What Not to Eat During Immunotherapy?

Immunotherapy drugs work by unleashing your own immune system against cancer, so anything that dulls, distracts, or disrupts that immune response can theoretically undermine treatment. The strongest dietary cautions center on alcohol, artificial sweeteners, and ultra-processed low-fiber foods, though the list of things worth watching is longer than most patients hear from their oncologist. Much of this research is recent and still evolving, but enough evidence has accumulated to make certain eating habits worth reconsidering during treatment.

Alcohol

Of all the dietary factors studied alongside immunotherapy, alcohol has some of the most direct evidence of harm. Research in both human patients and animal models found that alcohol use roughly doubled the risk of disease progression in people receiving anti-PD-1 therapy for lung and bladder cancer. The mechanism is not just general immune suppression: alcohol reduced the number of cancer-fighting T cells inside tumors and pushed remaining immune cells toward less effective subtypes. In lab experiments on both rodent and human T cells, alcohol disrupted the activation and killing functions those cells need to respond to immunotherapy.¹

A broader systematic review that looked at alcohol across multiple cancer treatment types found the evidence for immunotherapy specifically is still limited in volume, while the signal for harm to radiotherapy was clearer. The authors called for larger prospective studies but did not find evidence that moderate drinking is safe during treatment either.² Given the biological plausibility and the direction of every finding so far, avoiding alcohol during immunotherapy is one of the simplest and best-supported dietary changes you can make.

Artificial Sweeteners

This one surprises a lot of patients. Diet sodas, zero-calorie sweetener packets, and sugar-free products seem like neutral or even healthy choices, but a growing body of evidence suggests nonnutritive sweeteners can impair anti-PD-1 therapy. The mechanism runs through the gut microbiome: these sweeteners alter the composition and metabolic output of intestinal bacteria in ways that appear to blunt the immune response immunotherapy depends on. Researchers found the effect across multiple cancer types and concluded that limiting daily intake of nonnutritive sweeteners is clinically relevant during treatment.³

This does not mean you need to load up on sugar instead. Plain water, unsweetened tea, and coffee are all fine alternatives. The concern is specifically about the engineered sweetener molecules and what they do to gut bacteria that your immune system relies on.

Ultra-Processed and Low-Fiber Foods

If there is a single dietary theme running through immunotherapy research, it is the gut microbiome. The trillions of bacteria in your intestines help train and regulate the immune cells that checkpoint inhibitors are trying to activate. A diet heavy in ultra-processed foods and low in fiber starves the beneficial bacteria and encourages species linked to inflammation and immune suppression.

In a study of 128 melanoma patients receiving immune checkpoint therapy, higher dietary fiber intake was associated with significantly improved progression-free survival. The benefit was most pronounced in patients eating enough fiber without commercial probiotic supplements. When researchers tested this in mice, a low-fiber diet impaired the response to anti-PD-1 therapy and reduced the cancer-fighting T cells in the tumor environment.⁴ The same research group later showed in a randomized dietary trial that a high-fiber diet changed bile acid metabolism in melanoma patients’ guts in ways that favored treatment response.⁵

Practical guidance from a review covering the microbiome and immunotherapy recommended that patients minimize animal meat and maximize plant foods, aiming for at least 30 different plant types per week and more than 30 grams of fiber daily.⁶ That is a high bar for most people, but even moving in that direction helps. Fast food, packaged snacks, white bread, and processed meats are the biggest culprits for displacing the fiber-rich whole foods your gut bacteria need.

There is also a potential bonus: higher fiber intake may protect against some of immunotherapy’s own side effects. One study of melanoma patients on ipilimumab found that patients who ate more fiber relative to their total calorie intake tended to experience less diarrhea and colitis, common and sometimes severe side effects of that drug.⁷

Raw and Undercooked Foods

Immunotherapy does not suppress your immune system the way chemotherapy does, but many patients receive immunotherapy alongside other treatments, and the cancer itself can leave you more vulnerable to foodborne illness. The practical concern is food safety, and surveys show cancer patients routinely eat foods that carry real infection risk. In one study, more than half of cancer patients in treatment consumed eggs with runny yolks, about seven in ten ate restaurant salad bar items, and nearly nine in ten ate cold deli meats. Roughly a third ate raw cookie dough, and similar numbers ate soft cheeses.⁸

The foods most worth avoiding or handling carefully include:

• Raw or rare meat: ground beef, tartare, and undercooked poultry carry bacteria like Salmonella and E. coli that a weakened immune system handles poorly.
• Raw fish and shellfish: sushi, raw oysters, ceviche, and smoked fish served cold all pose risk from Vibrio, Listeria, and parasites.
• Unpasteurized dairy and soft cheeses: brie, camembert, queso fresco, and raw milk products are common Listeria sources.
• Raw sprouts: alfalfa, bean, and other sprouts are consistently flagged as high-risk by food safety authorities because bacteria can grow inside the sprout itself, where washing cannot reach them.
• Raw eggs: including in homemade dressings, mayonnaise, and cookie dough.

