Does sugar cause cancer?
Sugar phobia is alive and well in our culture. Demonized as a cancer-feeding, diabetes-causing, addictive molecule. Influencing some folx to throw out the candy stash and avoid all foods with added sugar. Morning shows, social media influencers and some medical providers collude with the no-sugar hype, misrepresenting the strength of studies and exaggerating the science. After multiple clients asked me, does sugar cause cancer? I decided I needed to take a closer look at the claim. Luckily, my colleague Suzanne Dixon, MPH, MS, RDN, an oncology writer, epidemiologist and dietitian, was willing to answer common questions on the topic.
* note, Suzanne uses language, including “obese” and “obesity” when quoting research. We both agree that this language is stigmatizing and problematic and inaccurately frames larger bodies as a medical condition.
Grab a beverage, because there is a lot to explore here:
Jamie: Hi, Suzanne. I’m excited to talk to you today about a topic that comes up in nutrition counseling sessions: sugar allows cancer cells to flourish.
Suzanne: You’ve just hit on one of the hardest nutrition myths oncology dietitians deal with. Once this idea that “sugar feeds cancer” or “sugar causes cancer” gets lodged in someone’s head, it’s very hard to convince them otherwise. However, there is NO truth to the idea that sugar causes cancer.
Jamie: Dismantling some of these food fears is really challenging. Especially when people believe it is a choice that could lead to disease or death. Let’s talk about the science. What are the studies telling us?
Suzanne: There are no controlled trials where people are fed X amount of sugar and others are fed Y amount of sugar, or say, for example, no sugar, and then followed to see who gets cancer. These studies do not exist.
Jamie: No studies exist to prove sugar causes cancer! So, how does this myth persist, even within the medical community?
Suzanne: Here is some background on why people think sugar causes cancer (which it doesn’t).
The only research that does exist on this topic are large population and epidemiological studies. There are two types of studies:
Cohort Studies: Groups of people report their dietary intake at multiple points over several decades and the group is followed to see who gets cancer (or heart disease, diabetes, etc) and who doesn’t. Researchers simply divide the group into low, medium or high sugar intake after the fact and then see if those in the “high sugar group” have more cancer. Of course, this is a very flawed way to understand sugar and cancer and it only proves an association, not causation.
And here’s the problem. The high sugar group also may exercise less, drink more alcohol, use tobacco, eat fewer fruits and vegetables, get less sleep, have any number of other health issues or engage in many other behaviors that also contribute to cancer risk. These are called confounders and they cause a lot of headaches in epidemiological research. Statisticians do their best to “control for” these factors when they analyze the data, but obviously, you can never control for everything that might be different between the low-sugar vs. the high-sugar groups. These are free-living humans who are going about their lives, and the differences between groups of people who eat low-sugar diets and those who eat a lot of sugar can be very large.
Case-control Studies: After people get cancer, researchers find “controls” to match to the cancer cases. They try to match these controls to the cancer patients in every way possible, except that they don’t have cancer. So they’ll match them on education level, age, gender, other illnesses, other dietary factors, tobacco and alcohol use, you name it. But as you can see, this runs into the same problem as cohort studies. There’s no way to match controls perfectly to cancer cases, so there may be other factors among the cases that are linked with why their cancer risk was higher. Things that you just can’t capture in a case-control comparison.
Plus, don’t forget, there is something called recall bias in case-control studies. Basically, if you ask someone about their “exposures” to potentially disease-causing factors after they have the disease, they are much more likely to remember the “bad” things than people who don’t have the disease. So cancer survivors are much more likely to report eating a lot of sugar, whether they did or not, compared with controls. This gives you a false result that the cancer patients ate more sugar, and in truth, they probably didn’t eat more sugar. Just being diagnosed with a life-threatening illness makes people a lot more likely to think of all the things they may have “done wrong,” that led to their disease. In reality, they likely ended up with cancer for all sorts of reasons we can’t even control on an individual level, such as genetics, environmental exposures, menstrual patterns and when and how many children they had (for women) and all sorts of things that have nothing to do with how they ate.
