Does sugar feed cancer?

Where the idea comes from: the Warburg effect

In the 1920s, the German physician and scientist Otto Warburg, who later won the Nobel Prize, made an important observation. Cancer cells consume far more glucose, the sugar in our blood, than normal cells do, and they use that sugar in a different way.

A normal cell "burns" glucose completely inside its mitochondria, the cell's power plants, and gets a lot of energy from each molecule. A tumor cell, by contrast, prefers a shorter, less efficient route, even when there is plenty of oxygen. It gets little energy from each molecule, so it has to take up much more glucose, sometimes as much as 30 times more than normal tissue. In exchange, it uses the pieces of glucose as raw material to build new cells and multiply. This became known as the Warburg effect.

This discovery is real and useful. It is what makes the PET-CT scan possible: the scan finds tumors precisely because they "light up" as they absorb a tagged form of glucose.

How the theory was misused

Over time, Warburg's observation was simplified until it became something else: "the sugar you eat feeds the tumor" and, from there, "cut out sugar and the cancer will starve". This idea spread across the internet and through many "anti-cancer" diets, but it is not what the science showed.

• Warburg described how tumors use glucose, not that dietary sugar makes tumors grow. A tumor takes up a lot of glucose because its genetic changes have altered how it works. The sugar in your food is not the cause of this preference.
• The tumor does not depend on what is on your plate. The body keeps blood glucose steady at all times, even when you eat no sugar, as I explain below.
• The tumor adapts. When glucose runs short, many cancer cells switch to other fuels, such as fats, proteins and ketone bodies.
• "Starving the tumor" failed even in the lab. In animal studies, diet alone did not cure cancer. In some models, including bowel tumors, it actually made the disease worse.

Today, scientists study tumor metabolism to develop new drugs and to understand how to combine nutrition with treatment. It is a promising field, but very different from the idea that cutting out sugar, on its own, treats cancer.

Why cutting out sugar does not "starve the tumor"

The body keeps blood glucose levels under tight control. Even if you eat no sugar at all, the liver makes glucose to keep the brain and other organs working. The tumor keeps getting what it needs. And when energy runs short, the first to lose out is usually the patient: weight, muscle and strength, which are exactly what help most in getting through treatment.

Where sugar really matters

The problem is not the spoonful of sugar in your coffee. It is too much sugar and too many calories, which over the years lead to weight gain, high insulin and inflammation. That environment encourages abnormal cells to grow.

• Excess weight is linked to at least 13 types of cancer, including bowel, liver, pancreas, stomach, post-menopausal breast and endometrial cancer.
• In a large US population study, people with chronically high insulin had about twice the risk of dying from cancer.

In other words, sugar raises risk indirectly, through weight and insulin, and not by "feeding" a tumor that is already there.

What about no-carb diets, such as the ketogenic diet?

The ketogenic diet cuts out almost all carbohydrates and is very high in fat. In the lab, it showed interesting results in some models, but it also made cancer worse in others, including bowel tumors. In studies with patients:

• The few comparative studies that measured how the cancer progressed showed no benefit.
• In several studies, many patients could not stick to the diet, and some lost a worrying amount of weight and muscle.
• In large population studies, low-carbohydrate diets based on meat, eggs and dairy were linked to higher cancer mortality. When the diet was plant-based, this added risk disappeared.

That is why the leading oncology and nutrition societies, such as ESPEN, ESMO and ASCO, do not recommend ketogenic or restrictive diets during treatment, except within clinical trials.

What to do in practice

• Choose real food: fruit, vegetables, whole grains, beans and lentils, fish and olive oil. The Mediterranean pattern has the strongest evidence.
• Cut back on added sugar, especially soft drinks, sweetened juices, sweets and ultra-processed foods. Whole fruit does not count here.
• During treatment, do not cut out food groups on your own. If your appetite is low, a sweet treat you can manage to eat may help you keep your weight up. The priority is not to lose weight or muscle, with enough protein at every meal.
• If you have diabetes, keeping your blood sugar under control is still important, together with your doctor.
• Keep moving. Regular physical activity lowers the risk of several cancers and helps you tolerate treatment.

What has evidence for prevention today: managing your weight, eating a good-quality diet and staying active. In a large study of people with overweight and diabetes, those who lost weight in a planned way had 16% fewer obesity-related cancers over 11 years. No "special" diet has done better than that.

Before changing your diet during treatment, talk to your care team and, if possible, to a dietitian with experience in oncology.

For information only. It does not replace advice from your care team or a dietitian.