【Supporting Your Treatment】Eating Tips During Chemotherapy, Strategies for Each Side Effect, and Foods to Be Careful With

“I’ve been sticking with my chemotherapy, so why isn’t the tumor shrinking much…”

“The side effects are so hard, yet the results aren’t what I hoped for. I’m worried about whether I should keep going like this…”

“Does what I eat during cancer treatment even matter? My doctor never mentioned it, so I haven’t given it much thought…”

Cancer treatment isn’t determined by the choice of drugs alone.

One idea that recent research is focusing on is that the “nutritional environment” surrounding cancer cells can influence how well chemotherapy works.

Reports from around the world increasingly suggest that three factors, blood sugar, chronic inflammation, and the gut environment, are closely tied to treatment outcomes.

In this article, I’ll unpack, from a scientific perspective, how eating habits can affect cancer treatment.

Whether or not you’re currently in treatment, I hope you’ll read to the end, including from a prevention standpoint.

How the “Nutritional Environment” of Cancer Cells Relates to How Well Chemotherapy Works

Cancer cells have a feature that sets them apart from normal cells: they consume large amounts of glucose as their main source of energy.

This is called the “Warburg effect,” discovered by Dr. Otto Warburg in the 1920s.

Because cancer cells can use sugar as fuel to multiply rapidly even in low-oxygen environments, high blood sugar is thought to easily become a “favorable environment” for them.

What Is the “Tumor Microenvironment” That Gets in the Way of Chemotherapy?

One concept that modern oncology has been paying close attention to is the “tumor microenvironment (TME).”

Cancer cells don’t exist in isolation. They form a “microenvironment” around themselves, a complex web of inflammatory cytokines, immune cells, blood vessels, fibroblasts, and more.

A growing number of studies report that this environment can make tumors more resistant to chemotherapy and weaken the ability of immune cells to attack cancer.

What’s even more surprising is that this tumor microenvironment can change significantly depending on blood sugar, inflammation levels, and the balance of gut bacteria.

Eating Habits Build the “Foundation” for Treatment

Of course, diet alone cannot cure cancer.

That said, research is steadily building up showing that the quality of the “foundation,” the body receiving the treatment, may affect how well the drugs work.

Whether a drug works depends not only on the type of drug but also on the environment of the body receiving it.

That’s exactly why supporting the body’s environment through diet is getting attention as a complement to treatment.

If the Drug Is the “Seed,” the Body Is the “Field”

It’s easier to picture the relationship between chemotherapy and the body if you compare it to seeds and a field.

No matter how good the seeds you plant, if the soil is poor or neglected, the sprouts will struggle to grow.

What shapes the condition of the “field” that is your body are factors tied to your daily diet, such as blood sugar, inflammation, and your gut environment.

How High Blood Sugar and Excess Insulin “Feed” Cancer Cells

When your blood sugar stays high, your pancreas releases large amounts of insulin.

Insulin’s job is to move blood sugar into cells, but at the same time it also activates signals that promote cell growth (such as IGF-1).

This mechanism, which causes no problem for normal cells, can end up being exploited by cancer cells as a switch for multiplying.

The Link Between Obesity, Metabolic Syndrome, and Cancer Risk

The World Health Organization (WHO) recognizes obesity and being overweight as risk factors for 13 types of cancer.

Obesity is said to cause chronic high insulin levels (hyperinsulinemia), high blood sugar, and inflammation, which together raise the risk of cancer developing and progressing.

In other words, the chain reaction set off by metabolic syndrome reaches all the way to cancer.

How Carb Restriction and Low-GI Diets Might Change the Treatment Environment

Several studies have reported that low-carbohydrate and low-GI diets may have a positive effect on treatment outcomes in cancer patients.

By stabilizing blood sugar and insulin levels, it may be possible to reduce the conditions that favor cancer cells and create an internal environment where treatment drugs can reach their target more easily.

However, because cancer patients are prone to malnutrition, excessive carb restriction can backfire in some cases.

It’s essential to proceed only while consulting your doctor or a dietitian.

Managing Blood Sugar

For this reason, people with diabetes or higher blood sugar may have their blood sugar managed alongside their treatment.

However, as I mentioned in the previous section, avoid drastically cutting carbohydrates on your own.

If you’re concerned about your blood sugar, work out how much carbohydrate you should eat together with your doctor or a registered dietitian.

