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Intermittent Fasting: Can it Help With Weight Loss?
Neil Iyengar, M.D.
January 22, 2021

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Dr. Neil Iyengar is a medical oncologist at Memorial Sloan Kettering Cancer Center whose practice focuses on people with breast cancer.

His research is looking at the links between metabolic health and cancer, specifically looking at how diet, exercise, and medicines can prevent cancers tied to obesity, such as breast cancer, and improve outcomes for people with inflammation in fatty tissues.

Listen to the episode to hear Dr. Iyengar explain:

  • what intermittent fasting is and how it was developed
  • why some people think intermittent fasting could be a tool to prevent breast cancer
  • the research done so far
  • the risks of intermittent fasting and what people who want to try it should consider

Running time: 21:40

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Show Full Transcript

Jamie DePolo: Hello. Thanks for listening. Dr. Neil Iyengar is a medical oncologist at Memorial Sloan Kettering Cancer Center whose practice focuses on people with breast cancer. His research is looking at the links between metabolic health in cancer, specifically looking at how diet, exercise, and medicines, can prevent cancers tied to obesity such as breast cancer and improve outcomes for people with inflammation in fatty tissues.

Today, we'll be talking to Dr. Iyengar about intermittent fasting and why some people think it could be a tool for breast cancer prevention. Dr. Iyengar, welcome to the podcast.

Dr. Neil Iyengar: Thank you so much for having me.

Jamie DePolo: So, to start, just in case anyone isn't familiar, I know there’s been a lot of news coverage of intermittent fasting. But could you just explain to us what it is and how it was developed? Like what was the rationale behind it?

Dr. Neil Iyengar: Yeah. Absolutely. So, intermittent fasting is a dietary modification technique that's unique in that it really prescribes timing parameters for eating rather than actually prescribing a food pattern. And so it delineates itself from other dietary approaches by not having that kind of structured macro- or even micronutrient parameters and really focusing on the timing.

It came from the philosophy and the realization that we are really surrounded by an obesogenic environment, meaning that food is always available to us, and it's easy to access food for many people but not always high-quality food. And we'll touch on that I’m sure, but also that food consumption has gone on to involve multiple parameters, not just the need to supply our bodies with calories, but also emotional aspects as well as filling time and other similar habits.

So I think that where intermittent fasting has really come from is this realization that we came from a place where food was not so readily available, and really, food served the purpose of fulfilling our energy needs, our caloric needs, and not much more than that. Based on that realization, the hypothesis arose that if we can really time our caloric intake, we may be able to get our bodies back to that original homeostasis that was originally intended to keep our energy balance in check and avoid metabolic disorders like obesity, like diabetes, and like heart disease.

Jamie DePolo: Okay. So, if I'm understanding you correctly, it doesn't really matter at all what a person eats, it's strictly when a person eats.

Dr. Neil Iyengar: I think in its most technical definition, that is correct, but I would say that most advocates of intermittent fasting will add on the caveat, of course, that one of the goals of intermittent fasting is weight control. So of course, we do need to be mindful of what we're eating and how much we're eating. But yes, technically that is correct that intermittent fasting does not prescribe a particular food pattern.

Jamie DePolo: Okay. So, just to let people know, it doesn't mean you can eat donuts for 5 hours a day and call it good, right?

Dr. Neil Iyengar: That's right. That's right.

Jamie Depolo: Okay. Now, I have read about a couple different types of intermittent fasting strategies, basically it's a difference in the number of hours that people would either fast or eat. Is one of those strategies more popular or considered the standard? Some people advocate, say, an 8-hour, what they call a feeding window, so when you eat. Some people say it should be 6, some people say 5. So is there a standard, or does it depend on the person and the goals and what someone is advocating?

Dr. Neil Iyengar: That’s such an important question, because I think as the scientific community studies intermittent fasting as a tool not just for weight loss but for prevention of chronic diseases, we really need to be clear that intermittent fasting is an umbrella approach, and it really encompasses different methods, which could have dramatically different results depending on the endpoint that we're studying.

There are some more popular approaches to intermittent fasting. One of the methods, or one of the driving philosophies, for intermittent fasting is to keep it simple and to find an approach that fits with your lifestyle. So, a very popular method is to pick certain days to fast. So, for example, people often pick a 5:2 ratio where people will eat normally for 5 days out of the week, but then eat very low calories, usually somewhere between 500 to 800 calories, 2 days out of the week.

There is also time-restricted fasting, or time-restricted eating, as you mentioned. I would say the popular approach in terms of time-restricted eating is to choose a 14- to 16-hour fast every day, where folks are fasting for 14- to 16-hours per day and restricting their eating to an 8- to 10-hour window. That's another very popular method.

There are a couple of other popular methods that are worth mentioning. There are stop-and-go methods, so these are methods where you pick a 24-hour period, usually at least two 24-hour periods, where you’re fasting, whereas the rest of the days you are eating normally. That can also sometimes be interpreted as alternate day fasting, where people will fast either for 24 hours or they will eat very few calories, typically in the low hundreds, every other day.

