Sugar causes cancer – another trope from little evidence

I am generally skeptical of any claim that anything causes cancer. That doesn’t mean I reject every claim that something or another causes cancer – smoking, sunlight, obesity, viruses, genes have all been linked to certain cancers. Now, there’s a claim that sugar causes cancer – is there anything there?

Sugar is considered a demon food, based on the irrational claims for it. But most of it has been scientifically debunked, so I really don’t pay attention to much of it. Yes, sugar is bad for diabetics. And yes, it may be one of the causes of the so-called “obesity epidemic.

One of the ongoing myths is that somehow sugar causes hyperactivity in children. But reviews have shown that it’s probably not correlated.

Can sugar cause cancer – is it even plausible? Time to find out.

All about sugar

I’m going to apologize, but this could be heavy science. But it helps understand what sugars are and do, instead of myths.

Before we can discuss whether sugar causes cancer or not, we need to have a better idea of what exactly is “sugar.” I daresay most individuals think that the sugar packets (see Note 1) that they open and put in their coffee is “sugar.” But sugar is more than that.

There are over twenty individual, naturally-found sugars, called monosaccharides, the basic units of all carbohydrates. Of all of those monosaccharides, only four play any significant role in human nutrition: glucosefructosegalactose, and ribose. The first three are the major energy sources for humans, while the last one, ribose, is critically necessary as the backbone of RNA and DNA molecules.

There are other monosaccharides found in nature that can be consumed, but they are nutritionally unimportant, and cannot be absorbed by humans. They can feed the gut flora, or can be enzymatically converted into one of the four basic sugars by those same gut flora.

So, I hope that’s clear. Only four sugars can be absorbed into the human body.

But it gets a bit more complicated. Many monosaccharide sugars can form disaccharides which result from two covalently bound monosaccharides.

Table sugar, the white stuff we put in our coffee, is called sucrose – a disaccharide made of one molecule of glucose bound to one molecule of fructose. Sucrose is also the main sugar in most other commercially purchased sugars that you find including brown sugar, molasses, beet sugar, and maple sugar.

Two of the other most common types of disaccharides are milk sugar, or lactose, which is glucose and galactose bound together; and maltose is two glucose molecules. There are a few dozen less common disaccharides which impart slightly different and unique tastes to certain fruits and vegetables.

Humans cannot absorb disaccharides – they are either quickly disassociated by water (or hydrolyzed) or acids plus enzymes within the gut. All that remain are simple monosaccharides.

So when you put sucrose in coffee, and consume it, it will be broken down into two sugars that actually can be used by the body – glucose and fructose.

But this is slightly more complicated (I know, you just want to know about how all this causes cancer). All carbohydrates are sugars. Starches are sugars. They are long chains, or polymers, of monosaccharides, almost always glucose.

Cellulose, which is a major component of paper, wood, natural plant fibers, and many other items, are glucose polysaccharides. So are insect shells.

Generally, these long chain polysaccharides cannot be broken down by humans, though our intestinal flora can use them for food. In addition, bulk fiber, an important part of your diet for intestinal motility, is generally long chains of saccharides.

Since humans really can only use 4 monosaccharides, humans have evolved a gastrointestinal tract that helps us process all the various disaccharides and polysaccharides to the point where we can actually use it. The gut has a variety of different enzymes that break down these starches and disaccharides.

For example, we cannot absorb sucrose directly. The gut breaks it down, using an enzyme called sucrase, into glucose and fructose, which is then absorbed into the blood. By the way, any disaccharide or polysaccharide that isn’t broken down remain in the gut, providing food for our gut bacteria, thereby maintaining a “healthy” digestive system. No I’m not advocating colon cleansing.

Now you’re a sugar expert.


Sugar causes cancer – the claim

The article in Shape, by K. Aleisha Fetters, claims that “a new study links sugar with breast cancer.” I have long ago given up reading websites as the last word on cancer. They’re generally not reliable, misinterpret real scientific evidence, or overstate the conclusions.

And here we go again.

The claims are based on an article recently published in the high impact factor journal, Cancer Research. The researchers investigated the potential relationship between dietary sugar and mammary gland tumor development in mouse models. The researchers concluded that, “we found that sucrose intake in mice comparable with levels of Western diets led to increased tumor growth and metastasis, when compared with a nonsugar starch diet.”

Scary, right?

