The diet regime of intermittent fasting recently caught my attention when listening to an episode of This American Life. And then a close friend told me he’s planning to switch from a low-carbohydrate diet to some form of intermittent fasting.
I got to wondering, though: Are the health-benefit claims from intermittent fasting backed up by scientific evidence?
Research studies have shown that reducing your daily caloric intake by 20 to 40 percent is an effective way to lose weight and improve cardiovascular and metabolic health. However, it’s very difficult to eat less every day for a long time. So people are looking for more manageable ways to improve their health, and many are turning to intermittent fasting — short periods of eating little to no energy-containing food and drink.
To learn more about intermittent fasting, I turned to fasting expert John Trepanowski, PhD, postdoctoral research fellow at the Stanford Prevention Research Center, for answers:
What are the health benefits of a calorie-restricted diet?
Calorie restriction is probably the most scientifically established diet regimen for improving health. The main benefits include improvements in risk indicators for cardiovascular disease and type 2 diabetes, which include reductions in total cholesterol level, blood triglycerides, blood pressure, carotid intima-media thickness, insulin and fasting glucose. The biggest limitation is that most people find it incredibly challenging, and some find it impossible, to follow a calorie-deprived diet for any notable length of time.
Why has intermittent fasting become increasingly popular?
Michael Mosley's "Eat, Fast and Live Longer" documentary on the BBC introduced millions of people to intermittent fasting. Beyond that, I think intermittent fasting is appealing to many people, because they can lose weight on the diet but still have guilt-free days of eating what they want on a regular basis.
There is an increasing number of studies that suggest that intermittent fasting is a viable approach to weight loss for some. But you will have to wait until the results of my doctoral thesis are published to see if intermittent fasting is as effective for weight loss as daily calorie restriction (shameless plug!). And no study to date has examined whether intermittent fasting is effective in people who previously tried and were unsuccessful at calorie restriction.
Can you give examples of different types of intermittent fasting?
The 5:2 diet is a particular form of intermittent fasting, with five consecutive "normal" days of no restriction followed by two consecutive days of eating only 25 percent of your energy needs. I believe there have been two studies on the 5:2 diet in humans, and both studies found that the benefits were mostly the same as calorie restriction, such as weight loss and decreases in insulin.
Time-restricted feeding involves reducing the window of time to anywhere between four to twelve hours that someone takes in calories each day. The theory behind this dietary plan is that we have a circadian rhythm that calls for food intake at times and no food intake at other times in order to experience optimal health. Continuously eating, without periods of no food intake, disrupts the circadian clock and leads to metabolic derangements — such as lowered energy expenditure and elevated glucose and insulin.
Time-restricted feeding could lead to weight loss by harmonizing our eating pattern with our circadian rhythm, or it could be simply due to the fact that there are fewer "opportunities" to take in energy. And some people will lose weight due to following any type of structured eating plan, regardless of the specifics.
It's very hard to do an accurate intermittent fasting study in humans, because it's really difficult to get an accurate measurement of what people eat at any particular time of day. The main disadvantage of time-restricted feeding is resisting the temptations that come from our 24-hour-access-to-food environment, but that disadvantage exists with all dietary plans.
What inspired you to study different diets?
I met a very inspirational professor, Richard Bloomer, PhD, at the University of Memphis. I helped him run some studies on the Daniel Fast, which is a more stringent form of veganism based on the biblical book of Daniel. From there I wrote some review articles on fasting and calorie restriction, and I decided to study a form of intermittent fasting called alternate-day fasting for my PhD.
As a postdoctoral research fellow at the Stanford Prevention Research Center, I'm now studying factors that predict weight loss success on low-fat and low-carbohydrate diets. I am also doing meta-research — basically "research on research" to find ways to do science better.
Have you ever fasted?
I have done the Daniel Fast. It's pretty tough. If you want to expand your cooking skills, I suggest doing the Daniel Fast. There's no way to eat anything on this diet that is both warm and appetizing without following good cooking principles.
A cautionary note: In his review of fasting studies, Trepanowski said daily calorie restriction and alternate-day fasting do not appear to increase eating and mood disturbances among research participants who did not have an eating disorder. However, it's best to speak with your physician before starting an intermittent fasting regimen, particularly for those with a history of or at risk for eating disorders.
Previously: Stanford dietitian explains how – not just what – you eat matters, Stanford seeks participants for weight-loss study and Study clarifies link between dieting, exercise and reduced inflammation
Photo by Jean Fortunet