And so those approaches are most common, and the theme there is either you're picking a day to fast or you're picking a large chunk of hours to fast.

Jamie DePolo: Okay, and this originally was looked at as weight loss. So how did it come about that people thought, “Well, maybe this could be a tool to prevent some of these diseases?” Was it just because of the overweight being connected to the diseases, or is there something else going on there?

Dr. Neil Iyengar: I think there are a couple of reasons why the appeal of intermittent fasting has poured over from just weight loss.

Certainly weight loss is a driving factor for studying this approach. We do know that that being overweight or obese does increase the risk of not just metabolic disorders like diabetes or heart disease, but it can also increase the risk of diseases like cancer. There are at least 13 different cancers that are associated with obesity. And so there has been a lot of research looking into weight-loss techniques either to prevent the development of these obesity-related cancers or to prevent a recurrence of an obesity-related cancer after diagnosis.

So, any successful weight loss strategy is certainly very attractive for study in preventing diseases, but there's more than that, of course. Part of what my team studies is how a chronic state of metabolic dysfunction can give rise to cancer cells. And we know that when your body is chronically bathed in high levels of insulin, high levels of glucose, high levels of inflammatory molecules that are common in metabolic disorders, these all provide the building blocks for the development and growth of cancer cells.

Approaches that help to reduce that chronic exposure to molecules that promote cancer growth like insulin or like inflammatory molecules are also very attractive. And intermittent fasting may be a way to put our bodies in periods of metabolic equilibrium where those high levels of insulin have been brought down dramatically. So, that kind of equilibrium or energy balance is also very attractive and very important and a reason why this approach is being studied for disease prevention.

Jamie DePolo: Okay. Thank you. That's a great segue into my next question. So, what kind of research has been done so far? What do we know scientifically about intermittent fasting?

Dr. Neil Iyengar: The research is pretty early in terms of prevention, or risk reduction, or for cancer. But for metabolic disorders, there has been a lot of activity and a lot of work that has been and is currently being done to look at intermittent fasting as a weight-loss strategy and as a way to improve diabetes or a way to prevent diabetes.

You know, the type of research that I would say we have the most data for is for weight loss. But that data's conflicting, and we certainly need more data to tease out some of the studies which have shown very different results.

And then when we start to get over to reducing the risk of metabolic disorders like diabetes and then also chronic illnesses like cancer, most of that data comes from pre-clinical research or studies that have not been done in humans. And so there is a lot of work being done right now to try to translate some of that data from pre-clinical models to human data.

Jamie DePolo: Okay, and when you say pre-clinical, are we talking cells and Petri dishes? Are we talking mice? What are we looking at?

Dr. Neil Iyengar: Yeah. At least in the cancer realm, we're talking mostly cells and Petri dishes. But there are some in vivo studies in mice that have been done as well, but certainly very early stage.

Jamie DePolo: Okay. Now, are there any drawbacks to intermittent fasting? Especially, you know, we have had some questions about it from our community. So, say somebody's currently being treated for breast cancer. Is there a risk for somebody who hasn't been diagnosed with cancer, and are there any extra or special risks for somebody who's in treatment?

Dr. Neil Iyengar: So, for someone who has not been diagnosed with breast cancer, I think that the risks of intermittent fasting are, one, the interpretation of how to do the intermittent fasting. We touched upon this earlier. I think that it's important to keep in mind that with any dietary modification technique, in order to lose weight, it's a mathematical equation. And essentially you need to be at a net energy negative. So take in less calories, or expend more calories, or a combination.

It's important to keep in mind, particularly with a diet like intermittent fasting, that not eating for a period of time, of course, doesn't give us a license to really gorge ourselves when we can eat. The danger with intermittent fasting is people get hungry! You're not eating for long periods of time, and so to maintain that control when your body is secreting hormones that are telling you that you're hungry and you need to eat, that's a challenge. And the idea with intermittent fasting is that as we train our bodies that that challenge becomes more and more surmountable, but that is a challenge.

The danger for people who have been diagnosed with cancer is that we don't have enough data yet. We don't know from a quality of life perspective, but also from a cancer perspective, how the diet will impact people. We are optimistic that it could be a successful weight-loss strategy for some people. And for some cancers, like breast cancer, weight gain is very common during breast cancer treatment, and so a weight-loss strategy like intermittent fasting may be helpful.

However, there are periods where people are near starvation, and if you're actively getting chemotherapy or if you're actively getting some kind of treatment that is associated with side effects, then putting your body in that state can make you even more vulnerable, potentially, to those side effects. There are studies that are going on right now testing whether or not that's true, but until we have that data I think we have to be very, very careful.

If somebody does want to try intermittent fasting, then my approach is to try it either after treatment like chemotherapy or radiation has been completed — the danger there is if your side effects get worse you might interrupt your treatment, you might not accomplish all of your treatment as originally set out, so either wait for that acute treatment to be completed — or do it very, very closely under the supervision of a nutritionist working closely with your nurse, with your doctor. And if these side effects start to get worse and the diet becomes more difficult to tolerate, then to consider switching to a different dietary modification approach.

