Here’s some food for thought about this buzzy eating regimen.
Intermittent fasting continues to get a lot of attention these days. Proponents look to numerous mice studies to show that fasting for more than 12 hours may increase your life, your alertness and your body’s ability to burn fat. But how does this translate to humans? Research on intermittent fasting is still preliminary, but science is offering some possible insights; however, it isn’t the silver bullet you might think it is if you read the praise of its most devout practitioners.
Recently, researchers at the University of Alabama at Birmingham looked at how intermittent fasting may affect our weight. 24Life spoke to Dr. Courtney M. Peterson, an assistant professor in the department of nutrition sciences at the university, about how our bodies react to this short-term fasting and what is the best way to give it a try if we choose.
24Life: What is intermittent fasting?
Courtney Peterson (CP): Intermittent fasting is a meal timing strategy, for which you just eat your food in a smaller time period each day, so you’re lengthening your fasting period from dinner one day to breakfast the next. You can do that by having either a later breakfast or an earlier dinner. There’s no one standard definition in terms of what constitutes daily intermittent fasting; however, the average American eats within a 12-hour period, so I tend to think of anything that’s less than nine or 10 hours of eating each day to be intermittent fasting.
24Life: What piqued your interest in researching intermittent fasting? And why should we pay attention to this style of eating?
CP: There was a really important mice study done at the Salk Institute that looked at how the time of day one eats affects health. The researchers divided the mice into two groups. Both groups ate the same total number of calories, but one group ate those calories by grazing throughout the day and the other group ate the calories by eating within an eight-hour period early in the rodent day.
What they found was that the rodents who grazed actually gained more weight, had worse blood sugar control, had higher levels of inflammation, and just across the board were much less healthy than the rodents who ate within eight hours early in the day.
This kind of blew my mind because you always hear that you should eat lots of small meals to keep your metabolism revved up. But if you actually look at the data that’s out there, the number of meals per day that you eat doesn’t really affect how many calories you burn. In the best studies on this, if you lock people in a room for 24 hours and measure how many calories they burn, it doesn’t matter whether they eat two meals a day or seven meals a day, they still burn the same number of calories.
But it looks like potentially the time of day when you eat may affect how many calories you burn. And this is driven by what’s called the circadian or biological clock that controls our metabolism throughout the day. It basically makes you better at doing certain things at different times of the day.
Conversely, your best blood sugar control is in the morning. That means if you eat the same meal for breakfast and dinner, your blood sugar won’t spike as high in the morning, but it’ll spike much higher in the evening. We also know that something called the thermal effect of food, how many calories you burn when you’re digesting your food, is also a little higher in the morning, and that might give you a little bit of a metabolic edge.
To make a long story short, I saw this study on rodents and thought, “This is phenomenal. If this works in people, it would help a lot of people. We really have to test this in humans to see if it works the same way.”
24Life: Do you burn more calories or fat during intermittent fasting?
CP: So just to backtrack, in my study, we took 11 overweight men and women and had them try intermittent fasting, during which they ate between 8 a.m. and 2 p.m. We were interested to know, does this help you burn extra calories like it does in rodents? What does it do to your appetite? And what might be going on to cause these benefits?
Unfortunately, what we found is you don’t burn more calories when you do intermittent fasting; however, it looks like you may burn more fat. We’re not certain of that yet because we saw only a 6 percent increase in fat burning, and it wasn’t quite statistically significant—meaning we don’t have a definite yes or no as to whether that’s a real effect. However, we suspect it might prove to be because during intermittent fasting, we found a really long period at night when fat burning was definitely higher, so that could be beneficial for weight loss.
We know that appetite is often triggered by how quickly your blood sugar levels fall, and that once you’ve hit a more fat-burning zone, your appetite levels are steadier. We found that during the intermittent fasting, hunger levels were much more stable. Participants didn’t have the dramatic swings in hunger.
24Life: Why didn’t people get hungry on intermittent fasting?
CP: In my small study, we had an 18-hour fasting window, so you’d think the participants would get much hungrier in the evening, but we found that the control group [those who weren’t fasting] actually got hungrier in the evening compared with the group that had already eaten all their food. What we think might be happening is that the control group members get to 8 p.m. and they still haven’t had dinner, so their bodies are registering that they are missing a third of their daily calories.
