When diagnosed with cancer, one of the first questions is — what should I eat now? And even more importantly — what should I not eat. This question is natural and correct, because nutrition directly affects the body's ability to fight the disease.

The internet is full of contradictory information. Many sources repeat the same outdated nutrition myths, funded by the food and pharmaceutical industries. This article presents a different perspective — based on the metabolic theory of cancer, established by Dr. Thomas Seyfried and increasingly supported by scientific research^PubMed.

One essential point: nutrition alone is not a substitute treatment. It cannot replace surgery, chemotherapy, or radiotherapy. However, proper nutrition can drastically change the environment in which cancer grows — or stops growing. And the wrong kind, on the contrary, feeds the cancer itself. Literally.

What not to eat when you have cancer

Let's start with what is truly harmful. There is no debate about these things.

1. Sugar and processed carbohydrates — cancer's food

Cancer cells consume glucose 10–200 times faster than healthy cells^PubMed. This phenomenon is called the Warburg effect — scientist Otto Warburg described it back in 1924 and won the Nobel Prize. Cancer cells produce energy by fermenting glucose, even when there is enough oxygen around. They are dependent on glucose — without it, they cannot function effectively.

This is precisely why PET scanning uses radioactive glucose — cancer cells "absorb" it much faster than healthy cells and therefore "light up" in images. If doctors use sugar to findcancer, doesn't it seem logical to stop feeding it supply?

A common question: “But doesn’t the body need glucose?” Partly — yes. Red blood cells and a small portion of the brain do use glucose. However, for that, you don’t need to eat any carbohydrates. Your liver produces all the glucose your body needs from proteins — this process is called gluconeogenesis. It is an evolutionary mechanism that has been working for hundreds of thousands of years — our ancestors didn’t have a bread shop. And the brain, once adapted to ketosis, obtains up to 70% of its energy from ketones — much more efficiently than from glucose.

In other words: carbohydrates in the diet are a non-essential nutrient. There is no such thing as a “carbohydrate deficiency” — only a deficiency of proteins and fats.

In his fundamental work “Cancer as a Metabolic Disease,” Dr. Thomas Seyfried showed that cancer is primarily a metabolic diseasenon-genetic^PubMedMitochondrial dysfunction forces cells to switch to fermentation — and glucose (along with glutamine) is precisely the fuel for this process.

Avoid: any sugar (glucose, sucrose, fructose, dextrose, maltose, and any other '-ose'), candies, chocolates, cakes, pastries, carbonated drinks, juices (yes, even 'natural' ones — they are liquid sugar without fiber), white bread, pasta, baguettes, yogurts with additives, cereals, instant porridges.

2. Processed food products

Semi-finished products, ready meals, chips, cookies, cheapest sausages and frankfurters — anything with a long list of ingredients containing substances you don't recognize. The problem is not the meat (more on that below), but what is added to it: nitrates, nitrites, sugar, soy flour, starch, preservatives, emulsifiers, flavor enhancers. High-quality meat products with a short, clear ingredient list are a completely different matter.

The studies cited to claim that meat causes cancer have serious methodological flaws. First, they rely on self-reported dietary questionnaires — people often inaccurately recall what they ate a year ago. Second, these studies do not distinguish unprocessed meat from processed meat products (sausages, frankfurters, smoked meats with preservatives). Third, there is the so-called healthy consumer bias: people who avoid meat also tend to exercise more, smoke less, consume more vegetables and less alcohol. A fourth flaw is that epidemiological studies show correlations, not causal relationships. The 2019 NutriRECS review, encompassing over 6 million participants, concluded that the quality of evidence is low and the absolute risk reduction is very small. ^[22].

A simple principle: if it contains more than 5 ingredients — you probably don't need it..

3. Alcohol

Alcohol is a recognized carcinogen — and not without reason. This is a particularly sensitive topic in Lithuania, as we are among the highest alcohol consumers in the world. Even small amounts increase the risk of esophageal, throat, and liver cancer^PubMed.

In cancer patients, alcohol is categorically not recommended. It damages the liver, which is already under increased strain due to medication. More details — Chemotherapy and alcohol.

