Debunking Diets: Keto, Intermittent Fasting, and More

If you’ve ever thought about starting a diet, you’re not alone. Studies suggest that more than 45 million Americans will attempt to diet every year. While it’s easy to find a regimen that claims you’ll lose weight and feel better, how do you know it’s safe or even good for you? The amount of misinformation about diets isn’t just confusing — it can be downright dangerous.

For cancer patients and survivors, this is especially true. Patients who are on active treatment or have ended treatment have different nutritional needs than others.

In this podcast, we address some of the most popular fad diets with Dana-Farber senior nutritionist Stacy Kennedy MPH, RD/LDN, CSO, from the keto diet to intermittent fasting. Plus, we get down to the heart of the matter: whether a patient on active treatment should even diet at all.


AUSTIN FONTANELLA: Let’s just jump right into it. Would you ever advise a patient who is in active treatment to begin a diet, if they weren’t already on one?

STACY KENNEDY, MPH, RD/LDN, CSO: So, in a technical sense, everybody is already on a diet because we’re all eating. When we think about diets, we’re thinking about an eating pattern that someone is following. That’s more of a technical nutritionist way of thinking about it — when most people hear the word ‘diet’, they’re thinking about something they read online, maybe a book that somebody showed them, and those kind of preset ways of eating that are ‘diets’, we generally do want to help everybody learn to adapt so that they’re eating their own diet, right? So, it’s kind of like your own version of something that might follow certain guidelines.

AUSTIN: OK, so just kind of going off of that, with that definition of what a diet is in mind, what makes them harmful to patients or just people in general?

KENNEDY: A lot of what we think of as diets tend to be pre-prescribed, they don’t take an individual into account — so for any given person going through treatment, it might have too much of certain nutrients, too little of other nutrients. It’s really like going into the store every single time you go shopping and thinking you’re going to find something in the one-size-fits-all aisle, and it’s going to fit you perfectly. You might even find a size that feels good, but you might need to go, and have it altered to really fit you.

That’s kind of the work we want to do in nutrition and working with our patients, is take something that seems like a great idea, this one-size-fits-all, and then tailor it to really fit the patient and suit their needs in the moment, especially as they’re going through treatment.

AUSTIN: For those patients, though, who might have read something online, there are a lot of popular diets out there, and I want to cover a few, the first being the keto diet. What should people know about it?

KENNEDY: I think when a patient comes in, and they’ve read about a diet, or they want to follow a diet, we’ll try to honor that as much as possible. So, we’ll try to see, with that diet, do we need to make any adaptations to help you? That’s something that we’ll do.

With the keto as an example, it’s very popular, and there’s a lot of information out there promoting the ketogenic diet as being helpful in terms of fighting against cancer. The origins of the ketogenic or the keto diet (as it’s mostly called) come from treating children for seizures. It is an effective tool in many cases for clinicians to use, under medical guidance, to help children in terms of managing their seizures.

From that there has kind of been this interest in looking at it in other uses. At the heart of it, it’s a high-fat eating plan. Seventy percent or greater of calories will come from fats with only a very small amount coming from carbohydrate and the rest being protein. What’s generally recommended is more of a balance — a higher amount of carbs, a lower amount of fats and proteins. The biggest change is very little carbohydrates and a lot of fats.

Now, this is a great example where a patient comes in, and they want to follow a keto diet. The first thing we start to think about is looking at that percentage of fats and what really might help that patient in the moment and what might they digest well. What might work for them, and fats in our diet can come from all different sources, right? It can be from what we call ‘healthy fats’, like an avocado or like from olive oil or olives. It might also come from fried food, junk food, fast food, lard, and processed meats—things that would not be as healthful.

There’s some interesting research looking at ketogenic diets as potentially having some merit in specific types of cancers. So, this is the other thing that’s really important for patients to think about — when you hear a study on the radio or you read something or somebody sends you something, you want to start to dive into the details a little bit.

So, with certain types of brain tumors, like with glioblastoma, in certain scenarios, like early stage, there are some studies going on now that are finding some very positive results, but it’s not at the point where we’re ready to start recommending that to everyone or even to patients with the same type of diagnosis. But we are certainly open-minded and starting to work with more patients within the neuro-oncology group who are interested in this and helping them adapt it to be healthy for them.

I’ll give another example — some patients who have gastrointestinal cancer may have a difficult time digesting fats. I’ve had the scenario where some patients who have a type of cancer where they really can’t digest very much fat at all, they heard about keto online and started trying to follow it and were just suffering from terrible side effects of pain and bloating. So, it was really important to focus on, well, we want your diet to be balanced in healthy fats, but maybe the keto diet isn’t right for everyone. We really want to look at those details and use the evidence in the research to really guide us.

It can sound confusing because, on one hand, we’re saying, “Well, we’re not prescribing this for everyone, but for certain people, if they’re interested in following it, we would be very supportive, and we would absolutely help them.” It’s kind of thinking about where the research is at and then how to personalize any of these eating patterns.

AUSTIN: Going back to what you had said earlier, really, if you have questions, you should bring it to somebody, because they should be able to tell you if that diet is right for you.

KENNEDY: Right, and there can be side effects. One of the side effects, because the intake of fiber and carbohydrates is so low, many people on a ketogenic diet might develop constipation. It might be a greater burden on their kidneys or other parts of their bodies. So again, it’s important to really look at the details and, of course, speak with your doctor and speak with your dietician, but don’t be afraid to speak up, because maybe you’re a person who could benefit from something, and you want to be able to bring that up.

