Q&A on fasting for weight loss with Krista Varady

Krista Varady, professor of nutrition at the University of Illinois Chicago, explains the basics of fasting. In a conversation with Science Sparks, she breaks down time-restricted eating and how it can be an effective way to lose weight. 

What is fasting? 

Fasting, in a really basic sense, is just a period of eating followed by a period of not eating. So very, very simple.  

What are the different types of fasting? 

Intermittent fasting is basically an umbrella term for two major types of diets. One is time-restricted eating, where people eat within, like, a shortened window of time, usually like six to eight hours. So with that, when you pretty much don’t eat anything ’til, let’s say, noon, and then from noon to 8 p.m., you can eat whatever you want. And then after 8 p.m. that night, you just don’t eat anything ’til noon the next day. And then the other one that’s a little tougher to do is called alternate-day fasting, and that’s when people literally fast every other day. So you have 500 calories on one day, that’s called your fast day, and then the next day you get to just eat freely. And the diet kind of flip-flops like that. So feast day, fast day, feast day, fast day, indefinitely.  

Is fasting an effective way to lose weight? 

We’ve been running studies for about 20 years now in both alternate-day fasting and time-restricted eating, and we found that both help people lose anywhere between like three to, some people lose like up to 10% of weight. But usually the average is like 5% weight over about a year. So it’s kind of on par with, like, just any other diet intervention out there. 

Does it matter what you eat when fasting? 

The weight loss is pretty much sheerly dependent on how many calories you take in. So we show that people naturally eat within about a 14-hour window. So essentially, all time-restricted eating is doing is kind of, like, just narrowing down, cutting out about six hours of eating per day. And by doing that, people naturally cut out anywhere from like three to 500 calories per day. So that’s what’s resulting in the weight loss. 

Basically, fasting just helps people eat less, and then that lowers people’s body weight. But obviously, I’m a nutrition professor, so like, what you like, eating a healthy diet is definitely, like, really important. You want to have lots of, like, fruits and vegetables, like lean proteins, like whole grains, so that type of stuff will help with longevity, and just like, lower cardiovascular disease and diabetes risk.  

Who should avoid fasting? 

So I wouldn’t recommend it for say, still in teenagers. It’s a little tricky, because eating disorders kind of set in at a young age — for most people, like between 12 and 25, the research shows — so I would be a little cautious with implementing it in a younger population.