Glycemic Levels and Cancer Recurrence

Glycemic Levels and Cancer Recurrence

Written By: Dr. Adam McLeod, ND, BSc (Hons)

I tell virtually every cancer patient that they should avoid sugar as much as possible. Some doctors insist that sugar has no effect on cancer. This is simply not what the scientific literature states. If you are trying to fight cancer or prevent the recurrence of cancer, then you should make an effort to reduce your sugar intake.

Study after study has demonstrated a direct connection between sugar intake and cancer risk1,2,3,4,5. There are a wide range of cancers which are associated with increased sugar intake. Cancer cells often have significantly more insulin receptors than normal cells. In other words, they respond very rapidly to insulin and they will always be more effective at grabbing sugar from the blood stream and utilizing it as an energy source. Cancer cells will always grab the sugar before normal cells due to this fundamental shift in their metabolism.

The sugar acts as a direct source of energy for the cancer cells. These abnormal cells are often dependent on a constant supply of sugar, which is pushed through anaerobic glycolysis to provide them with energy. Essentially the sugar acts as fuel which directly stimulates the growth of cancerous cells. The fundamental challenge is that normal cells also require sugar and it is simply not possible to eliminate sugar completely.

It turns out that although sugar acts as fuel to cancer cells, the mechanism for the enhanced tumour growth from sugar is different than you would expect. There is a big difference in the metabolism of a food rich in simple sugars compared to a food that contains complex carbohydrates. When you eat a food rich in simple sugars such as candy, the body rapidly absorbs the sugar. This causes a rapid and significant elevation of the sugar concentration in your blood. In response to this sugar spike, the pancreas secretes insulin, which circulates through the entire body in an effort to bring the sugar levels back to normal.

Insulin interacts with the receptors on the surface of both normal and cancerous cells. Upon interacting with the cells, it helps them to pull sugar in from the blood until the blood sugar level drops back to a normal level. Remember that cancer cells have more insulin receptors, so they will always take advantage of this insulin spike more effectively than normal cells. It is this spike in insulin and insulin-like growth factors that stimulate the growth of cancerous cells2. In other words, it is not the sugar content that is stimulating growth; it is the response to sudden increases in sugar levels.

Complex carbohydrates are metabolized very differently in the body. They do not cause a sudden spike in blood sugar levels. The sugar in complex carbohydrates is slowly released as the food passes through the gastrointestinal tract. As the sugar is being slowly released, it is also being metabolized by cells within the body at a similar rate. As a result, it is not necessary for the pancreas to secrete as much insulin because there is no spike in blood sugar that needs to be controlled.

Despite the overwhelming evidence, some skeptical health care professionals insist that avoiding sugar makes no difference because everything we consume has sugar in it. Although it is true that virtually everything we eat contains some sugar, this simple logic is completely incorrect and demonstrates a lack of understanding of the mechanism. The sugar is not directly stimulating the growth of cancer, but there is no question that our body’s response to sugar does stimulate cancer.

Sugar, Inflammation and Recurrence

There are several key metabolic changes that occur in the body when exposed to simple sugars such as in candy. High levels of sugar in the blood seem to inhibit the function of the immune system and stimulate inflammation5,6. This inflammation is not localized; it is a true systemic inflammatory response. There are countless studies which strongly suggest that chronic inflammation is a significant factor in the development and in the progression of cancer. This inflammatory response makes it easier for cancer cells to evade detection by the immune system and it enhances the rate of spread. Any effective cancer treatment plan must address systemic inflammation and make a significant effort to control it in a balanced way.

Obviously when fighting cancer, it is critical to use every tool at your disposal to keep the immune system strong so that it can focus on the task at hand. Hyperglycemia (high levels of sugar in the blood) inhibits the function of the immune system on a number of different levels. It is important to recognize that this immune suppressing effect is not something that would be readily detected from any blood work. The number of white blood cells and neutrophils in the blood will remain the same however; these cells will not be working as effectively. The immune cells will not attack cancer cells as effectively when they are exposed to high levels of sugar.

It is logical that if sugar inhibits the immune system and stimulates inflammation, then you would expect high levels of sugar to be associated with an increased cancer risk. The correlation between high glycemic diets and cancer risk is well established. It is essential that patients looking to prevent recurrence of cancer adhere to a low glycemic diet. There was a recent study conducted on women with a history of breast cancer. In this study researchers looked for a connection between fasting blood glucose levels and risk of cancer recurrence. There was a strong correlation between high fasting blood glucose levels and cancer recurrence7. In other words, the women who consistently had high levels of sugar in their blood had a higher risk of developing cancer. This is really not surprising given what we know about the relationship between sugar and cancer.

The connection between sugar and cancer is both logical and well supported by data. What can be done to decrease the levels of sugar in the blood? There are a number of different pharmaceutical options, the most common being Metformin, which is associated with a decreased cancer risk (although the mechanism for this anti-cancer effect may not be related to sugar). The safest approach is making modifications to your diet so that you are not putting large amounts of sugar into your body in the first place. Start reading labels and become familiar with the foods that you are putting into your body. If it looks sugary and tastes sugary, then it is probably sugary and it is best to avoid it. The first step is very obvious; avoid putting simple sugars into your body. Beyond dietary controls there are a number of different pharmaceutical options, the most common being Metformin, which is associated with a decreased cancer risk (although the mechanism for this anticancer effect may not be related to sugar).