Jamie: This is really helpful in understanding research flaws. Associating any nutrient to a disease is challenging. Like you said, it’s difficult to control for all the factors, including socio-economic status, access to medical care, oppression, trauma histories, etc. There is so much that isn’t captured in the research. What I’m hearing from you is the claim that sugar causes cancer is misleading.
Suzanne: The only studies on this topic are observational and don’t prove cause and effect. Another problem is there are many observational studies looking at this topic and they don’t agree with one another!
When the only evidence we have is observational (vs. a high-quality randomized controlled trial), what researchers and epidemiology folks do is to look for consistency. Even though no single observational study can prove that factor X caused disease Y, if you have many, many of these studies, and they all agree with one another, that is some confirmation that maybe the association really does cause the disease through some real, physiological pathway.
And here’s the thing with sugar and cancer. These observational studies are all over the map. Some show more sugar intake is associated with higher risk of some cancers. Other studies show NO association at all, meaning, the groups of people with high sugar intake actually don’t have more cancer in a good portion of these studies.
This is a good indication that there really is no causative connection between sugar and cancer.
In other words, if sugar really did increase cancer risk, the studies would more consistently show this. But they don’t!
When studies are all over the place, that’s pointing toward the fact that if there is an effect (on risk of cancer), it’s so small we can’t even measure it. And most likely, there isn’t an effect of sugar creating much higher cancer risk.
Jamie: This is the problem when media headlines only sharing one study. Plus, the fact that writers aren’t looking critically at the research, like you are. Are there any studies worth taking a closer look at?
Suzanne: There is actually one really excellent controlled trial looking at fat, carbs, protein in the diet and risk of breast cancer recurrence. It’s called the WINS – Women’s Intervention Nutrition Study. In the early 2000s, researchers randomized nearly 3,000 women with a history of breast cancer to follow either a general healthy diet or a very, very low-fat diet.
The control, healthy diet was your standard, “government issued” approach – make sure you get five servings of fruits and veggies per day. Eat no more than 30 to 35 percent of calories from fat. Limit saturated fat to fewer than 10% of total calories, etc.
The low-fat intervention diet had the women aim for 15 percent of calories from fat. So this was VERY low fat. Per the methods, the low-fat group had a lot support to achieve the study goals. The diet was provided through: “centrally trained, registered dietitians who applied behavioral, cognitive, and motivational counseling techniques. The low-fat eating plan was implemented in an intensive phase with eight biweekly (up to month 4), individual counseling sessions followed by a maintenance phase (month 5 up to and including year 5) with registered dietitian visits every 3 months and optional monthly group sessions. Self-monitoring (daily fat gram counting and recording), goal setting, and motivational interviewing strategies were key components.”
The intervention group didn’t meet the 15 percent calories from fat goal, but they did eat much, much less fat than the control group. It was something like 22% fat vs. 32% of calories from fat. This was a huge different (10% difference in where calories are coming from), so even though the intervention didn’t quite make it to the 15 percent goal, they did succeed in significantly limiting fat.
So, if they were only eating about 1 in 5 of their calories from fat (that would be 20%), what else where they eating. CARBS! Lots and lots and lots of carbs. This group of breast cancer survivors was essentially eating a very low-fat, moderate protein, high carb diet. And looking at the papers that came out of that study, I can tell you, they were eating plenty of simple carbs, too.
So here’s what happened. As a whole group, the low-fat ladies had 24% LOWER risk of breast cancer recurrence compared with the breast cancer survivors who ate a general, healthy diet. And consider estrogen receptor negative (ER-) cancer. It is the toughest to treat and most aggressive. Survival rates tend to be much lower for women with ER- breast cancer compared with women with ER+ breast cancer.
So among women with the most aggressive type of breast cancer, the women eating the low-fat, high-carb diet had 42 percent (FORTY TWO) reduced risk of cancer recurrence compared to women in the control group.
I always explain this study because honestly, if carbs and sugar were SO bad, those women should have done worse, not better. But they did MUCH better on the low-fat, high-carb diet.
Does this mean I think fat causes cancer? No, of course not. But the findings lend very strong evidence to the idea that carbohydrates and sugar DO NOT increase cancer risk.
