Note: You should always consult with a doctor, nutritionist, or dietitian before trying intermittent fasting, or any diet.
Recently there’s been a lot of buzz around intermittent fasting. With so much information available, we are splitting this into two parts. (Read part 2 here.)
In today’s post, let’s define what intermittent fasting is, the different types and popular variations trending, and what scientific evidence says about intermittent fasting and weight loss.
What is intermittent fasting? Is it just another quick diet fix?
The world of diets is noisy and saturated with the latest fads that come with quirky or strict rules around certain foods. You’ve most likely heard about intermittent fasting. Right now, this is one of the world’s most popular health trends, the main idea of this diet is alternating between periods of fasting and eating normally.
Tracing back thousands of years, fasting has been historically done within cultural or religious groups, and is still being practised today, such as fasting during the month of Ramadan as observed by Muslims. The idea of intermittent fasting isn’t new.
Today, intermittent fasting has become a popular way to lose weight. Perhaps so because this is simpler to follow compared to other fad diets; you don’t have to change what you eat, but when you eat. Intermittent fasting is technically not a diet, but an eating pattern – a smaller time frame to eat the usual amount of food.
Advocates of intermittent fasting claim that fasting is the ‘metabolic key’ to unlock weight loss, lower inflammation, reduce risk of heart disease and diabetes, and perhaps even extend your life.
What are the types of intermittent fasting?
There are at least 7 ways to do this – from skipping one meal a day to fasting for a full 24 hours; broadly categorised into time-restricted fasting, alternate-day fasting and modified fasting.
For time-restricted fasting, a very popular variation is the 16:8 fast. This means 16 hours overnight fast, with an 8-hour eating window. Someone could be skipping breakfast and consuming only lunch and dinner, or skipping dinner and consuming breakfast and lunch instead. Those following the 16:8 fast might progress to 18:6 or 20:4 fasts and alike.
Alternate-day fasting, as the name implies, involves alternating days between fasting and eating normally. Variations include total abstinence from any calorie intake on fasting days. On those days, non-caloric drinks such as no-sugar black coffee, tea and water are allowed.
The 5:2 diet is a popular form of modified fasting, where you eat normally for 5 days and fast for 2 non-consecutive days. The term fasting in this case is misleading as one is technically not absolutely abstaining from food like a true fast, because you’re allowed to consume up to one-quarter of your usual calories intake on those ‘fasting’ days. This is about 400 calories per day for women, and 550 calories per day for men based on median figures for Singaporeans’ calorie intake.
What does the scientific evidence say about intermittent fasting and weight loss?
In short, some people lose weight, others don’t, and it doesn’t seem to be any better or worse than any other diet.
According to a review conducted on eleven randomised-controlled trials (the gold standard of research) comparing intermittent fasting and traditional calorie-restriction diets studied in overweight adults over at least 8 weeks, nine out of these 11 studies showed no significant differences in weight or body fat loss between the groups (Rynders, Thomas, Zaman, & Pan, 2019).
If there were any difference or benefit of intermittent fasting over other diets, it is not of much significance.
All diets fail poorly in the long term.
We know that long-term sustained effects of weight loss is important and all diets fail poorly in the long term. But what about intermittent fasting diet? There is currently not enough evidence to conclude this.
Other research so far has demonstrated that restricting calories in animals tends to extend lifespan and improve tolerance to metabolic stresses in their bodies (Robertson & Mitchell, 2013; Harvie & Howell, 2017). However, this is less convincing in human studies.
The bottom line
As noted earlier, some benefits of restricting calories may have been observed in animal studies (Robertson & Mitchell, 2013). However, those have not been demonstrated in humans. At present, it is inconclusive whether intermittent fasting is superior to other weight loss methods in terms of total weight loss, biological changes, reduced appetite, and compliance.
Additional high-quality trials and human studies with longer follow-up of more than a year are needed to show the benefits and direct effects of intermittent fasting. Currently, there is no conclusive recommendation on intermittent fasting for weight loss.
In the next post, we’ll cover what the science says about intermittent fasting’s extensive list of health claims, is it safe to do intermittent fasting and should you jump on it? Till then.
Harvie, M., & Howell, A. (2017). Potential Benefits and Harms of Intermittent Energy Restriction and Intermittent Fasting Amongst Obese, Overweight and Normal Weight Subjects—A Narrative Review of Human and Animal Evidence. Behavioral Sciences, 7, 4.
Health Promotion Board. (2020). What should my daily calorie intake be? Retrieved from https://www.healthhub.sg/live-healthy/1341/how-much-to-eat-at-each-meal
Robertson, L. T., & Mitchell, J. R. (2013). Benefits of short-term dietary restriction in mammals. Experimental gerontology, 48(10):1043-8.
Rynders, C. A., Thomas, E. A., Zaman, A., & Pan, Z. C. (2019). Effectiveness of Intermittent Fasting and Time-Restricted Feeding Compared to Continuous Energy Restriction for Weight Loss. Nutrients, 11, 2442.