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What You Should Know About Keto and Cancer

As a cancer dietitian, I can say with confidence there’s not a single food or diet that can prevent or treat cancer on its own. The keto diet is no exception, but that hasn’t stopped the flurry of excitement around its potential benefits. The keto diet has been around for a very long time as a treatment for people with epilepsy, but it hit the mainstream in recent years as a way to lose weight, manage diabetes, and even treat cancer. With so many claims out there, I’m here to break down what you should know about keto and cancer.

There is some evidence to support the keto diet as a complement to cancer treatment, but it’s not for everyone and can come with serious side effects. It needs to be considered with a lot of caution because it could actually really hurt you if it’s not right for your unique situation. Always talk to your care team and a registered dietitian like me before making big changes to the way you eat, like starting a keto diet.

What is the keto diet?

The keto diet is all about eating more fat and less carbs. Here’s a specific breakdown of how much fat, protein, and carbohydrate the diet should contain:

  • High in fat: 80% or more of total calories

  • Moderate in protein: 15% of total calories, or 1 gram per kilogram of body weight

  • Low in carbs: about 5% of total calories, or 40-60 grams per day of low GI foods or non-starchy vegetables (to give you a reference point, a cup of blueberries is about 22 grams and a cup of broccoli is about 6grams)

You’ll notice that keto looks really different from the way a lot of people eat. It’s also really different from current recommendations that 45-65% of daily calories come from carbohydrates, 10-35% from protein, and 20-35% from fat. If you want more information on the basics, check out this article I wrote for Food & Nutrition Magazine and this one in the Huffington Post.

It’s easy to get overwhelmed and confused by all of the variations of keto that call for different percentages of each nutrient. Just know that what all true keto diets have in common is that they’re high enough in fat and low enough in carbs that the body can enter something called ketosis. Ketosis sounds really scientific but it’s actually pretty simple (and essential) to understand.

Normally, the body uses carbohydrates as its main source of fuel. But a carb-restricted ketogenic diet shifts the body’s metabolism so that it burns fat and makes something called ketones, which the body then uses for fuel instead of carbohydrates. This switch to burning fat and ketones for energy instead of carbohydrate is called ketosis. It can take several days or more to achieve ketosis, and the best way to know for sure is with a blood or urine test.

I’m talking about this more below but during the time that your body is adjusting to ketosis, you could experience flu-like symptoms or dehydration. If you’re already feeling ill or being affected by treatment, this could really compound the negative. As you learned here, it can also fuel further weight loss so, more to come, but I want to really highlight some of the challenges of this diet in cancer care for you!

Foods you can and can’t consume

Keep in mind that even though carbohydrates are allowed, they’re limited to around 40-60 grams per day and should come mostly from non-starchy vegetables.

Foods to consume:

  • Non-starchy vegetables: leafy greens, cauliflower, broccoli, zucchini, radishes, Swiss chard, bok choy, cucumber, bell peppers, tomatoes, asparagus, Brussels sprouts, cabbage, celery, eggplant, mushrooms

  • Fruit: small portions of some berries like strawberries, raspberries, blackberries, blueberries

  • Proteins: Yogurt, meat and poultry (chicken, turkey), eggs, fish and shellfish

  • Fats: Avocado, nuts and seeds, olives, vegetable oils like olive and coconut, cheese, butter, cream, mayonnaise

  • Sweets: Dark chocolate (at least 70% cacao) *check the label because some may have too much sugar/carbs

Foods to limit or avoid:

  • Whole and refined grains, including products made with grains: bread, bagels, crackers, biscuits, muffins, pancakes, bulgur, rice, oats, tortillas, pasta, breakfast cereals, grits, popcorn, cornmeal and cornbread

  • Starchy vegetables: potatoes, parsnips, yams, squash, all types of beans, corn, peas, lentils

  • Most fruits except for the berries listed above

  • Sweeteners like sugar, maple syrup, agave, honey or anything else that will raise your blood sugar.

Side effects

When you consider that keto forces the body to do something it wasn’t designed to do all the time -- burn fat instead of carbohydrates as the main energy source -- it’s not hard to believe that some unpleasant side effects will come into play. Here are the most common complaints:

  • Constipation: This happens because Keto limits a lot of high-fiber fruits, vegetables, and whole grains

  • Nutrient deficiencies: Carbohydrates like whole grains, fruits, vegetables, beans, milk and yogurt are rich in vitamins and minerals but are limited on the keto diet, so you might become at risk for deficiencies in nutrients like vitamin D, calcium, iron, and prebiotics

  • The keto flu: After a few days on keto, as a result of the body switching gears to burning primarily fat instead of carbohydrate, some people will get headaches and muscle cramps or feel dizzy, weak, tired, dehydrated, irritable, down or depressed

  • Kidney damage: higher-protein diets make the kidney work extra hard, which is especially dangerous for people with kidney disease or who have had a kidney transplant. Keep this in mind, too, if you’re on a treatment that could damage your kidneys.