4. Seed oils and trans fats

Sunflower, soybean, rapeseed, and corn oils are unstable polyunsaturated fat compounds that oxidize when heated, forming aldehydes and other toxic substances^PubMed. Trans fats (margarine, many fried products) promote systemic inflammation^PubMed — and inflammation is cancer's best friend.

Avoid: all seed oils, margarine, fast food (fried in seed oil), chips, 'light' products. Use: butter, lard, olive oil (cold-pressed), coconut oil, avocado oil — these are stable fats that do not oxidize when heated.

5. Grain products and excess processed carbohydrates

Bread, pasta, cereals, rice, potatoes — all of this turns into glucose in the body. The glycemic index of white bread is 75 — almost the same as pure sugar (65–100). Your body does not distinguish — sugar is sugar, whether it comes from cake or from 'healthy' bread.

When dealing with cancer, carbohydrate intake should be minimized. More on this in the ketogenic diet section below.

What to eat when dealing with cancer

Now for the good news. The body tends to heal itself — you just need to give it the right fuel and stop putting in what harms it.

1. Fatty wild fish

Wild salmon, mackerel, sardines, herring, cod — the best source of omega-3 fatty acids (EPA and DHA). Omega-3 suppresses inflammation, slows tumor growth, and helps maintain body weight during treatment^PubMed. Patients receiving sufficient omega-3 experience less muscle mass loss during chemotherapy^PubMed.

Eat only wild fish. Farm-raised fish is fed grains, given antibiotics, growth hormones, and synthetic dyes (astaxanthin analogs) to make the flesh appear pink. This is not food — it is an industrial product. The only exception is certified organic fish, but even that is rarely found. 2–3 servings of wild fish per week — minimum.

2. Quality meat

Natural, unprocessed meat is one of the most nutritious foods a person can eat. Beef, mutton, lamb, poultry, game meat — these are a complete source of protein, iron, zinc, vitamin B12, and creatine. These substances are vital for immunity and tissue repair — especially during treatment.

You may have heard that 'red meat causes cancer'. This claim is based on epidemiological studies in which unprocessed meat was not separated from processed meat products (sausages, hot dogs, semi-finished products with additives). Epidemiology shows correlations, not causal relationships^PubMed. Humans have eaten meat for 2.5 million years — it was precisely an animal-based diet that allowed the development of the human brain^PubMed.

The problem was never meat. The problem is what what the food industry does to meat: adds sugar, nitrites, soy, starch, preservatives, and calls it a "meat product."

Important to know: the claim that red meat causes cancer is based on studies with serious methodological flaws. First, the main experimental studies were conducted on rats that were first injected with azoxymethane – a potent carcinogenic agent – and had calcium removed from their diet. In other words, the animals were already primed for cancer development, regardless of what they ate. Furthermore, of the six experimental studies cited by the WHO, four were conducted by the same research group. Second, epidemiological studies do not distinguish unprocessed meat from processed products containing preservatives and nitrites. Third, they exhibit healthy consumer bias: meat eaters are more likely to smoke, exercise less, and consume more processed food. The 2019 NutriRECS review (6 million participants) concluded: the quality of evidence is low, and the absolute effect is very small. ^[22].

Ironically and tellingly: red meat consumption in the US has decreased by one-third since the 1970s ^[USDA], yet over the same period, colorectal cancer incidence among people under 50 has increased by 50% – from 8.6 to 12.9 cases per 100,000 population ^[SEER]. Moreover, colorectal cancer, which at the end of the 20th century was the fourth leading cause of death among young men, is now the first ^{[ACS 2024]}. Over the same period, consumption of sugar, processed carbohydrates, and seed oils has dramatically increased.

3. Eggs

Eggs — one of the most perfect foods. Complete protein, choline (essential for cell membranes and liver function), vitamins A, D, E, K2, B12, iron, selenium. Eat them with the yolks — that's where the majority of nutrients are.

