Obesity & Cancer – let’s get this straight shall we?


You might have been lead to believe by recent Cancer Research UK (CRUK) campaigns and from other equally reputable (and some probably less so) sources that we now have concrete evidence that being obese (in and of itself) is a cause of cancer; the way that we’re sure that smoking is. This though, is not a fact. Cancer is such an emotive topic and the causes, treatment and life-changing effects it can have are vastly complicated. But to link obesity and smoking together as equal causes of cancer is just wrong. Read more for why and what CRUK could have done better.

The facts

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

It’s really hard to study cancer. This is because in many people it can take years to develop and be this 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 cancer – it just gives 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 this will do anything to your risk.

Also, the report that shows cancers that have the greatest associated increased risk with obesity (oesophageal and uterine) only considered those with a BMI >40 – that’s around 4% of the population, meaning this campaign is missing the target for 96% of us. But they don’t explain that do they? Instead they reinforce fat phobia by making a bold, unsubstantiated claim that anyone with a BMI >30 (the definition of obesity) is at greater risk of cancer.

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 cancers.

We don’t have a way 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 effort to get people to 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 from scientific and healthcare professionals – good luck trying it by yourself!) the rate of total weight regain 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, chooses mostly less nutritious foods is still 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 focussing on the positive steps that we can take to better health? For example, helping people to eat a more varied diet, have better access to ways to move their bodies, breastfeed their babies etc. But instead we spend time and money contributing to fat phobia and weight stigma that we know are detrimental to physical and mental health and 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 a singular choice. 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 are more likely to be viewed as having less energy, less friends, are less likely to be happy etc. 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 people wouldn’t put on weight 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, poor mental and emotional wellbeing and 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.

For details of how I can support you on your journey to a healthier relationship with food through personalised online nutrition consultations check out my website Non-Diet Nutrition.

“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

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