Another helpful dietary change is increasing your fibre intake. When you consume fibre, it essentially slows the release of sugar into the blood stream. This results in less insulin being secreted and consequently less stimulation of any residual cancer cells. The data on fibre consumption and cancer prevention is mixed but generally positive. In one large study on fibre intake and breast cancer recurrence (known as the HEAL cohort), it was determined that fibre decreased risk of recurrence, but the improvement was not considered statistically significant7. Another study concluded that higher levels of fibre consumption provided significant benefit to overall survival, but this benefit was not necessarily related to cancer8.

Many patients immediately focus on avoiding gluten when they get the diagnosis of cancer. It is important to mention that avoiding gluten is not usually a critical component of a diet designed to fight cancer. Generally speaking you want to avoid foods that will stimulate inflammation in the body and in some people consumption of gluten certainly triggers a systemic inflammatory response. In these patients who are sensitive to gluten they should certainly make an effort to avoid it. In those who are not particularly sensitive to gluten, going gluten free is not the number one priority. We have to focus on getting the essential nutrients into the cells so that they can more effectively fight the cancer.

It is also worth pointing out that many of the gluten free foods are very high in sugar. In many of the better-tasting gluten free products, there are significant amounts of sugar added. In the context of cancer, this added sugar will cause more problems than any benefit that would be gained from the absence of gluten. If avoiding gluten makes you feel healthier and more vital, then by all means, avoid it. It is critical to recognize that just because it is gluten free does not mean that it is healthy. You need to make a conscious effort to avoid sugar and read the labels of the foods that you are putting into your body.

The sugar content of fruits is generally not a concern. In my experience, most patients could benefit from having more fruits in their diet. Any negative impact from the sugar in fruits is far outweighed by the positive effects of the nutrients and the natural antioxidants. There are some fruits which are exceptionally rich in sugar. These sugar rich fruits such as mangos, kiwis, bananas and dried fruits should be consumed in moderation. It can be helpful to look at the glycemic load (not the glycemic index) of your favourite fruits and modify your diet accordingly to reduce your intake of sugar. It is not necessary to strictly avoid these sugar rich fruits but by eating them in moderation you can substantially reduce your overall sugar consumption.

The bottom line is that it is not hard to connect the dots. When you consume high levels of sugar, it has a number of effects on the body. It promotes inflammation, weakens the immune system and stimulates the growth of cancerous cells. If patients consume a low glycemic diet, then they are less likely to develop cancer and any cancer cells that are present will not grow as quickly. Fibre helps to further enhance a low glycemic diet by reducing your body’s response to sugar. At the end of the day the goal is to develop a diet plan that you can maintain for the rest of your life. There is no benefit adhering to a strict diet for only a short period of time. When it comes to cancer prevention, it is best to develop a simple and sustainable long-term treatment plan that you can easily maintain.

Dr. Adam McLeod is a Naturopathic Doctor (ND), BSc. (Hon) Molecular biology, First Nations Healer, Motivational Speaker and International Best Selling Author. He currently practices at his clinic in Vancouver, British Columbia where he focuses on integrative cancer care. https://www.yaletownnaturopathic.com


1) Augustin, L. S. A., et al. “Dietary glycemic index and glycemic load, and breast cancer risk: a case-control study.” Annals of Oncology 12.11 (2001): 1533-1538.

2) Franceschi, S., et al. “Dietary glycemic load and colorectal cancer risk.” Annals of Oncology 12.2 (2001): 173-178.

3) Michaud, Dominique S., et al. “Dietary sugar, glycemic load, and pancreatic cancer risk in a prospective study.” Journal of the National Cancer Institute 94.17 (2002): 1293-1300.

4) Gnagnarella, Patrizia, et al. “Glycemic index, glycemic load, and cancer risk: a meta-analysis.” The American journal of clinical nutrition 87.6 (2008): 1793-1801.

5) Qi, Lu, and Frank B. Hu. “Dietary glycemic load, whole grains, and systemic inflammation in diabetes: the epidemiological evidence.” Current opinion in lipidology 18.1 (2007): 3-8.

6) Turina, Matthias, Donald E. Fry, and Hiram C. Polk Jr. “Acute hyperglycemia and the innate immune system: clinical, cellular, and molecular aspects.” Critical care medicine 33.7 (2005): 1624-1633.

7) Belle, Fabiën N., et al. “Dietary fiber, carbohydrates, glycemic index, and glycemic load in relation to breast cancer prognosis in the HEAL cohort.” Cancer Epidemiology Biomarkers & Prevention 20.5 (2011): 890-899.

8) Kroenke, Candyce H., et al. “Dietary patterns and survival after breast cancer diagnosis.” Journal of clinical oncology 23.36 (2005): 9295-9303.

9) Contiero, Paolo, et al. “Fasting blood glucose and long-term prognosis of non-metastatic breast cancer: a cohort study.” Breast cancer research and treatment 138.3 (2013): 951-959.

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