4. Organ meats

Liver, heart, kidneys — nature's multivitamin. Liver contains more nutrients than any other food: vitamin A, B vitamins (especially B12 — up to 3000% of the daily value per 100g), iron, copper, CoQ10. If you can, eat them at least once a week.

5. Animal fats

Butter, lard, goose fat, duck fat — these are stable, saturated fats that do not oxidize when heated and do not pollute the body. In a ketogenic diet, fats become the primary energy source – and cancer cells cannot effectively use ketones, which the body produces from fats^PubMed.

6. Fermented foods

Sauerkraut, kimchi, kefir — foods rich in probiotics. The gut microbiome directly affects the immune system, and a healthy microflora helps the body recognize and destroy altered cells^PubMed. Choose naturally fermented products, not pasteurized ones — pasteurization destroys beneficial bacteria.

7. Bone broth

An ancient food that is becoming popular again — and for good reason. Bone broth is rich in collagen, glycine, proline, and minerals. Glycine suppresses inflammation and helps repair the intestinal lining — this is especially important after chemotherapy, which damages the digestive tract.

What Cancer Dislikes

In short: cancer dislikes everything that deprives it of glucose, strengthens your immune system, and reduces inflammation..

Ketosis. When the body no longer has glucose (because it stops eating carbohydrates), it switches to burning fat and produces ketones. Healthy cells use ketones as energy perfectly — but cancer cells cannot do this because their mitochondria are damaged. They depend on glucose fermentation. Remove the glucose — and the cancer cell is left without fuel. More on this in the next section.

Fasting. Short-term fasting (24–72 hours) activates autophagy — a process during which the body 'eats' damaged and senescent cells. Yoshinori Ohsumi won the Nobel Prize in 2016 for discovering the mechanisms of autophagy. Fasting before chemotherapy can protect healthy cells from damage, making cancer cells more vulnerable to treatment^PubMed.

Physical Activity. Exercise is not nutrition, but its impact is no less significant. Scientific data shows — moderate-intensity physical activity (30 min/day) reduces the risk of cancer recurrence by up to 40–50%, depending on the cancer type^PubMed. Exercise lowers insulin levels, improves mitochondrial function, and strengthens the immune system.

Strong Immunity. And this is the main weapon. Your immune system detects and destroys altered cells daily. NK cells (natural killers), T-lymphocytes, macrophages — this is your army. The problem is that cancer cells know how to 'hide' — they send 'don't attack me' signals. Natural immunomodulators found in medicinal mushrooms help the immune system recognize and respond to these signals. More — Lentinan — How It Works.

Ketogenic Diet and Cancer

The ketogenic diet is not a new fad — it is a return to how humans ate for most of evolution. And it is precisely in cancer that it has the strongest scientific basis.

The Warburg Effect — Why It Works

Otto Warburg received the Nobel Prize in 1931 for discovering that cancer cells produce energy by fermenting glucose, even when there is enough oxygen around (aerobic glycolysis). Their mitochondria are damaged — unable to efficiently carry out oxidative phosphorylation^PubMed.

Dr. Thomas Seyfried (Boston College) expanded this theory and showed that cancer is primarily a metabolic disease. His work demonstrates that by restricting glucose and glutamine, the growth of many cancer types can be slowed or stopped^PubMed.

What clinical studies show

The ketogenic diet in oncology is increasingly being studied, and the results are impressive:

• A 2018 clinical trial with glioblastoma patients showed that the ketogenic diet combined with standard treatment prolonged survival^PubMed
• A systematic review (2018) confirmed that the ketogenic diet is safe for cancer patients and may have anti-tumor effects^PubMed
• Seyfried et al. demonstrated that the glucose-ketone index (GKI) is a reliable indicator of metabolic status — the lower the GKI, the more unfavorable the environment for cancer^PubMed
• The ketogenic diet reduces insulin and IGF-1 — both hormones promote cancer cell growth^PubMed

What a ketogenic diet looks like in practice

Key principles:

• Fats: 70–80% of calories — butter, lard, olive oil, coconut oil, fatty fish, fatty meat
• Proteins: 15–25% — meat, fish, eggs, organ meats
• Carbohydrates: less than 20–30 g per day
• No sugar, grains, potatoes, fruits (except small amounts of berries)
• Regularly check blood ketone levels (with a ketone meter)

Seyfried recommends aiming for GKI < 2.0 — this is therapeutic ketosis, where cancer cells experience the greatest metabolic stress.

