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Obesity is the biggest cause of cancer in the UK, after smoking.

But this isn’t well known.

When people do hear this, we’re often asked: ‘I’m already overweight, will losing weight reduce my risk?’

And with 2 in 3 UK adults either overweight or obese, it’s an important question. But while it sounds logical that losing weight would reduce the risk, proving this isn’t easy.

When studying people, separating those who lose weight intentionally from those who lose it because they’re already ill can be tough. On top of that, losing weight and keeping it off is hard.

But this hasn’t stopped researchers from hunting for answers. And the good news is, research so far tells us that weight loss is beneficial when it comes to reducing cancer risk.

Understanding weight gain

Years of research have shown that the more weight gained, the higher the risk of cancer.

Most of this evidence comes from studies that have used body mass index (BMI) as a measure of body fat.

But BMI can only provide a snapshot of someone’s weight.

Other studies have looked at how long someone is overweight. And the results suggest that the longer someone is overweight, the higher their risk

Based on this, losing weight (and keeping it off) means you stop accumulating more risk, and reduce your risk compared to what it would be if you gained more weight. So losing weight does help, both with cancer risk and your general health.

But this doesn’t fully answer our question: can an increased risk go back down to the level it would have been had the extra weight never been there?

Weight loss through surgery

One of the most effective, although extreme, ways for people who are very overweight to lose weight is bariatric surgery. This covers a range of surgical techniques, such as stomach stapling or surgically bypassing large parts of the gut.

Because people lose a lot of weight after surgery, and keep most of it off, it’s more likely researchers will find an effect on cancer risk if it’s there.

It’s also more likely any effect would be due to weight loss itself, rather than lifestyle changes that reduce cancer risk. These studies also help untangle the effects of losing weight intentionally and weight loss due to illness.

Results from studies post-surgery are mixed, but overall they suggest that people who undergo bariatric surgery do have a reduced risk of cancer compared to those who don’t.

The strongest evidence so far is for women, but evidence is growing in men too.

A study that combined the results of 6 others found a staggering 45% reduction in cancer risk among formerly obese people who had bariatric surgery. But when they split the results by gender, this finding only remained in women.

A more recent US study, which included over 2500 cancer cases, also found a reduction in cancer risk in people who had surgery based on 3.5 years of follow up. And the reduced risk was seen for a range of cancers, including breast, colon, pancreatic and womb.

So the results so far are promising, and suggest weight loss can reverse increased cancer risk.

But there are limitations. Firstly, major surgery isn’t the solution for everyone. And it’s possible that people who have surgery differ in ways these studies don’t account for. And weight loss through surgery could have different effects to weight loss by other means.

Weight loss outside the operating room

A 2012 review looked at 6 weight loss studies and 5 of these linked intentional weight loss with a reduced risk of cancer.

But a more recent study, looking at the results of weight loss trials (mostly low-fat diets), didn’t find they reduced cancer risk. But the quality of evidence for cancer was rated as very low – overall the original trials only included a small number of cancer cases (103 in total) and the average amount of weight lost after 3 years was small.

These findings illustrate how difficult it is to study weight loss in the real world. So the evidence isn’t as strong. But what’s there is promising, although as with surgery studies the strongest evidence is for women – specifically for breast cancer.

How might weight loss help?

A rigorous 2016 review of how extra fat affects the body found good evidence that intentional weight loss affects key ways obesity is thought to cause cancer: namely hormones and inflammation.

And studies since have also found this.

But we don’t yet fully understand all the ways obesity causes cancer. So there’s still more to know about how weight loss could reverse these effects.

Why is there seemingly more of an effect in women?

There are many possible explanations. It could be that because these female cancers are common, there are more cases to study. This increases the chance of finding an effect if it’s there.

But it’s also possible that cancers strongly linked to sex hormones, such as womb and breast, are more quickly affected by weight loss, whereas for other cancers it may take longer to see an effect. For example, weight loss can quickly reduce levels of oestrogen in the body, and high levels of oestrogen are almost certainly how obesity causes womb and breast cancer.

Cancers more common in men, such as bowel cancer, may take longer to see an effect. This might explain why this study showed no impact of weight loss on bowel cancer risk after 7 years of follow up. This study also couldn’t distinguish between intentional and unintentional weight loss.

What’s likely is that weight loss affects different parts of the body in different ways, and this is reflected in how it might affect cancer risk. This makes sense, as weight gain affects cancer risk differently for different cancers. Studies in the future will need to take this into account.