Jamie DePolo: Okay. Thank you. I'm curious, too, in some of these studies, are they looking at the quality of the diet when people are eating? Because as we talked about earlier, it's not a license to just eat donuts all the time when you can eat. I’m assuming people still have to be mindful of nutrients and making sure they’re the right nutrients and the right combinations?

Dr. Neil Iyengar: That's right. A lot of the studies are looking at what people are eating, but it's challenging also because you want to tease out, what is the effect of the intermittent fasting strategy versus what is the effect of implementing a change in the types of foods that you're eating? So, for example, if you are running a clinical trial where you're testing a plant-based diet, let's say, you may not prescribe certain times for eating that diet, and it's a very straightforward question: Is that type of diet helpful?

Whereas if you are implementing an intermittent fasting intervention, that becomes challenging, because if you start to tell people what they should eat, then you're really testing two interventions. You are testing the intermittent fasting timing intervention, and you're testing the type of food that people are eating. So then it becomes challenging to tease out which is it that actually led to the results that you're seeing.

And so the answer to that is, again, let's go back to that energy-balance equation and remember that quality of food is indeed important, but ultimately at the end of the day it really matters how much energy you’re taking in and how much you're expending.

Jamie DePolo: Okay. That's a great point, too, and for some of these dietary studies, I'll be very honest with you — not necessarily intermittent fasting but sort of all dietary studies — I personally am skeptical, because a lot of them rely on the people who are in the study accurately reporting what they ate. And I know a little bit about human nature, and part of me believes that if somebody, say, went way off the diet, they may not report how far off the diet they went because they want to please the researcher or they don't want to look like they failed. So, I'm always a little bit skeptical about dietary studies just for that reason.

Dr. Neil Iyengar: That's such a great point. That is exactly a major drawback of any type of lifestyle intervention, including dietary studies. You know, we and other groups are trying to do better and to control for that, so one of the things that we do in our dietary studies is, yes, we do rely on people to tell us what they’re eating. We have also tried to implement tools that make it less burdensome. You imagine if you're a person who is enrolled on one of these studies and donating your time, now to fill out a bunch of questionnaires and keeping track of what you're eating is a major commitment. So we’re trying to use technology and apps and photo journaling tools to help people take pictures of whatever they eat so that we can do the work on the back end and calculate their caloric intake.

And perhaps even more exciting is that the scientific community is working on blood tests that help give us the sense of what people are eating. So, we can look at small molecules and metabolites in the blood to give us a sense of the type of diets and foods that people are consuming.

Jamie DePolo: Okay. Now, I’m not sure, so I'll ask you. Have you ever recommended intermittent fasting to any of your patients, or are you still kind of on the fence?

Dr. Neil Iyengar: Yeah. Great question. So, I have not recommended it yet, and that goes back to the sense of, we need two things. We need safety data in the cancer setting, and we need efficacy data in humans in the cancer setting before I start recommending it.

That being said, if I am working with somebody who is struggling to lose weight, we do explore a number of different options. We work closely with a nutritionist on our team, and I usually start off with that concept of, “Let's look at the overall energy balance and see how can we can put you at a slight net negative so that we can induce a healthy amount of weight loss that's sustainable over a longer period of time.” And if, for that person, intermittent fasting is a more attractive approach, then that is something I am not going to shut out, we'll certainly consider it.

But like I said earlier, we have to be very careful. And what I do point out to my patients, which I'd really like to emphasize here, is that there are pre-clinical data in mice to suggest that intermittent fasting may actually help to reduce the size of breast cancer tumors, especially hormone-receptor-positive breast cancer tumors. There are also studies to suggest, again in mice, that intermittent fasting may help to prevent resistance to [hormonal] therapy and other breast cancer therapies.

So those are all very exciting findings, they just need to be validated in humans. I feel like I keep swinging back and forth like a pendulum, but I will say that there was one study in humans that was published last month with disappointing results where intermittent fasting, time-restricted eating, fasting 14 to 16 hours a day, compared to no dietary intervention wasn't associated with weight loss, there was no difference between the two approaches. I say all of that to say, it's a very quickly shifting landscape, and we need to keep an eye on the data very closely.

Jamie DePolo: Okay. It sounds like, too, you touched on this but I just to summarize. If somebody was interested in trying this, it doesn't sound like you recommend that they would do it completely on their own, that they should probably talk to their doctor, talk to a nutritionist. Get some advice on how to do it?

Dr. Neil Iyengar: Yes. I couldn't agree more. I think that it is an important conversation to have with your physician, particularly if you're undergoing cancer therapy. And it is important to talk about that also with a nutritionist because there may be some important tips there. For example, there are some foods that provide more satiety than others, and certainly, with a diet like intermittent fasting, it helps to eat foods that make you feel full so that you are equipped to fast. So, little tips like that, not just for safety, but also for setting you up or setting a person up for success can be really helpful.

Jamie DePolo: All right. Thank you so much, Dr. Iyengar. I appreciate your time and your insights.

Dr. Neil Iyengar: My pleasure. Thank you for having me.

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