Fact or folklore?
1. High blood sugars cause cancer.
2. People with cancer shouldn’t eat sugars because the sugars cause the cancers to grow faster.
Scientists are still trying to answer this basic question. Does diabetes cause cancer? And if so, what types of cancer?
Numerous studies have shown that although as diseases, diabetes and cancer are poles apart, there are several links between them. The association that people with diabetes are more likely to develop cancer may be because there are several common risk factors which raise the risk for both diseases.
Age. As you grow older, your risk for developing cancers and diabetes are increased.
Male sex. Overall, males are more likely to develop cancer and type 2 diabetes.
Obesity. Overweight people are more likely to develop cancer and diabetes. Losing weight definitely reduces the risk for diabetes, but there is no evidence that losing weight combats cancer
Diet. There is a definite causative association between eating processed foods and red meats and both diseases.
Smoking is associated with several types of cancer and diabetes and worsens some complications of diabetes
Exercise. There is some evidence that exercise prevents both diseases.
Race. African Americans and non-Hispanic whites are more likely to develop cancer. They are also at a higher risk for developing diabetes
In other words, both conditions are likely to occur together, but does one cause the other? That is the fundamental question researchers are trying to figure out – to unravel the complex web that links diabetes and cancer. And if diabetes does increase the susceptibility of cancer, what types of cancer do occur as a result of diabetes?
In cancer patients who do have diabetes, the diabetes is often diagnosed shortly before or after their cancer diagnosis, suggesting that diabetes may be a symptom of developing cancer, rather than its cause.
Some of the hallmarks of diabetes are changes in how cells make and process energy, an increase in certain hormones in patients with diabetes, and persistent chronic inflammation in diabetics. All of these can have cancer-promoting effects, stimulating cells to grow and divide.
Cancers of the pancreas, liver, uterus, colon and rectum, kidney, breast, stomach, thyroid, lung, and esophagus are twice as likely to develop in patients with diabetes. And diabetics have a lower incidence of cancer of the prostate than non-diabetics.
It is the association of pancreatic cancers with diabetes that is most intriguing. Cancers diagnosed after many years of diabetes may have occurred as a result of diabetes, which makes the case for screening diabetics for cancer all the more strong. At this moment in time, we do not have a definitive answer to the question of what comes first – diabetes or pancreatic cancer. Much like the chicken and egg debate, there is evidence for it on either side. In fact, some researchers are now suggesting that the answer may actually be both. 8 in 10 people with pancreatic cancer have some level of intolerance to sugar, and up to 5 in 10 have frank overt diabetes.
In some people, diabetes may develop as a result of pancreatic cancer interfering with how the pancreas works. This is probably the case in patients who are diagnosed with pancreatic cancer shortly before or after being told they have diabetes.
In a small number of other cases, diabetes may eventually lead to pancreatic cancer. This could be the case in those who have lived with diabetes for a longer time and are diagnosed with pancreatic cancer many years after finding out they have diabetes.
A recent study suggests that those who control their blood sugar levels well may be at lower risk of pancreatic cancer than those who don’t.
In any case, the indisputable fact is that people with diabetes have an increased chance of developing many types of cancer, and an increased risk of dying from it. In addition, many of the life-style choices that increase the risk of diabetes may simultaneously increase the risk of cancer.
There are several theories as to why researchers believe that diabetes may cause and help in the spread of cancer.
High insulin levels are thought to be one of the causes. When the body produces too much insulin, the risk of cancer goes up. People with type 2 diabetes are insulin resistant. Initially, in diabetics, enough insulin is produced by the pancreas in response to a carbohydrate meal, but the body cells being resistant to the insulin are unable to use it as effectively, leading to high blood sugar levels. This high production of insulin lasts for years while the disease progresses. The pancreas makes insulin, which is in turn is sent straight to the liver. So these two organs deal with particularly high insulin levels. It may be the reason why these organs are particularly prone to cancer in patients with diabetes.
High insulin levels increase the production of sex hormones by the ovaries and decrease the production of testosterone. The higher estrogen levels and the lower testosterone levels may be why cancer of the ovaries, uterus, and breast are linked to diabetes, and why diabetics have a less incidence of prostate cancers.
People with many insulin receptors on their cancer cells have a worse prognosis than those with minimal receptors. Even though a cancer cell's insulin pathways don't work in diabetics, cancerous tumors get around that roadblock and trigger the insulin receptors regardless. As a result, cancerous tumors metastasize and spread to other parts of the body.
Insulin encourages cell growth, and discourages cell death, enhancing the proliferation of cancer cells. Insulin makes cancer cells more invasive, and more likely to metastasize.
There also is some arguable evidence that some specific long acting insulins (Lantus) elevate the risk of developing cancer.
Besides the high insulin levels, the high blood sugar levels may also cause cancer cells to proliferate, since cancer cells are very capable of absorbing glucose from the blood, without the need for insulin. The oral anti-diabetic drug Metformin is associated with lower risks of developing cancer, and there are numerous studies showing improved survival rates with those already having cancer who are taking Metformin.
Treatment of some cancers with steroids are associated with a higher level of blood sugars, causing worries that it may fuel the growth and spread of cancers.
There is a strong feeling among researchers that tumors seem to love sugar. They feel it makes the cancers grow and proliferate. The PET scan (positron emission tomography scan) uses a small amount of radioactive tracer – typically a form of glucose to detect metastasis in cancers. All body tissues absorb this, but cancer cells absorb greater amounts. It may be one reason researchers have concluded that cancer cells grow faster on sugar.
Another contributing factor may be chronic inflammation. Elevated levels of C- reactive protein, a potent marker of inflammation is often seen in patients with diabetes and cancer. And chronic inflammation is often found in diabetics.
The bottom line?
We are still not sure if diabetes causes cancer. But one thing is sure. Lifestyle changes that prevent or reverse diabetes, will certainly cut cancer risk.
(The author is a former head of the department of cardiothoracic surgery at Christian Medical College and Hospital, Vellore. He is currently the chief administrative of icer, and head of the cardiothoracic surgery department at Pushpagiri Heart Institute, Tiruvalla.) Also read: Health | Matters of the Heart | Alcohol and coronary artery disease