Jamie: Thank you for sharing this study. Unfortunately, what I’m seeing, even among some dietitians and bloggers, is to name “added sugar” as the culprit for weight gain, then linking “obesity” to cancer and other diseases. Can you speak to this?
Suzanne: Here’s something encouraging. Another controlled trial among 3,300 breast cancer survivors (WHEL – women’s healthy eating and living study – my grad school advisor is the principal investigator on this study!) looked at diet, “obesity”, physical activity and risk of breast cancer recurrence. This was really, really important, because for years, women in larger bodies were told, “You’re more likely to get cancer again because you’re fat.” It was terrifying for these women! And again, there was an association (not proven causation) between being “overweight” at diagnosis and higher likelihood of getting cancer again.
So this WHEL study examined the women and categorized them based on diet, physical activity and body weight. What they found was pretty amazing and very encouraging. Basically, “obesity” was only a risk factor for getting cancer again in the women who also had a poor diet and who were very sedentary.
Among “obese” women, those that ate at least five servings of fruits and vegetables per day and exercised the equivalent of at least 40 minutes of brisk daily walking did NOT have an increased risk of recurrence compared to “thinner” women.
In other words, even if the women in larger bodies did not lose a single pound, simply eating well and moving their bodies regularly completely erased that increased recurrence risk that appeared to be due to “obesity.” It wasn’t due to “obesity” at all.
Jamie: I’m so sick of the link between health and weight. Inevitably when we tease out the behaviors, the benefits of healthy behaviors are there regardless of size. Couple that with the recognition around weight stigma as the problem and the real impact it has on wellbeing.
What about all of the people saying carbohydrates and sugar are so bad that the only solution to health is the ketogenic diet. I’ve seen a lot of promotion of this diet for cancer prevention or treatment. Is there any science to that?
Suzanne: I think the ketogenic diet has some therapeutic uses, meaning with close dietitian supervision and to treat very specific medical and metabolic conditions, it may be something worth trying. However, this idea that everyone, or even anyone should be in ketosis all the time for health is simply not supported by good research. And as far as cancer risk, there isn’t any evidence cutting all sugar and carbohydrates out of the diet, which is essentially what a ketogenic diet is, reduces risk.
A very comprehensive 2019 research review paper conclusively found no solid evidence of a connection between moderate amounts of added dietary sugars and cancer risk. None! So anyone promoting the keto diet to “prevent cancer” is on pretty thin ice in terms of what the research actually tells us!
For anyone with a history of cancer, I am pretty dismayed and disappointed people are promoting, with no evidence to back the recommendation, that these folks should try a ketogenic diet.
It’s important to keep in mind, sure, cancer cells use carbohydrates and sugar for energy, but they also use protein and fat! In this sense, sugar does not specifically “feed” cancer any more than other nutrients. In fact, some emerging research points to potential harms of a ketogenic diet for people with cancer. Animal and cell studies hav found ketones, the primary fuel the body uses on a ketogenic diet, may promote the growth and spread of cancer cells! Clearly, the picture is complex and no two people are alike. But until we have more research on the potential benefits, but especially the HARMS of a ketogenic diet for people with cancer, I would not recommend people try it! As they say, “don’t try this at home.”
As a final note, the ketogenic diet can be exceedingly difficult to maintain. Certainly, for anyone with a history of disordered eating — and that includes a lot of people! — this type of restrictive diet is absolutely not an option. And even for people think, “Just this once, I can do this diet…,” if a person eats just a few too many grams of carbohydrates, this can stop ketosis. You may end up with an unbalanced, high-fat, low-fiber diet that is devoid of healthy vitamins, minerals and phytonutrients. And then a person isn’t even getting the purported “benefits” of being in ketosis, to offset all that fat and lack of healthy nutrients.
And finally, side effects of this diet can include constipation, fatigue, muscle cramps, bone loss and kidney stones! This is why I always stress it’s a therapeutic diet, not a lifestyle! It should never be treated as a lifestyle, regardless of what the latest instagram-famous person tells you.
Jamie: Thank you so much for taking the time to provide some clarity around the sugar/cancer myth. You are a wealth of knowledge and I so appreciate it!