Chronic Inflammation Becomes a “Shield” That Protects Cancer

One of the hottest topics in cancer research today is “the relationship between cancer and inflammation.”

Over the past few years, study after study has reported a startling finding: a state of chronic inflammation can provide cancer cells with a “shield” that helps them escape attacks from the immune system.

Inflammatory Cytokines “Tame” the Immune System

Cancer cells release inflammatory substances (such as IL-6 and TNF-α) that turn nearby immune cells into a “tumor-promoting” type.

Immune cells that should be attacking cancer cells end up, because of the inflammatory environment, working in a way that helps the cancer grow.

Isn’t it rather shocking that this is also thought to affect how well immune checkpoint inhibitors (cancer immunotherapy) work?

Calming Inflammation Through Diet Can Change the Treatment Environment

Diets high in processed foods, refined carbohydrates, and trans fats tend to promote chronic inflammation.

On the other hand, multiple studies have confirmed that the Mediterranean diet and anti-inflammatory diets (centered on oily fish, olive oil, vegetables, and nuts) can lower inflammatory markers.

Managing inflammation through diet during treatment could serve as a support that helps get the most out of treatment. I hope you’ll keep this perspective somewhere in the back of your mind.

Inflammation in the Body and Nutritional Status

When you have cancer, chronic inflammation (a state like a small fire that keeps smoldering) is thought to occur more easily in the body.

Some research suggests this inflammation is involved in loss of appetite and muscle loss.

If you’re feeling well enough to eat fish, simply including it in your regular meals is plenty.

Tips for Adding Anti-Inflammatory Foods to Your Diet

Eating to calm inflammation doesn’t mean you need to stock up on special ingredients during treatment.

Adding a little something to your usual meals, like grilled fish, canned fish, a salad dressed with olive oil, or a handful of nuts, is enough.

When you have no appetite, put getting energy from whatever you can eat first.

Once you’re feeling more settled, it’s fine to add these foods little by little.

The Latest Research: How the Gut Environment Influences Chemotherapy and Immunotherapy

In recent years, a discovery that has been rapidly gaining attention in cancer research is that “gut bacteria influence how well cancer treatment works.”

A series of studies published in journals such as Science showed that patients with greater gut bacterial diversity tend to respond better to immune checkpoint inhibitors.

The gut environment and cancer treatment may seem unrelated at first glance, but they’re actually quite deeply connected.

Gut Bacteria “Train” Immune Cells

Gut bacteria do more than just help with digestion. They’re also deeply involved in educating and activating immune cells.

It has also become clear that certain gut bacteria (such as those in the genus Bifidobacterium and Akkermansia) may boost the activity of T cells (the immune system’s main fighting force) and strengthen their attack on cancer cells.

Conversely, there are also reports that when the balance of gut bacteria is disrupted by antibiotic use or a diet centered on processed foods, treatment can become less effective in some cases.

Dietary Fiber and Fermented Foods Help Balance the Gut Environment

Including foods like these in your daily diet may help support your treatment by way of your gut environment.

The Gut Environment and Treatment

Because gut bacteria are involved in how the immune system works, research into the relationship between the gut environment and cancer treatment has been moving forward in recent years.

Of course, antibiotics are used when a doctor decides they’re needed, so you should never refuse them on your own.

That said, specific guidance, such as what kind of diet would improve treatment results, is still not known.

Also, during periods when your white blood cell count is low, hospitals may give different guidance on how to handle fermented foods, yogurt, and similar items.

Even if you want to pay attention to your gut environment, check with your medical team first before adding these foods.

The Basics of Eating During Chemotherapy, and Foods to Be Careful With

From here, let’s look at the practical points to keep in mind when it comes to eating during chemotherapy.

The Goal: “Don’t Lose Weight or Muscle”

During chemotherapy, there may be times when side effects reduce your appetite or make it hard to eat as much as you’d like.

When you eat less, you lose weight, and you tend to lose muscle along with it.

If you think of your body as a car, energy is the gasoline and muscle is the engine.

If both the gasoline and the engine run down, it becomes hard to make it through the long road of treatment.

That’s why a major goal during treatment is making sure you get enough energy (calories) and protein.

If you start losing weight quickly, it’s important to let your medical team know early.

Don’t Restrict Your Diet on Your Own

Online and in books, you may come across extreme claims like “people with cancer should cut out carbs” or “fasting makes cancer disappear.”

