While there is no magic diet that will cure or prevent cancer, intervention in a patient’s diet will help them maintain an acceptable quality of life – as long as that intervention comes from a medically qualified professional with no vested interests. Which brings us to the case of Australian blogger Belle Gibson, who is facing legal action for “unconscionable conduct” after promoting a book in which she talked of curing herself of multiple cancers by eating the right things. The publisher, Penguin, paid more than A$264,000 (£160,000) for the book and Gibson also made more than A$420,000 from app sales before it emerged that she had never had cancer. Her claims to have donated over A$300,000 to charities also turned out to be false.

In contrast, a recipe book for cancer patients experiencing weight loss from University College, Cork, and one for patients with swallowing difficulties from Breakthrough Cancer Research have been developed by dieticians and are available free of charge.

What is an oncology dietician?

Oncology dieticians provide expert, evidence-based advice to cancer patients to help maintain their strength, wellbeing and quality of life. They can, for example, develop plans using fluids or tube feeding to help maintain weight and muscle mass. They also address side effects of treatment that affect a patient’s ability to eat. Qualifying as an oncology dietician involves a four-year undergraduate degree in human nutrition and dietetics, with a hospital placement. Alternatively, people with a related science degree can obtain a postgraduate qualification in dietetics, followed by at least three years of post-qualification hospital work.

Ruth Kilcawley and Fiona Roulston are oncology dieticians. Frustratingly, says Kilcawley, much of the misinformation about diet and cancer her patients receive comes from well-intentioned loved ones, while Roulston’s patients often get information online. “If you enter ‘nutrition and cancer’ into Google, you get over 123m results. With so much conflicting information, it makes it so confusing and overwhelming for cancer patients at an already stressful time.”

What about a vegan diet?

There isn’t currently any strong evidence that eating too much red meat causes cancer. Some studies have shown a weak possibility that red meat may be associated with colorectal cancer in men. But an association is not a cause. There is some evidence that vegan diets may aid weight loss in certain people (though this may partly relate to vegans simply being more aware of what, and therefore how much, they eat). However, a healthy diet hinges on avoiding overindulgence and ensuring that we get enough nutrients to meet our essential needs. The few scientific studies on veganism and vegetarianism refer to a lack of data on vegan diets, but some vegan diets have been shown to result in calcium levels that are below minimum dietary regulations. In a small study of men with early-stage prostate cancer, a vegan diet resulted in vitamin D deficiency.

“Unnecessary diet restrictions of food groups are not recommended during cancer treatment,” says Roulston. Kilcawley adds: “The negative consequences include accelerating muscle loss. This leads to reduced ability to tolerate treatment, inadequate micronutrient intake (particularly calcium, which may compound bone mineral density issues with some types of chemotherapy), lack of energy, low mood and fatigue.”

But surely we all know that sugar is carcinogenic

Eating too much fatty and sugary food is not good for anyone. But the idea that cancer has a sweet tooth and that a patient can “starve” their cancer by cutting all sugars and carbohydrates is a gross oversimplification for a complex disease that behaves differently from patient to patient. It simply isn’t the case that cake feeds cancer while eggs feed healthy cells, and cutting carbs completely puts patients at increased risk of malnutrition. A study of more than 1,000 patients across three Irish hospitals found alarming levels of malnutrition amongst cancer patients.

The case study of a 65-year-old woman who recovered from a brain tumour has been used to promote the idea that a low-carb, high-fat “ketogenic diet” is a cure for cancer. However, this patient had also received standard chemotherapy and radiotherapy. Disturbingly, a proponent of this diet, Dr Thomas Seyfried of Boston College, has said it “can replace chemotherapy and radiation for almost all cancers”.

Nutrition is complex. Patients should seek advice on their individual needs from a qualified professional. No competent professional will ever tell a patient to forgo evidence-based medicine in favour of a diet.

What about supplements and superfoods?

Anything that alters how your body works is a drug, even if it’s “natural”. Supplements not recommended by a dietician can interact with other drugs, including anti-cancer therapies, and make them less effective. The Memorial Sloan Kettering Cancer Centre urges patients to stop taking any herbal supplements before starting treatment and lists some of the dangerous effects such “natural remedies” can have. The term “superfoods” is simply part of a marketing strategy, not based on science.

“There is huge manipulation of people’s emotions going on when someone tries a fad diet, hoping against hope it will cure them, and then subsequently fails because it is restrictive, unpalatable, or the patient is simply too ill,” says Kilcawley. “They risk blaming their own failure in keeping to the diet on any progression of disease, which is excessively cruel of those promoting diets without sufficient evidence that the diet if kept to would be effective. I see this a lot.”

Further information: Cancer Research UK, Macmillan and the Irish Cancer Society are all reliable sources for advice and support.