Are there links between sugar, cancer and diabetes?

Dietitian, Berna Harmse, unpacks the evidence on the effects sugar has on our bodies and the direct and indirect links it has with cancer and diabetes.

Researchers are continuously investigating the connection between sugar and cancer. Unfortunately, the topic causes a lot of anxiety and misinformation in the media and on the internet. There is no strong evidence that directly links sugar to increased cancer risk, but there is an indirect link.

What is the indirect link?

All the cells in our body, including cancer cells, need sugar (glucose) from our bloodstream for fuel. We get this blood glucose from carbohydrate-containing foods, including fruit, vegetables, starch, wholegrains and dairy. Some glucose is also made in our bodies from protein.

Sugar doesn’t make cancer grow faster. As stated in a Mayo Clinic article, “All cells, including cancer cells, depend on glucose for energy. But giving more sugar to cancer cells doesn’t speed up their growth. Likewise, depriving cells of sugar doesn’t slow down their growth.”

Eating high-sugar foods increase our body weight and body fat, which is linked to some kinds of cancer. For that reason, the American Institute for Cancer Research recommends increased intake of wholegrains, vegetables, fruit and beans; and reducing intake of sugary beverages and sweets.

Influence on weight and metabolism

Let’s look at the influence on weight and metabolism. The Academy of Nutrition and Dietetics reinforces that much research shows that higher insulin (a hormone) levels and related growth factors may influence cancer cell growth the most, as well as increasing the risk for other chronic diseases.

Different types of cancer cells have high amounts of insulin receptors, making them respond more than normal cells to insulin’s ability to promote growth.

All the food we eat gets broken down to smaller bits, in the process we call digestion. Glucose sits in the veins, and insulin working like a key, unlocks the veins so the energy can get to the rest of the body to be used.

When insulin levels are high, it is a signal to the body that there is plenty of food available, and that these kilojoules should be used to grow and build reserves for future times of starvation.

Insulin levels rise quickly when we eat unrefined carbohydrates (white bread and sweets) and leads to a drop in blood glucose levels. Low blood glucose is the biggest appetite stimulant in the world. It makes you overeat, which again causes a release of more insulin and thus a cycle of eating more and gaining weight and body fat continues.

Inactivity and being overweight also increases insulin levels, and so insulin resistance is caused. If no intervention is launched at this stage, diabetes soon follows.

Is there a link between cancer and diabetes?

Research is being done to investigate the direct link between cancer and diabetes, with some researchers speculating that the underlying metabolic factors, like long-term stress and the inflammation that comes with it, underpins some of the patterns behind it.

So, what do I eat considering all this information?

If you can keep your food portions controlled and maintain your weight, you are on the right track.

Avoid refined carbohydrates, like take-aways, white bread products, and sweetened foods and beverages. Rather choose high-fibre carbohydrates, like wholegrains, fruit, vegetables and legumes. Higher fibre foods are the cornerstone of blood glucose management.

The five-a-day approach is still best – try to have at least two fruits and three vegetables per day, or vice versa. This ensures the adequate intake of antioxidants which plays a big role in terms of fighting and preventing chronic diseases.

Vegetables and salad should take up half of your dinner plate, and carbohydrate and proteins should be the side dishes of the meal.

Try to have breakfast every morning and do not skip meals

Aim to do some form of physical activity most days of the week.


Berna Harmse is a private practicing dietitian in Cape Town, she holds a MSc in Dietetics and has a special interest in oncology nutrition. She is also an external lecturer at Stellenbosch University Division of Human Nutrition.

Diabetes and cancer – is there a link?

An estimated up to 4,6 million people are living with diabetes in South Africa and an alarming 60 000 new cases of cancers are reported annually, according to the South African National Cancer Registry. Dr Jay Narainsamy says it is important to delve into the link between these two prevalent conditions in the hope that this understanding may lead to better lifestyle choices and positive changes in clinical management.

The link between diabetes and cancer was considered as early as 1959. A report in the New England Journal of Medicine, in March 2011, looked at causes of deaths in patients with diabetes. The article estimated cancer-related deaths at 7 per 1000 person-years and 4 per 1000 person-years among men and women respectively. Diabetes was associated with an increase in cancer-related deaths involving the pancreas, ovaries, liver, colorectum, breasts, lungs and bladder.

Common risk factors

Diabetes and cancer have common risk factors, some of which are modifiable and some not. Non-modifiable risk factors include: age, gender and ethnicity, with increased risk in older people, men and in the African American population in the United States. Modifiable risk factors include: obesity, diet, physical activity, smoking and alcohol.

Obesity is linked to the development of insulin resistance and Type 2 diabetes. It is thought that the high levels of insulin produced by the body to compensate for insulin resistance and obesity-associated inflammation may precipitate cancer development. In addition, diabetes itself (especially if not controlled) may cause vascular damage and an inflammatory state, which may create an environment for tumour development.

Diets low in processed and red meats, with a high content of vegetables, fruits and whole grains aid in lowering the risk of developing certain cancers. A healthy diet may also lead to weight loss and reduce the risk of developing insulin resistance and diabetes. Increased physical activity has shown to reduce the risk of certain types of cancer as well as improving overall health. Smoking and alcohol are both associated with the development of cancer as well as diabetes.

Metformin and cancer

On a further positive note, the oral diabetes medication metformin – the first-line drug of choice for patients with Type 2 diabetes – has been shown to inhibit abnormal cell growth, and has potential anti-cancer properties. Further studies are currently underway to assess the interaction between metformin and cancer.

The link between diabetes and cancer in other classes of oral diabetes agents are, however, less conclusive.

Injectable insulin and cancer

On the opposite spectrum, injectable insulin was thought to be associated with an increased risk of cancer development. However, this has not been conclusively proven and risk is probably better evaluated in the context of duration of diabetes, other oral diabetes agents already on board, and poor glycaemic control.

The link between diabetes and cancer

There is undoubtedly a link between diabetes and the development of certain types of cancer. With this in mind, doctors must ensure routine screenings for at-risk patients are completed timeously. They also need to be vigilant for ‘red flag’ complaints and act promptly to investigate these problems.

Reduce your risk

While further research needs to be done on the link between diabetes and cancer, the positive message is foundational lifestyle therapies for diabetes – healthy eating, increased physical activity, weight loss, not smoking, and first line pharmacological therapy, metformin – may have the additional benefit of reducing your cancer risk.


Dr Jay Narainsamy is a specialist physician and endocrinologist.