But let’s take a look at this research from several different views:

  1. This is a mouse model. Around 90% of research done on these models never amount to anything. There are many reasons for this lack of usefulness, but it’s mostly in the physiology of the animal.
  2. On the hierarchy of evidence based research, animal research ranks pretty low.
  3. This is primary research that hasn’t been supported by other research in other labs. And it has not been established in human clinical research.
  4. Although the authors attempt to establish some sort of plausibility (certain genes are expressed and suppressed in the mouse tissue). However, I remain highly skeptical that a basic human nutrient (glucose and fructose, the hydrolyzed monosaccharides from sucrose) would disrupt cells that radically. Remember, the four essential monosaccharides that humans can absorb are closely controlled physiologically, remaining in homeostasis except under certain conditions, like diabetes.
  5. The authors did not show that blood sugars were causal to the gene changes.

I am a long way from believing or accepting that high sugar diets lead to breast cancer, especially in one published article. I cannot stand this type of lazy science journalism, which seems to rely upon a daily scan of PubMed articles, writing about ones that support your prior conclusions about anything. Including sugar.

Ironically, Ms. Fetters then referred to an International Journal of Cancer study that analyzed dietary sugar and cancer risks.  The prospective cohort study examined the sugar consumption and observed cancers in 435,674 patients. Did anyone at Shape actually read this article? Because I did.

The authors of the study clearly concluded that “none of the investigated sugars were associated with increased risk of colorectal cancer or with any other potentially IGF-I-related cancer (i.e. breast, prostate, pancreatic or endometrial cancer).”

In other words, in a massive and well developed epidemiological study, they couldn’t find a link between sugar and breast cancer. Huge prospective cohort study trumps mouse study, any day.

But in the world of cherry picking, Ms. Fetters chose to use only the data that supported her a priori beliefs about sugar and cancer. She claimed that, “people who consumed more added sugar had higher rates of esophageal adenocarcinoma, small intestine cancer, and cancer in the lining of the lungs.”

Now this is in contradiction to conclusions of the researchers themselves – ” in our large prospective study, we found no association between dietary sugars measured by an FFQ and risk of colorectal or any other major cancer.”

Now to be far to Ms. Fetters, who probably has no background in real science or statistics, she may have be tripped up by the secondary conclusion of the researchers, “the effects observed on some of the rare cancers, such as pleural, esophageal and small intestinal cancer, as well as on ovarian cancer, are intriguing and open questions on possible mechanisms, but may have been due to a chance, and warrants further investigation.”

Pleural, esophageal, and small intestinal cancers are extremely rare, so data must be carefully read to determine if there is a link let alone causality. The confidence interval for the incidence of those cancers was rather large, because of the rarity of the diseases.

Let’s look at a disease like small intestine cancer. This year, there will be an estimated 9400 new cases of the cancer in the USA. That converts to an incidence of about 3.9 per 100,000 adults – that’s extremely rare. Prostate cancer, for example, has an incidence of about 146 per 100,000.

And even if the data said that the incidence increases to 5 per 100,000, the absolute increase is so small, a real science writer would ask “can that all be attributed to sugar?” Well, we actually know what probably causes small intestine cancer – high fat diet, Crohn’s disease, celiac disease, and an inherited colon polyps.

A real science writer would think, “hmmmm, I wonder if a high fat diet is correlated with a high sugar diet.” Did sugar contribute to the risk of cancer? Or was it just a confounding factor to the actual cancer causing component of the diet?

But in general, the research just didn’t find a causal relationship between high sugar consumption and cancer.


What we can say?

Generally, the causes of cancer that are within your control are very few, just a handful. Get your HPV and HepB vaccinations. Don’t smoke. Stay out of the sun. And don’t be obese – and if sugar is related to obesity, then it may be indirectly related to cancer.

I fundamentally fail to see biological plausibility between a simple carbohydrates (glucose and fructose) and cancers. These sugars are inherently necessary for cell energy, various metabolic pathways, and several other physiological purposes. And generally, outside of a handful of metabolic conditions, like diabetes, sugar is under very good control in your body. Once again, people think that the human physiology is pathetically weak and subject to sudden destruction.

According to the MD Anderson Cancer Center, sugar doesn’t cause cancer:

It’s true that sugar feeds every cell in our body — even cancer cells. But, research shows that eating sugar doesn’t necessarily lead to cancer. It’s what sugar does to your waistline that can lead to cancer.



  1. I know some of you insist on those brown packets of sugar for your coffee. It’s still just plain white sugar contaminated intentionally with a byproduct of sugar refining called molasses, which is still mostly sugar.
  2. This article was originally published in January 2016, and has been updated and republished for World Cancer Day.


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The Original Skeptical Raptor
Chief Executive Officer at SkepticalRaptor
Lifetime lover of science, especially biomedical research. Spent years in academics, business development, research, and traveling the world shilling for Big Pharma. I love sports, mostly college basketball and football, hockey, and baseball. I enjoy great food and intelligent conversation. And a delicious morning coffee!