  • Social isolation: The restrictive nature of the diet can make it hard to dine or socialize with friends and family, to the point where some people might avoid social gatherings all together

While keto is generally not recommended for people with certain diseases and conditions like liver and pancreatic disorders, it’s become a hot topic in cancer care. Even though this is a newer area of research, limited studies are shedding light on potential benefits of keto for cancer treatment and prevention. Let’s explore!

What You Should Know About Keto and Cancer

Before we get to what the research says, let me explain why people are so interested in keto diets for cancer. Cancer cells need fuel to survive and grow. Their preferred fuel is glucose (sugar), which comes from the carbohydrate foods you consume. The theory is that very low-carb diets like keto will essentially cut off the cancer cell’s fuel supply and cause them to die off. Healthy cells are able to burn fat for fuel when carbohydrate supply is low (aka when on a keto diet), but cancer cells not so much. Researchers have been putting this knowledge to the test in order to understand if keto diets can, in fact, be used to treat and prevent cancer.

So what have we learned so far about keto’s effectiveness in cancer? Basically, there’s a lot of hope and interest but not enough evidence to back it up...yet. Most of the research out there has been done on animals, and these studies do show some promising results for certain cancers like brain, colon, pancreatic, prostate, ovarian, lung, and endometrial. But results from animal studies should only be used to guide future research on humans, not to form conclusions.

There are a few human studies out there which also show promise, but they’re pretty small and have important limitations. Let’s take a look.

  • A 2018 study done on 45 women with ovarian and endometrial cancer showed that 12 weeks on the keto diet was able to shift the body’s metabolism in a way that made it hostile to tumors.

  • In a separate study done on those same 45 women, researchers found that the keto diet improved physical function, increased energy, and lowered food cravings.

  • A few studies done on certain brain cancers show that keto might help delay its progression and create longer remission times. One study of 53 patients showed that the six people who were following the keto diet had lower blood glucose levels. But these studies are small and vary in terms of quality.

This is not an exhaustive list of every study measuring keto’s impact on cancer, but it gives you an idea of where the research is at -- promising but limited. Studies so far show that keto can be safe and tolerated (at least by the small groups of people in these studies), and improve quality of life during treatment, but they don’t prove that it can directly impact tumor growth or cancer progression.

The bottom line is that all of the potential is just not matched by the evidence quite yet. The evidence we do have from many long-term and well-done studies actually shows that plant-based diets made up of mostly vegetables, fruits, whole grains, nuts, and beans -- many foods that keto cuts out -- reduce the risk of cancer. With all that in mind, I actually DO work with clients who are on a ketogenic diet with cancer. I also work with folks who are plant-based during cancer treatment; there’s no one right way for all people so I love being able to offer a variety of options for my patients.

Challenges with keto and cancer

Just like any diet, keto might work for some people with cancer but not others. It can be incredibly challenging and even be harmful for some people, and side effects are important to consider. People with cancer are at risk for all of the same side effects mentioned above, like kidney damage, constipation, and nutrient deficiencies, but there are additional and unique challenges for this population.

  • Weight loss: Many people choose to do keto for its short-term weight loss benefits, and this is a big concern for people in active cancer treatment who might already be struggling with unintentional weight loss.

  • Tolerance: It’s common for people to have a low tolerance for such a high-fat, low carb diet. The last thing I want is for people who are already struggling with a poor appetite during treatment to have another reason to not eat enough.

  • Nutrition concerns: Carbohydrates like fruits, vegetables, and whole grains are packed with cancer-fighting antioxidants. People on keto are less likely to get enough of these important nutrients in their diet.

  • Social and environmental issues: Being on keto requires a lot of planning in advance and preparing food at home, so you’ll likely need a lot of support from friends and/or family. It can also be incredibly difficult to maintain for a long period of time because it’s so restrictive.

  • Other side effects: This includes dehydration from ketone production, fatigue, and nausea.

The bottom line on keto and cancer

At this time, none of the major cancer organizations recommend the keto diet because it lacks research. The current evidence-based guidelines for cancer prevention and treatment recommend a plant-based diet rich in vegetables, fruits, whole grains, nuts, and beans. That’s not to say you can’t have a conversation with your care team about the benefits and risks of keto if you are interested in trying it. You’ll also need a dietitian like me who can help you develop an individualized nutrition plan and make sure you’re implementing it safely and correctly. If you have any questions, please feel free to comment below and I’ll be happy to answer them.

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