What to eat after chemotherapy

Chemotherapy destroys not only cancer cells but also healthy, rapidly dividing cells — those in the blood, intestines, and oral mucosa. After treatment, the body is depleted and requires specific support.

First days after chemotherapy

When nauseous and without appetite:

• Bone broth — easily digestible, rich in collagen and minerals, soothes the intestines
• Ginger — scientifically proven anti-nausea effect. 1–2 g of fresh ginger in tea^PubMed
• Light broths — chicken broth with salt (to restore electrolytes)
• Soft-boiled eggs — excellent protein, does not irritate the intestines

Body recovery (1–4 weeks)

When nausea subsides, restore:

• Protein — eggs, fish, quality meat. Protein is essential for tissue repair. Oncological patients are recommended 1.2–1.5 g/kg of body weight per day^PubMed
• Iron — liver, red meat. Chemotherapy often causes anemia — heme iron from animal sources is absorbed 5–10 times better than plant-based iron
• Zinc — meat, seafood, pumpkin seeds. Zinc is essential for immune recovery
• Probiotics — naturally fermented sauerkraut, kefir. Chemotherapy devastates the gut microbiota
• Vitamins D and K2 — vitamin D deficiency is an epidemic in Lithuania due to our climate. Low vitamin D levels are associated with a poorer prognosis in many types of cancer^PubMed. It is recommended to check the level and supplement to 40–60 ng/ml

More about body recovery after chemotherapy — Chemotherapy — body recovery.

Dietary supplements for cancer

Key principle: dietary supplements are not medicines. However, there are supplements supported by serious scientific research that can help the body fight the disease and withstand treatment.

Lentinan — a polysaccharide from the Lentinula edodes mushroom

Lentinan is a polysaccharide extracted from the medicinal Lentinula edodes mushroom (Lentinula edodes). In Japan, it has been officially registered as a cancer treatment drug since 1985 — not a supplement, but a drug.

A 2018 meta-analysis reviewing 38 randomized clinical trials with 3117 patientsshowed^PubMed:

• Chemotherapy + Lentinan increased the response rate from 43.3% to 56.9%
• 1-year survival improved by 46% (p<0.001)
• Side effects reduced by 27% (p=0.007)
• Quality of life improved

Another major meta-analysis (Wang 2017, 1423 patients) confirmed the same results^PubMed.

Lentinan works through the immune system — it activates NK cells, T-lymphocytes, and macrophages, helping the body recognize 'hidden' cancer cells. More on the mechanism — Lentinan — how it works.

Omega-3 (EPA/DHA)

Omega-3 fatty acids suppress inflammation and help maintain body weight during chemotherapy. Clinical data show — patients receiving omega-3 experience less muscle mass loss^PubMed. The best source is fatty wild fish, not capsules.

Vitamin D

Low vitamin D levels are associated with a worse prognosis in many types of cancer. In Lithuania, vitamin D deficiency is an epidemic. Check your levels and supplement to 40–60 ng/ml^PubMed.

Which supplements are best for cancer and where to get them

Not all supplements are equal. The difference between cheap mushroom powder and a concentrated polysaccharide extract is the same as the difference between herbal tea and a pharmaceutical preparation. Several key criteria:

Extraction method – only water-based extraction yields polysaccharides that activate immune cells. Alcohol (ethanol) extracts do not do this ^[1].

Multi-mushroom formula – different medicinal mushrooms activate different immune system mechanisms (NK cells, macrophages, T‑lymphocytes). A single mushroom extract cannot achieve this ^[2].

Antioxidant component – astaxanthin, the strongest known antioxidant, protects cellular DNA from oxidative stress, which is particularly intensified during chemotherapy ^[3].

A formula combining all these components in one capsule is more effective than individual ingredients.

Frequently asked questions