You do not need to eat only bland, overcooked food. The goal is simply to cook things to safe internal temperatures, avoid known high-risk items, and wash produce thoroughly. Many oncology teams provide a food safety handout, but if yours has not, it is worth asking.

Herbal Supplements and Interactions

Herbal supplements sit in a regulatory gray zone, and many cancer patients take them without telling their oncologist. A multicenter study of cancer patients receiving immune checkpoint inhibitors found widespread use of herbal products, and some carry real risks. St. John’s Wort is a well-known offender because it strongly affects the enzyme system that metabolizes many drugs, potentially changing how your body processes immunotherapy agents. Reishi mushrooms, another popular supplement among cancer patients, can cause digestive upset and in some cases liver toxicity.⁹

The broader issue is that “natural” does not mean “harmless alongside immunotherapy.” Many herbal products have immunomodulatory effects of their own, and those effects can either overstimulate the immune system (increasing autoimmune side effects) or dampen it (reducing treatment effectiveness). The safest approach is to tell your oncology team about every supplement you take, including common ones like turmeric, echinacea, and green tea extracts in capsule form.

Concentrated Soy Products

Soy foods are generally considered part of a healthy diet, but there is reason for caution about consuming large amounts of concentrated soy supplements during immunotherapy, particularly for hormone-sensitive cancers. The isoflavone genistein, which is the main phytoestrogen in soy, has been studied for its own anticancer properties. However, in a mouse melanoma model, combining genistein with anti-PD-1 therapy did not improve outcomes beyond what either treatment achieved alone, and the combination actually lost effectiveness over time compared to immunotherapy by itself.¹⁰

More concerning, earlier research demonstrated that even low concentrations of genistein could induce a protein in breast cancer cells that blocks natural killer cells from destroying them. Dietary levels of genistein achievable through regular soy consumption were enough to trigger this protective shield in human breast tumors grown in mice.¹¹ This is not a reason to panic about eating edamame or adding tofu to a stir-fry. The concern is mainly about concentrated soy isoflavone supplements, which deliver doses well above what you would get from food. If you enjoy soy foods in normal quantities, that is likely fine, but high-dose soy supplements are worth discussing with your oncologist, especially if you have a hormone receptor-positive breast cancer.

The Probiotic Paradox

Probiotics present a genuinely confusing picture for immunotherapy patients. On one hand, a meta-analysis found that probiotics could counteract the damage that antibiotics do to immunotherapy outcomes in lung cancer patients, roughly halving the risk of progression.¹² On the other hand, the melanoma fiber study found that the greatest survival benefit came from high fiber intake without probiotic supplements. Mice given probiotics actually showed impaired responses to anti-PD-1 treatment.⁴

How can both be true? The likely explanation is context. When antibiotics have wiped out gut bacteria, probiotics may help restore some baseline function. But in a person with an intact microbiome, introducing large doses of a few commercial probiotic strains might actually reduce the diversity that immunotherapy benefits from. The practical takeaway: if you have not been on antibiotics recently, reaching for a fiber-rich diet of diverse plants is a safer bet than loading up on probiotic capsules. If you have needed antibiotics during treatment, talk to your oncologist about whether targeted probiotics make sense for your situation. Fermented foods like yogurt, kimchi, and sauerkraut deliver microbes in a more natural context and with additional nutrients, which may be preferable to supplements, though direct trial evidence for fermented foods during immunotherapy is still sparse.

Vitamin D and Nutritional Status

While the article’s focus is what to avoid, severe nutritional deficiencies are themselves something to steer clear of during immunotherapy. Vitamin D is the best-studied example. A meta-analysis found that higher blood levels of vitamin D were associated with substantially better overall survival in cancer patients on immunotherapy. Vitamin D supplementation was also linked to longer survival, though the effect was smaller.¹³ In melanoma specifically, patients who had normal vitamin D levels or brought their levels up with supplementation had a response rate of 56%, compared to 36% in those with low levels who were not supplemented. Progression-free survival nearly doubled.¹⁴

The picture is not perfectly clean. Some studies have noted paradoxical results, with certain vitamin D-related signals associated with immunosuppression in specific contexts.¹⁵ Still, the weight of evidence tilts toward checking and correcting vitamin D deficiency during treatment. A restrictive diet that eliminates dairy, fortified foods, and fatty fish could easily leave you deficient.

More broadly, poor overall nutritional status and muscle loss are associated with worse immunotherapy outcomes across several cancer types.¹⁶ Crash diets, extreme cleanses, or any eating pattern that leads to significant weight loss and muscle wasting could harm you more than any individual food. Maintaining adequate protein and calorie intake is essential, even when nausea or appetite loss makes eating difficult.

