fasting provides potential health benefits. One theory is that during the time that we fast, our cells get to practice being in survival mode and learn how to better cope with daily stress. Fasting may provide protection to our nerve cells and might reduce our risk for certain diseases such as cardiovascular disease, diabetes, and various neurological disorders. In terms of weight loss, when you don’t eat for 10–16 hours, your body starts to use its stored fat for energy, and ketones are released into the bloodstream and used for energy.
Research on fasting as a weight loss technique has been ongoing for over a decade. There are several types of fasting routines that have been studied. In this article we will discuss two methods: Intermittent Fasting (IF) and Time Restricted Feeding (TRF).
Intermittent Fasting (IF):
The technique of Intermittent Fasting requires alternating between Fasting Days and regular eating days called Feeding Days. On the Fasting Days people consume one mid-morning meal of around 500-600 calories, and then on Feeding Days they are instructed to eat as they normally would. There are various fasting routines. One popular method is Alternate Daily Fasting, where Fasting and Feeding Days are done on opposite days. Other IF routines suggest only 2 Fasting Days each week.
In 2019 a review article was published that looked at the results of 11 weight loss studies that directly compared IF to more traditional calorie controlled diets. Results found that both methods provide very similar weight loss. The amount of weight loss for both methods ranged between 3-8%. Previous studies that have looked at metabolic changes with IF have found improvements in cholesterol, triglycerides, fasting glucose and insulin levels. These metabolic improvements were found to be similar to those from previous weight loss trials. There is limited research on IF and weight maintenance at this time.
Time Restricting Feeding (TRF):
The concept of Time Restricted Feeding (TRF) is a bit different from Intermittent Fasting. TRF looks to increase the time we fast overnight every day. Instead of eating over a 12-hour period, different forms of TRF have its followers eating over a 6 to 10 hour timeframe, and going 14 – 18 hours without food. While TRF is not currently as popular as IF, it might be less restrictive and easier to follow. When this method was researched there were generally no dietary restrictions given during the eating period, but study subjects still tended to lose some weight.
Carefully controlled long-term studies on TRF are limited, but research has shown modest weight loss of <3% in shorter trials. More research is needed to see if TRF is a useful weight loss strategy long term, and if it could improve weight loss outcomes or help with maintaining weight when combined with other healthy eating strategies. An interesting controlled feeding trial done in 2018 with men with pre-diabetes showed that even without weight loss, a TRF routine may improve insulin production, insulin sensitivity, and blood pressure.
Pros of Fasting
Cons of Fasting
Scientific reviews that have compared the various studies on TRF and IF agree there needs to be more research into the overall and long-term effectiveness of these methods. Many different protocols and timeframes were used when researching fasting, so there are no set protocols or standards for following these methods. There’s also no consensus on whether IF or TRF should be used alone or combined with other weight loss or healthy eating methods.
Everyone is an individual with unique health needs. What might be safe for some can be unsafe for others. That is why it is so important to talk with your medical doctor and / or a registered dietitian before starting any new diet or lifestyle.
Rynders ,C; Thomas, E; Zamon, A; Pan, Z; Catanacci,V; Melanson,G. Effectiveness of Intermittent Fasting and Time Restrictive Feeding Compared to Continuous Energy Restriction for weight loss. Nutrients 2019, 11, 2442; doi:10.3390/nu11102442
Mattson, MP; Longo, VD; Harvie, M. Impact of intermittent fasting on health and disease processes. Ageing Res Rev. 2017 October ; 39: 46–58. doi:10.1016/j.arr.2016.10.005.
Longo, VD; Panda, S. Fasting, circadian rhythms and time restricted feeding in healthy lifespan. Cell Metab. 2016 June 14; 23(6): 1048–1059. doi:10.1016/j.cmet.2016.06.001.
Sutton, EF; Beyl, R; Early, KS; Cefalu, WT; Ravussin, E; Peterson, CM. Early Time-Restricted Feeding Improves Insulin, Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metabolism 2018, 27, 1212–1221.
on March 3, 2022. Kelsea is a Registered Dietitian with her Master's degree in Nutrition from Bastyr University in Kenmore, WA, and is one of our Health Coaches.