Obesity and cancer - shall we get this straight?

04 Sep 2019

The facts

We don’t know that obesity is a cause of cancer

It’s really hard to study cancer. This is because for many people it can take years to develop and progression is influenced by a mind-blowing array of factors ranging from family history (genetics) to various lifestyle factors. We therefore can only really study cancer using observational studies; this means taking a group of people, learning about them and their life at one point, following them up years later and then trying to relate cases of cancer with individual factors. This type of research can only give us associations with what might be causing the disease as we can’t know for sure whether the factor at the initial time-point was what actually caused the cancer. This just gives us an idea of what might be going on. 

Looking at obesity we have evidence from these types of studies that cancer is associated with 12 different types of cancer. This gives a global rate of only 9% of all cancers being associated with obesity. But this doesn’t tell us whether excess fat (essentially what we mean when we say ‘obesity’) is responsible for the difference in risk. There are MANY other differences between someone in a bigger and smaller body that often aren’t accounted for in academic studies; such as the effects of yo-yo dieting, weight stigma and other lifestyle factors. Research is being done to understand where the associations come from but we currently don’t really know.

We don’t know if reducing body weight reduces risk of cancer

We don’t have evidence to show that lowering body weight actually reduces risk of cancer, as there’s little research into this. We mostly just compare people in a bigger body to people in smaller bodies; it’s a huge leap to say that reducing weight will do anything to your cancer risk.

Also, for some cancers being at a higher weight might even be protective (e.g. breast cancer in pre-menopausal women). And having a greater fat (and therefore energy reserve) is certainly associated with greater ability to survive the gruelling treatment often required to cure cancer.

We don’t have ways to help people lose weight and keep it off

Since 1980, the obesity rate has doubled in 73 countries and increased in 113 others. And in all that time, no nation has reduced its obesity rate, despite the ‘war on obesity’ and continued efforts to help people lose weight.

Research looking at ways to help people lose weight gives a bleak picture. Although weight can be lost initially (with intensive interventions and huge amounts of support – good luck trying it by yourself!) the relapse rate is almost 100% at 3-5 years. Take a look at my previous blog on why diets just don’t work.

We have non-stigmatising ways to help people reduce risk of cancer

A person of 'normal' weight who is not active and who chooses mostly less nutritious foods could still be more likely to have a greater risk of cancer than a higher weight person who chooses mostly nutritious foods and is active.

Why aren’t we focusing on the positive steps that we can take to better health? For example, helping people to eat a more varied diet, to move their bodies or breastfeed their babies. Instead we spend time and money contributing to fat phobia and weight stigma which we know are detrimental to physical and mental health and emotional wellbeing.

Why the new campaign sucks


Smoking is a behaviour, body weight is an outcome – therefore you can’t tie them together in the way CRUK are doing. For the most part smoking is a singular choice, for the most part body weight isn’t. You can’t just quit being obese the way people can quit smoking.

Obesity isn’t a lifestyle choice – there are more than 100 factors that determine how much we weigh and how resistant our body is to weight loss – it’s not to do with lack of willpower. Body weight isn’t completely under individual control. We have to get rid of this deeply held belief.

When CRUK were testing the effects of the campaign they found that there was no change in people’s views on those in bigger bodies BUT their perception was already negative, i.e. people in bigger bodies were more likely to be viewed as having less energy, less friends and less likely to be happy. So it isn’t helping us to remove stereotypes around people in bigger bodies either.

The campaign promotes weight stigma – if shaming people for their weight worked then no-one would be obese because we live in an inherently fat shaming culture. So it doesn’t work does it? Weight stigma has many knock on consequences not least increased risk of health issues because of fear of further stigma from healthcare professionals, increased likelihood of putting on further weight due to emotional eating and fear of being judged when exercising.


What could CRUK have done better?

Promotion of non-stigmatising healthcare

Promotion of the importance of nutrient-dense food being accessible to all people

Promotion of the importance of access to joyful movement for all people

Promotion of health behaviours that we know can reduce risk of cancer

Celebration of the diversity in body size we have

Here’s hoping this blog has given some insight into the debate on obesity and cancer. For more on why this campaign is so problematic have a read of this Open Letter to Cancer Research UK.

“There’s a lot we can do right now to improve fat people’s lives - to shift our focus for the first time from weight to health and shame to support” - Michael Hobbes