Green Tea Compounds and a Nuance Worth Knowing

Green tea polyphenols, particularly EGCG (epigallocatechin gallate), have gotten attention for potentially enhancing immunotherapy. In lab and animal studies, EGCG suppressed PD-1 expression on T cells, which is essentially doing some of the same work that anti-PD-1 drugs aim to accomplish. Mice given EGCG showed reduced PD-1 on their T cells and increased tumor cell death.¹⁷

This sounds like good news, and moderate green tea consumption is almost certainly fine. But there is an important nuance: if EGCG is suppressing PD-1 on its own, there is at least a theoretical question about whether it could interfere with the dosing or monitoring of PD-1-targeted drugs. High-dose green tea extract supplements are a different proposition from drinking a few cups of tea. This has not been tested in clinical trials with immunotherapy patients, so it falls into the “mention it to your oncologist and do not megadose” category rather than a firm prohibition.

Fasting-Mimicking Diets and the Ketogenic Question

Some patients explore fasting or ketogenic diets during immunotherapy, having heard that these approaches might boost treatment. The preclinical evidence is genuinely interesting but needs context.

Fasting-mimicking diets, which are short cycles of very low calorie intake designed to trigger some of the metabolic changes of fasting without complete food restriction, have shown promise in mouse models of difficult-to-treat cancers. In poorly immunogenic breast cancer, periodic fasting-mimicking cycles expanded a population of T cells that immunotherapy could then reactivate, shifting tumor metabolism and improving treatment response against tumors that normally resist immune checkpoint therapy.¹⁸ Detailed immune profiling showed that the fasting cycles enriched a specific type of early exhausted T cell that, unlike fully exhausted cells, can still be reawakened by immunotherapy.¹⁹

Ketogenic diets have also shown preclinical effects. In prostate cancer models resistant to checkpoint therapy, a cyclic ketogenic diet helped overcome that resistance, enhancing immune cell infiltration into tumors through mechanisms that included attracting cancer-fighting T cells and shifting immune-suppressive cells toward anti-tumor roles.²⁰ Similar results have appeared in bladder cancer models, where a ketogenic diet enhanced the effectiveness of anti-PD-L1 treatment.²¹ A review across preclinical cancer studies confirmed increased activation of tumor-fighting immune cells in response to ketogenic regimens.²²

Here is the caution: these are almost entirely mouse studies and lab experiments. Human trials of ketogenic and fasting-mimicking diets during immunotherapy are only beginning, and there are real risks to attempting them without medical supervision during cancer treatment. Severe calorie restriction can accelerate muscle loss, which as noted earlier is itself linked to worse immunotherapy outcomes. If you are interested in either approach, raise it with your oncology team rather than experimenting on your own.

The Salt Surprise

One counterintuitive finding: high-salt diets might not be harmful to immunotherapy the way many patients assume. Mouse studies found that a high-salt diet increased sodium concentrations in tumor tissue, which actually suppressed immune-blocking cells and reactivated T cell anti-tumor activity.²³ This does not mean you should start piling on the salt. High sodium intake carries its own cardiovascular and kidney risks, and the mouse findings have not been validated in human immunotherapy patients. But it is a good reminder that the relationship between diet and immune function is not always what common sense would predict, and blanket “eat clean” advice can miss the complexity of how specific nutrients interact with the immune system.

Omega-3 Fatty Acids and What Fats to Choose

Not all fats are equal during immunotherapy. A preclinical study compared fish oil rich in omega-3 fatty acids against soybean oil (rich in omega-6) and found a trend toward improved outcomes when fish oil was combined with radiation and anti-PD-L1 therapy, although the result was borderline statistically significant.²⁴ The omega-3 to omega-6 balance matters because omega-6 fatty acids tend to promote inflammation through different pathways than the immune activation immunotherapy needs, while omega-3s may support a more favorable immune environment.

In practical terms, this means heavily fried foods cooked in corn, soybean, or sunflower oil are worth reducing, not because any single serving will derail your treatment but because a chronically omega-6-heavy diet may create a less hospitable environment for the immune response your drugs are trying to generate. Fatty fish like salmon, sardines, and mackerel, along with walnuts and flaxseed, shift that balance in a potentially favorable direction.

What Your Oncologist Probably Will Not Tell You

Most oncology appointments are short, and dietary counseling tends to get squeezed out by discussions of scans, bloodwork, and side effects. Many patients leave with little more than a vague instruction to “eat well.” The research suggests that advice undersells how much diet actually matters during immunotherapy. The gut microbiome is not a peripheral player; it is deeply involved in determining whether checkpoint inhibitors work or fail.

A few things are worth keeping in mind that do not fit neatly into any single food category. First, antibiotics are the biggest known disruptor of gut health during immunotherapy, and while you cannot always avoid them, you should push back on unnecessary prescriptions and ask whether a narrower-spectrum antibiotic could be used when one is needed. Second, dietary diversity seems to matter as much as any single food: the recommendation of 30 or more plant types per week is not arbitrary, since microbiome diversity correlates with immune diversity.⁶ Third, individual variation is enormous. Two patients eating identical diets can have wildly different microbiome compositions and immunotherapy responses. The guidelines above represent the best general advice the evidence supports, but your oncology team can help tailor them to your specific situation, treatment regimen, and cancer type.