During the eight years Stacy Shawhan has worked as an oncology dietitian, she has heard many questions from her cancer patients about how their diets influence their prognosis. But one question has come up more than the rest: will consuming sugary foods and drinks feed my cancer cells, making my condition worse?

“Cancer patients are so vulnerable, and some of them are just terrified to eat,” said Shawhan, who practices at the University of Cincinnati Cancer Center in Ohio in the US. “They think, ‘If I stop eating sugar, then I can starve my cancer’.”

The “sugar feeds cancer” narrative goes back to the 1920s, when a German physiologist noticed that some tumour cells consumed more glucose than healthy cells did. Soon after, low-sugar diets sprang up claiming to cure cancer. Recent polls from the US and Europe suggest about a third of cancer patients actively avoid sugar.

While experts say that diets high in added sugars may increase your risk of cancer over a lifetime, cutting out all sugars doesn’t actually fight existing tumours. “Every cell requires glucose, our brain requires glucose,” said Philipp Scherer, a diabetes researcher at UT Southwestern Medical Center in Dallas, US.

In other words, the best way to eat if you have cancer — or are trying to lower your risk of it — is a balanced, healthy diet.

Sugar clearly isn’t a carcinogen, Scherer said. There’s no evidence showing that eating sugar will cause cancer itself (like, say, smoking cigarettes would). Besides, Scherer added, “many, many cancers prefer to use fat as their primary energy source, so even the idea that cancers prefer glucose is not quite true.”

Still, a limited yet growing body of evidence has linked the overconsumption of added sugars (found in cookies, cakes and soft drinks) to cancer. For example, a large review of studies published in 2018 cited several that linked added sugar and sugary beverage consumption to an increase in cancer risk.

Excess sugar consumption has been shown to spark chronic inflammation in some people, which can damage cells that may then become cancerous, Shawhan said. Overconsumption of added sugars has also been shown to lower immunity, which can allow cancer cells to more easily spread. And consuming excess sugar can alter metabolism in ways that may lead to obesity and diabetes, conditions known to increase the odds of getting cancer.

Once you’re diagnosed with cancer, eliminating sugar doesn’t seem to slow or halt cancer growth in most cases, Shawhan said.

For the most part, experts agree that there’s no need to abstain from sugars that appear in whole foods. But Natalie Ledesma, an oncology dietitian at the UCSF Helen Diller Family Comprehensive Cancer Center in San Francisco, US, notes that consuming excess added sugar has been associated with worse outcomes in patients with certain solid tumors like breast, colon and prostate cancers. Other cancers may also be impacted, she said, but research on rarer cancer types has been limited.

It’s also important that cancer patients manage their diets without becoming afraid of food, said Dr Santosh Rao, an integrative oncologist at University Hospitals Connor Whole Health in Cleveland, US. Up to half experience muscle loss as a result of their disease. And sometimes things that doctors recommend for patients during rounds of gruelling treatments — like Ensure, electrolyte drinks or even potatoes — can contain a lot of sugars, Shawhan said.

While all people should avoid diets high in added sugars, cancer patients with certain metabolic diseases should be especially vigilant because it can affect their prognosis.

“Patients with poorly controlled diabetes tend to have more aggressive breast cancer,” for example, Dr Rao said. And a meta-analysis suggested that obese patients were more likely to die from colon, breast and uterine cancers.

The best way to lower your cancer risk, and to eat if you have a cancer diagnosis, is to follow a healthy diet that has plenty of whole fruits and vegetables. Mediterranean diets meet these goals and help reduce cancer risk, some studies show. Pairing carbohydrates with protein, fibre and fat (a dab of peanut butter on an apple slice, for example) prevents spikes in glucose that can, over time, wreak havoc on our metabolism and increase cancer risk.

Generally speaking, Shawhan said, it’s alright to indulge in a little added sugar, even on a daily basis, as long as you’re getting essential nutrients from the rest of your diet.

Meanwhile, research on sugar substitutes and their influence on cancer and cancer risk is inconclusive. Experts suggest avoiding them until we learn more.