It seems to us that maybe the timing of the meals matters less for appetite, and what your body is really doing is mostly counting up how many total calories you have throughout the day. There may be some benefits from eating early in the day because you never get to that point when you’re ready to binge in the evening because you’re still missing a lot of the calories you need.
That’s what we think is going on, but we don’t know for certain. We can’t quite prove that yet. That’s our best theory.
24Life: Did the subjects have a hard time adjusting to eating in a six-hour window? And how would you suggest someone start an intermittent fasting program?
CP: Yes, they did. We found the participants said it typically took them about two and a half weeks to adjust. They said it was very challenging, and the harder part of it was eating enough food in that six-hour period—stuffing themselves with food was actually much harder than going without food in the evening.
I recommend to people interested in trying intermittent fasting to start with measuring how much food you typically eat, and then start with a nine-hour eating window and gradually work your way down to whatever’s comfortable. You could stay at nine if you really like it, or you could move to eight and see if that works for you.
Something to keep in mind: There is some interesting data suggesting that basically no matter what your baseline is now, like, say you’re eating over 15 hours a day, if you knock that down to 11 hours a day, it looks like you’re getting benefits.
24Life: Does it matter what time of day you have your eating window?
CP: We think that the time of day that you have your eating window is really important. In a human study of intermittent fasting done by Mark Mattson of the National Institute on Aging, participants either ate a normal three meals a day, or they tried eating between 5 p.m. and 9 p.m., a four-hour window at night. They found that the participants who ate between 5 p.m. and 9 p.m. at night lost more weight and lost more body fat. So that’s fantastic.
The funny thing is, unfortunately, their blood sugar control got worse and so did some of their cardiovascular disease risk factors. What I think is going on in that study is they got the benefits of intermittent fasting [weight loss and fat loss], but their health got a little worse. They weren’t eating at the right time of day with respect to their circadian or biological clock. I mentioned earlier that we know blood sugar control is better in the morning, and your metabolism is a little better in the morning, so I think that’s what happened. And I think examining the times of day eating windows occur is the future of research in this area.
24Life: Can you still get benefits of intermittent fasting even if you don’t do it daily?
CP: I would encourage people who want to try it to not see it as an all-or-nothing approach because they can try it and get some health benefits. Or at least that’s what we think based on a rodent study, in which rodents did the intermittent fasting on weekdays and they just ate normally on the weekends. The researchers found the rodents still got most of the benefits. So that to me is really encouraging because it means that in my daily life, I can do intermittent fasting on Mondays through Fridays and then eat whenever I want to eat on the weekends.
24Life: Is there anyone who shouldn’t be doing intermittent fasting?
CP: I recommend that only adults try this. Children should not try it because they’re growing, and we know intermittent fasting slows down cell growth rates. For the same reason, it is not appropriate for pregnant women.
Also, anyone with a serious disease like congestive heart failure or diabetes should check with a doctor.
The Different Faces of Intermittent Fasting
There are a number of intermittent fasting strategies. Here are some popular ones:
Alternate-day modified fasting. This involves doing a modified fast every other day, typically eating about 25 percent of your daily calories every other day.
Alternate-day fasting (a complete fast every other day). This isn’t sustainable for most people—you just get super hungry and don’t adapt over time.
The 5/2 diet. Very popular in the U.K., this diet basically involves having a modified fast two days a week. On your fasting days—women eat about 500 calories and men eat about 600 calories—you basically have a super small breakfast and super small dinner.
Eating breakfast like a king, lunch like a prince and dinner like a pauper. This is another way you can effectively change your meal timing, even though you’re not changing the timing of your meals, you’re changing how many calories you’re eating at each meal. With this type of program, you’re trying to front-load more calories at breakfast or lunch. A couple studies in Israel found that this helped people lose more weight.
Considerations to Support Your Fast
Especially when fasting, nutrition and hydration are important considerations. Some popular resources for support include:
– Greens Powder (like greens+) with water
– BCAA (branched chain amino acids) powder (or 5 capsules)
– Green tea or herbal tea, in addition to plenty of water
Photo credit: 123RF, Julien Tromeur