Medicine: Doctors warn cancer patients against keto diets


After the diagnosis Krebs the shock is deep. And the subsequent therapies such as surgery, chemotherapy or radiation also put enormous strain on the patients. In order not to feel at the mercy of the tumor, many of those affected also want to do something against the disease – and follow one of the many so-called “cancer diets”. The German Cancer Society (DKG) has also observed that these are being accepted more and more. The ketogenic diet, a special form of low-carb nutrition, is currently particularly popular.

“Around every second to third patient comes to my consultation with questions about low-carbohydrate diets or is already following them,” says Nicole Erickson, nutritionist and dietician at the Comprehensive Cancer Center of the LMU Munich. Most of the time, they would get their suggestions from the Internet or from guidebooks. “But there are also doctors who recommend this to their patients,” says Jutta Hübner from the DKG. But there are increasing indications that speak against trusting in this diet in the case of cancer.

The carbs frowned upon on the keto diet are found in sugars, but also in starchy things like potatoes, bread, and pasta. The diet provides these only in the smallest amounts – at most 20 grams per day. For this, patients should eat high-fat foods. Depending on the diet, 60 to 90 percent of the energy intake must consist of fats such as cream, butter, oil or nuts; the rest from protein such as meat, fish, eggs or dairy products and low-starch vegetables.

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The idea behind it sounds plausible at first. The majority of tumor cells prefer carbohydrates – more precisely: glucose as an energy supplier, they cannot tap into other fuels such as fat and protein, or only with difficulty. The body cells, on the other hand, can do that. In the event of a carbohydrate deficiency, the liver easily converts fat and proteins into ketone bodies, which serve to supply energy throughout the body. This gave rise to the idea that you could starve cancer cells by not eating sugar and other carbohydrates. However, laboratory tests show: “In the case of a glucose deficiency, tumor cells grow more slowly for a short time, but then faster,” says Hübner.

“The diet during cancer therapy should keep the weight stable and taste good.”

There is also a lack of evidence from clinical studies that prove a healing effect in humans. “Although there are some small studies, these have only been carried out with a small number of subjects,” said Erickson. In a statement by the DKG, the doctors come to the conclusion that a ketogenic diet has no direct effect on tumor growth and metastasis and neither improves the effectiveness of the therapies nor increases the tolerability of chemotherapy. “But all of this is promised by advocates of the diet,” says the oncologist Hübner. The experts therefore unanimously advise against such a diet regime in a current statement.

On the contrary, a ketogenic diet can also be dangerous for cancer. “Patients usually lose weight with the diet,” says Erickson. Weight loss is undesirable in cancer patients. Because up to 80 percent of those affected are already malnourished anyway, a consequence of the therapies that often lead to nausea and loss of appetite. But cancer itself also wears people out. Tumor cells secrete cytokines that lead to a breakdown in muscle mass. The poor general condition also thwarts movement – a vicious circle.

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However, malnutrition is not only associated with a miserable quality of life but also with a poorer prognosis. Numerous studies have shown that a below-average nutritional status prolongs hospital stays, makes chemotherapy and radiation less well tolerated and shortens lifespan.

On top of that, the patients suffer from the restricted food choices of the keto diet, it causes additional stress and restricts social life – this is also known from various studies. “And that’s exactly what must not happen,” says Erickson. “The diet during cancer therapy should keep the weight stable and taste good.” If an underweight patient has a craving for cake, sweets or convenience products and fast food, he should also enjoy this without regret.

In any case, nutritional advice is recommended, as shown by a 2012 study by Paula Ravasco, a nutritionist at the University of Lisbon. It has been proven that this also leads to a stable weight. But: “In Germany, there is unfortunately a lack of nationwide offers of scientifically sound advice,” complains Hübner.

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