Curious about intermittent fasting? Here's what experts say you should know
Research on weight loss during intermittent fasting is still emerging
Intermittent fasting (IF) isn't a new way to eat, but researchers and experts say it's an area that has potential.
It is important to know that research on intermittent fasting "is still in its infancy," said Amy Kirkham, an assistant professor in the University of Toronto's clinical cardiovascular health department. She has also led several studies on time-restricted eating, a form of IF.
Intermittent fasting is generally defined as the cycle of eating and then fasting.
The length of fasting can vary, depending on the person or approach.
"The idea is not deprivation or to go into excess, but to balance the feeding and the fasting because both are very essential for us," Dr. Jason Fung, a nephrologist and author of several books on IF, told CBC's The Dose guest host Dr. Peter Lin.
Anar Allidina, a registered dietitian based in Richmond Hill, Ont., says that intermittent fasting is "like a reset" for our bodies. The break from eating prompts our bodies to cleanse itself and get rid of more old cells, she adds.
Fung and others say there is some promising research showing the health benefits of IF, like improved cardiovascular health.
Research has shown that many of the health benefits of fasting are usually seen between the 14 to 16-hour mark, says Allidina.
"Studies have shown that during this time that you're fasting, [it] can have really important markers in your metabolic health, for example with cholesterol, with blood sugars and inflammation. So it can really help with lowering those levels," she said.
But before you even consider fasting, Allidina and Fung emphasized that it isn't for everyone.
So if you're interested, here's what experts say you should know about intermittent fasting.
Is it safe?
For most people, it is absolutely safe to pause eating for periods of time, says Allidina.
"Giving your body that break is absolutely OK and it's actually good for you," she said.
Those who shouldn't try intermittent fasting are:
- Anyone with a history of an eating disorder.
- Anyone who is underweight or malnourished.
- Pregnant women.
- Women who are breastfeeding.
Allidina and Fung recommend speaking with your health-care provider before trying intermittent fasting.
Is there only one way to intermittently fast?
There are several different approaches to intermittent fasting.
Time-restricted eating (TRE) is a common way as it limits when you eat your meals and snacks to a specific time period.
Fung says the most common fasting strategy is 16 hours of fasting and eating within an eight-hour time period.
"So you might eat for example from 11 a.m. to 7 p.m. or you might do it early, say 9 a.m. until 4 p.m. There's various ways to do it, but that's one of the more popular schedules," he said.
Another approach is the 5:2 method, where you eat normally for five days and then restrict calories two days a week to about 500 calories a day for women and 600 calories for men.
Alternate day fasting, or ADF, is when someone consumes food during an eight-hour period and then doesn't eat the next day, which translates to roughly 36 hours of fasting.
Fung adds there is flexibility with intermittent fasting.
"There's pluses and minuses of all of those strategies. So it's not like one is right and one is wrong. It's finding what really works for you," he said.
Are there health benefits?
Yes, but it depends on the length of the fast and fasting type.
Anecdotally, Fung and Allidina have heard from people who tried intermittent fasting that they feel more alert and energized, and less tired.
Research on other health benefits is varied.
University of Illinois researchers who published a review of clinical trials found that the three major types of intermittent fasting — TRE, the 5:2 diet and ADF — can cause "mild to moderate weight loss" in those who were overweight and obese.
They said that mild to moderate weight loss was a change of about one to eight per cent from baseline. But, they said ADF and the 5:2 diet are "the only fasting approaches that produce clinically significant weight loss," according to their review published in the peer-reviewed Annual Review of Nutrition journal in 2021.
They went on to say that these regimens "may also improve" some aspects of cardiometabolic health such as blood pressure.
Korean researchers who published their systemic review and meta-analysis in the peer-reviewed Nutrients journal in 2020 found that time-restricted eating can help some shed some pounds and have better cardiovascular health.
Fung says people may lose weight while fasting because the body uses two different types of energy: sugar and fat.
When the body runs out of glucose (the main type of sugar in the blood), it'll turn to fat stores, a process known as metabolic switching.
Yet, there is research — including a study published a week ago in the Journal of the American Heart Association — that suggests intermittent fasting approaches may not be better for weight loss than restricting calories.
A study published in the New England Journal of Medicine last year showed that among 139 obese participants, time-restricted eating with calories restricted was "not more beneficial" than daily calorie restriction.
Other researchers who led a randomized clinical trial and published results in 2020 found that time-restricted eating did not show significantly different weight loss nor cardiometabolic benefits compared to the controlled group.
In another study published in 2017 in the peer-reviewed JAMA, researchers did not find alternate-day fasting better for weight loss or weight maintenance compared with daily calorie restriction.
Kirkham says more studies on intermittent fasting are needed.
"We certainly do need more research to really fully understand all the different parameters and its potential health effects and certainly its safety within specific populations," said Kirkham, who was recently awarded funding by Diabetes Canada to research which intermittent fasting period best impacts blood-sugar control.
More research is needed on intermittent fasting approaches, especially on the long-term effects, according to several researchers who have published studies.
If I want to try it out, how can I start?
Before anyone starts intermittent fasting, Allidina suggests people ensure their diet is full of essential nutrients.
"Once that's done, then you can bring in the intermittent fasting slowly, starting with the 12-hour fasting and increasing it up to 14 to 15 to see how you feel with that," she said.
She adds that fasting doesn't need to happen every single day in the beginning, as it will take time to build it into your schedule.
There are also free apps that can help people keep track of their intermittent fasting, Kirkham says.
Most people cope with the eating schedule change after that first week, she adds. When starting out, it's important to remember that minor symptoms like headaches, feeling hungry or irritability are common.
"It may be a bit of a shock to the system initially, but I think if you try it for two weeks … and if you don't feel better then maybe you have your answer," said Kirkham.
"Like any health intervention, it's not a one-size-fits-all."
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