Busting Myths in Oncology: Carbohydrates are Bad

The idea that sugar feeds cancer is one of the most common and concerning myths people encounter. The truth, though, is more nuanced and more reassuring.

It is correct that cancer cells use glucose for energy, but so does every healthy cell in your body. Your brain, muscles, and immune system all rely on it. If you were to cut out all sources of carbohydrates, your body would still produce glucose from protein and fat, because it is essential for life. That means it’s not possible to “starve” cancer by removing sugar from your diet. 

The ketogenic diet (KD) has attracted attention as a potential strategy to slow tumour growth by targeting cancer metabolism. This concept originates from a phenomenon known as the Warburg effect. The Warburg effect refers to a discovery made by Otto Warburg in the 1920s. He observed that cancer cells tend to rely on a form of energy production called glycolysis, even when oxygen is plentiful. In simple terms, they use glucose differently from healthy cells. This was an important finding for cancer biology, as it revealed how rapidly growing cells adapt their metabolism.

Most of the research supporting this idea is still in the early stages. Animal studies and small clinical trials have shown that KD can slow tumour growth and may enhance the effects of standard therapies for certain cancers, like glioblastoma and colorectal cancer. Clinical trials are often small, vary widely in their methods, and typically combine KD with conventional treatments. There’s no clear proof yet that KD improves survival or tumour control in humans. Major oncology societies do not recommend KD as a standard cancer therapy. 

Equally important are the practical and emotional considerations. KD can be challenging to sustain, and restrictive diets can lead to weight loss, nutrient deficiencies, and digestive issues, especially during or after treatment when maintaining strength is critical. Beyond the physical effects, there’s the emotional toll. Diets framed around “starving cancer” can inadvertently imply that a person’s eating habits caused their disease or could cause it to worsen. For someone already navigating fear and uncertainty, this message can create unnecessary guilt and shame.

There’s also the social isolation that often comes with rigid food rules. Avoiding shared meals, stressing over ingredients, or constantly worrying about “breaking the diet” can increase overall stress. And we know that chronic stress affects sleep, cravings, immune function, and recovery, potentially offsetting any theoretical metabolic benefits the diet might offer.

During conventional treatment, your body is working incredibly hard, recovering from procedures, managing side effects, and keeping your immune system resilient. I believe adequate nourishment is ESSENTIAL to support this work. You need enough calories, protein, and nutrients to maintain energy, preserve muscle mass, and promote healing. Strict or highly restrictive diets can undermine these goals, leading to fatigue, weight loss, muscle loss, and slower recovery. Prioritizing balanced, consistent nutrition is one of the most powerful ways to support your body through treatment.

This isn’t to say the ketogenic diet is inherently harmful or never appropriate. In some carefully monitored, individualized cases, I have recommended it to patients, as it can serve as a complementary strategy. But like most things in cancer care, nutrition is nuanced. Oversimplified messages like “sugar feeds cancer” don’t reflect the complexity of the science, and more importantly, they can burden people with unnecessary fears. A balanced, compassionate, and evidence-informed approach serves you far better than fear-driven extremes.

Here’s what I recommend most often to my patients: a modified Mediterranean diet. This style of eating is rich in whole foods, loaded with produce, contains lots of healthy fats and is mostly made from scratch. I recommend modifying it by including only low glycemic index foods/grains, especially in those with a higher BMI, insulin resistance or diabetes. I recommend organic when possible. If the person has other comorbidities, from hypercholesterolemia or hypertension, to the more complex mycotoxin illness or chronic infections, I will adjust my recommendations to support these other comorbidities. Depending on the cancer type, I’ll guide patients toward foods with targeted benefits like broccoli and cruciferous vegetables in breast cancer, orange vegetables in renal cancer, or tomatoes and flaxseed in prostate cancer. I also teach ways to pair carbohydrates with protein, fat, and fibre to balance blood sugar and limit insulin spikes.

Carbohydrates are not the enemy; they’re a key part of recovery, energy, and tissue repair. In fact, carb-loading before surgery has even been shown to improve recovery outcomes.

Please, if you only take one thing away from this blog, let it be that nutrition in cancer care isn’t about fear and restriction, it’s about fuelling resilience.

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