AUSTIN: Another diet I know that’s pretty popular now is intermittent fasting, and I think that’s more about the times when you can eat rather than exactly what you’re eating. So, for you, when somebody comes to you and wants to follow intermittent fasting, what kind of advice do you have for them?

KENNEDY: So, intermittent fasting is all about the timing of meals, and there are different methods for intermittent fasting. The most common one you’ll see is called ‘time-restricted feeding’ — that’s when you give a window of not eating, generally overnight, and kind of extend that beyond what would be typical.

There are other versions of intermittent fasting. Sometimes people will take 24 hours or 36 hours out of their week and not eat and maybe only have water. Other people will follow where, two days a week, they have a very restricted amount of calories — so maybe only 500 calories — and then have less or no restrictions on the other days.

In terms of where we might kind of want to think about it, that time-restricted feeding, giving yourself a break overnight, there certainly can be a lot of merit to that. Now, how long should that window be, is it right for everyone? That’s when we start to get individual. We know that eating a lot of food late at night, right before you go to bed, could set you up for reflux and heartburn, and maybe it’s not even the best in terms of weight management. There are a lot of reasons to give yourself a couple hours without eating before you even go to bed. So, for someone who wanted to think about the time-restricted feeding, you would look at, at least 12 but maybe 14 and, for some people, they’re doing a 16-hour window without eating.

Now, for a lot of patients going through treatment, that would not necessarily be recommended in the same way. You don’t want to put yourself in a situation as a patient going through treatment where you can’t meet your nutrition needs or you’re starving, or you’re triggering nausea or other symptoms.

The data on intermittent fasting is very interesting. There is some preliminary research that is possibly suggesting that some people may benefit, but it’s really too early, and the risks of malnutrition are great. So, we’re not at the point where we’re telling people that they should be fasting for 24 hours before they come in for chemotherapy. Again, on an individual basis, these are all conversations to have, but if it’s something that seems of interest to you, I would say let’s start by saying, “The kitchen is closed after dinner. I’m not going to snack at night, and I’m going to wait and eat in the morning.”

I think the other thing we’re seeing with diabetes management is that circadian rhythm intermittent fasting, where that 8-hour window (or smaller 6-hour window) of eating is earlier in the day. So, there might be benefit — and again, we’ve all heard this for years that you should kind of eat your bigger meals earlier, so some of the newer research might be supportive of that. Again, if this is something of interest, you might think of flipping your bigger meal, instead of being at dinner, to being at lunch or at breakfast. There are a lot of ways that we can work within this. We’re not telling patients to ‘fast’ and not eat at all at this point.

AUSTIN: So, we kind of talked about two diets that maybe you have to make some modifications to, and it really depends on what your diagnosis is. So, when patients ask, “What kind of diet should I follow?” or “What kind of diet should I be looking for?” — what are some diets you would recommend that people take into account?

KENNEDY: So, where the research is strongest is recommending plant-based diets. That could be somebody that could be a vegetarian. You could be vegan, or 100% plant based. Mediterranean diet, which is a very plant-forward diet that includes foods like fish and eggs as well for protein sources. Building that plant-based diet where at least 50% of your plate is plants at each meal is a great way to focus on healthy eating, kind of what people call ‘clean eating’, and really staying where the evidence is based and looking at those plant foods to really fuel our immune system, fuel our energy levels, our metabolism.

So that’s what we recommend. Those aren’t necessarily out-of-the-box diets so to say, but I think that the focus on plant-based eating is something that’s going to be very important. Now, you could be a plant-based eater and follow a keto diet or follow intermittent fasting. Again, we can really personalize all of these things, but focusing your energy on eating more plants is absolutely what we’re going to be recommending the most right now.

AUSTIN: Well, it’s interesting you brought up the plant-based diet because more and more people are turning to vegetarianism or veganism. If somebody was a vegetarian or a vegan before they were diagnosed with cancer, and they wanted to continue that very strict diet moving forward through treatment, is that something you would recommend for them to do? Are there any things that they need to think about as they move forward?

KENNEDY: Again, it would be more on a personal level. If you were already following a vegan diet or a vegetarian diet, you can absolutely continue that during treatment. We might make some modifications. We might want to make sure you’re getting enough protein. We might want to add in specific plant-based foods that are high in minerals. With any diet, you need to pay attention to what you’re eating and the balance of those nutrients.

We would really look at it the same way we would anyone else’s eating pattern and make those personalized recommendations, but a patient should feel confident that, yes, what you believe in and feel strongly about we can help you adapt — you don’t need to abandon your way of eating completely.

AUSTIN: And I’m sure that will put a lot of people at ease hearing that. I know you had said that you don’t have to ignore really all diets, but when you’re looking at one and deciding if you wanted to follow it or not, are there red flags that you should say, “Hmm, maybe this isn’t for me?”

KENNEDY: Yeah, I think that’s a great question. So, red flags can be overpromising. False claims or promises of cures, those are going to be red flags, absolutely. I think other red flags are when, over a long period of time, if you’re restricting an entire group of nutrients that we know our bodies need, that can be problematic. If a diet is extremely absolute and sort of pitched as ‘forever’ (“You should never eat another gram of sugar ever again!”), that might be a red flag that the evidence may be lagging behind the proclamations.

AUSTIN: All right, and my last question, Stacy, before we let you go is all these rules and things you had brought up, when it comes to survivorship, should people be thinking the same way? Or can they make more adjustments or let more things in when it comes to their diet?

KENNEDY: Right. I think you have more ability to test things out, have a little bit broader of a focus on what you’re eating when you’re finished with treatment in many parts because you have less symptoms. But I think, yes, the cornerstone of what we talked about would be what we recommend for survivorship as well.

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