But if you impose restrictions like these on your own during treatment, you may lose weight and muscle and find it harder to continue your treatment.

Cancer cannot be treated with diet alone.

If you want to make major changes to what you eat, always talk to your doctor or a registered dietitian beforehand.

Grapefruit and Other Foods That Don’t Mix Well with Medication

Grapefruit is known to contain compounds that interfere with how some drugs are broken down, which can cause too much of the drug to stay in your body.

Some cancer drugs, mainly oral ones, also require caution when combined with grapefruit.

Even among other citrus fruits, some, such as pomelo (buntan) and oroblanco (sweetie), may contain similar compounds.

Check with your pharmacist or doctor about whether there are any foods you should avoid with your specific medications.

Always Consult Your Doctor Before Taking Supplements or Health Foods

If you’re currently taking anything, bring the actual product or a photo so the name is clear, and have your doctor or pharmacist take a look at it.

Be Wary of Claims Like “Eat This and Your Cancer Will Disappear”

During cancer treatment, anxiety can make you want to pin your hopes on a particular food or diet.

But many stories like “this food made my cancer disappear” or “I stopped chemotherapy and switched to diet therapy” have little scientific basis.

Stopping treatment on your own or replacing it with diet therapy is extremely dangerous.

When you come across information that catches your attention, don’t judge it alone. Ask your doctor, “What do you think about this?”

Eating Tips for Each Chemotherapy Side Effect (Nausea, Diarrhea, Low White Blood Cell Count, and More)

The side effects of chemotherapy vary depending on the type of drug and the individual.

Here, let’s look at commonly recommended tips for each of the typical side effects that affect eating.

When You Have Nausea or Vomiting

Nausea can start right after treatment and last for several days.

If you’ve been prescribed anti-nausea medication, it’s important to take it at the times you’ve been instructed.

Hot dishes tend to spread their smell along with the steam, so letting them cool before eating may make things easier.

If the nausea continues to the point where you can’t even keep fluids down, there’s a risk of dehydration, so contact your hospital as soon as possible.

When You Have Diarrhea

When diarrhea continues, your body loses water and electrolytes (minerals such as sodium and potassium).

Make rehydration your top priority, and drink room-temperature water, mugicha (barley tea), or oral rehydration solution in small, frequent sips.

For meals, easy-to-digest, stomach-friendly foods are best, such as okayu, udon, white fish, tofu, bananas, and grated apple.

Until your stomach settles, it’s better to go easy on greasy foods, very cold drinks, high-fiber foods like burdock root and mushrooms, dairy products, and alcohol.

If you have diarrhea many times a day, can’t keep fluids down, or have a fever, don’t try to handle it yourself; contact your hospital right away.

Watch Out for “Food Poisoning” When Your White Blood Cell Count Is Low

Depending on the type of chemotherapy, the white blood cells that protect your body may temporarily decrease.

White blood cells are like your body’s “security guards,” so when their numbers drop, your defenses against bacteria and mold get thin.

During this time, even levels of germs that wouldn’t be a problem when you’re healthy can make you sick.

For this reason, some hospitals may give food safety guidance like the following.

How strict you need to be depends on your white blood cell count, your treatment, and your hospital’s policy.

Restricting more than necessary leaves you with fewer foods you can eat, so check with your medical team about what applies in your case.

Frequently Asked Questions About Chemotherapy and Eating Habits

Using Diet to Support a Body That’s Fighting Cancer: Small Steps You Can Take Now

How well chemotherapy works isn’t determined by the type of drug alone.

The perspective that the environment of the body receiving the treatment (blood sugar, inflammation levels, and the balance of gut bacteria) may influence treatment outcomes is likely to become more and more important in the years ahead.

Using diet to improve this environment, even a little, is a realistic approach that researchers around the world are paying attention to.

Key Points to Keep in Mind When Eating During Cancer Treatment

Cancer treatment is something you go through together with a team of specialists, including doctors, nurses, and dietitians.

Think of improving your diet strictly as a “supporting” measure, and bring it in while consulting your doctor.

On the other hand, for those who want to prevent cancer or start supporting their body’s environment now, I believe rethinking your eating habits based on the scientific evidence behind fasting is a very effective approach.

Supporting healthy blood sugar, inflammation levels, and gut environment leads not only to cancer prevention but to better health throughout your body.

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