Prevention is still best, but weight loss is worth it

So, the answer to our original question – does losing weight reduce cancer risk? – seems to be: yes.

If you are overweight, you can reduce your risk by avoiding gaining more weight.

And overall, all the research carried out so far suggests that an increased risk can start to fall with weight loss.

Plus, the best way to lose weight for most people is by eating and drinking healthily and moving more, all of which can reduce the risk of cancer independently.

But the fact remains that losing weight and keeping it off can be incredibly hard. So this must be supported by public health measures (like the sugary drinks tax) that make healthy choices easy for everyone, both to prevent weight gain, and to help those lose it who need to.

Never gaining extra weight in the first place is still best for reducing cancer risk. But we know that’s not possible for everyone – and it doesn’t help people who have already gained weight. So having evidence that weight loss could help is good news.

Emma Shields is a senior health information officer at Cancer Research UK

Comments

Emma Shields March 26, 2018

Hi David,
Thanks for your comment. The evidence linking obesity and cancer is tied to body fat, rather than muscle mass. BMI is the most common measure of body fatness. But as you say, there are limitations because BMI isn’t a direct measure of body fat. So for people such as professional athletes or body builders, it isn’t an accurate measure. But for those of us who don’t fall under these groups, it is a good guide to body fatness. Good quality studies take account of all the things that could affect the relationship between body fatness and cancer, such as diet and physical activity.
Best wishes,
Emma, Cancer Research UK

David March 25, 2018

BMI is a pretty poor measure of being overweight. Some of our fittest, most athletic, and physically powerful people are classed as overweight by BMI, yet presumably they would be classed as doing “everything right” in terms of nutrition, exercise and general lifestyle.

I would be interested to know if the evidence can tell us if being overweight / obese (by BMI) because of lean muscle mass increases your risk of cancer? Should the article above really refer to fat gain and inactivity (i.e. not “weight gain” per se), or does it apply to all types of weight gain irrespective of level of physical activity?

i.e. does an inactive individual whose BMI ranks them as overweight/obese have the same increased risk of cancer as an individual whose BMI ranks them overweight/obese when they are a fit and active individual (e.g. rugby player, powerlifter)?

How do you tackle this when looking at the evidence?

Harvey Gund March 18, 2018

An interesting piece of research.

At the, tender, age of 37, I was luckily, during a routine blood test for another serious medical condition, diagnosed with cancer. Witihn 48 hours, I was on an ‘operating table’, and underwent rounds of, thankfully, successful chemotherapy.

My tumour was located in my neck, and due to me being, say, fifteen pounds ‘overweight’, wasn’t, initially, noticeable.

Whilst being overweight wasn’t the factor for my cancer, it didn’t help with the diagnosis, operation and subsequent recovery!

All th best to all of you, and good-luck with your health, life, diets, etc., etc..

Micha March 17, 2018

I’m not 100% sure about it – as people who are overweight or obese are usually because of poor diet, overeating, bad food choices. Therefore, it could be more related to this than just being overweight. Eg if eating bacon increase the risk of cancer and people who are bigger often consume it- reason could be ‘the bacon’ not being overweight. Would be nice to have access to full study.

Stephanie March 17, 2018

Hi Emma, potentially good news for people like me. I was very overweight at 17stone (164cm). Between 2010-2011 I lost 6.5 stones through adopting a healthy diet and regular exercise and I have kept the weight off ever since. Knowing the effort that I had to put in it would have been soul destroying to not have improved my odds of worrying diseases like cancers.

Emma Shields March 15, 2018

Hi Eugenie,

Thanks for your comment. As highlighted in the post, studying the effect of weight loss on cancer risk is difficult. Unfortunately, more people gain weight than lose it. So there is far more evidence for the effect of weight gain on cancer risk. But the evidence we have so far does suggest that losing weight reduces cancer risk, and there is good evidence intentional weight loss can reverse the keys ways obesity is thought to cause cancer.

Things that can cause cancer – whether that’s smoking, obesity, alcohol or the sun – increase an individual person’s risk of cancer, but this doesn’t mean that everyone who is exposed to these things will definitely develop cancer. Lots of different things affect our personal risk of cancer, including how old we are and our genes. But across large populations of people studied through research, things like smoking and obesity have been shown to cause more people to develop cancer.

You can read more about the evidence behind our campaign on our website.

Best wishes,
Emma, Cancer Research UK

Eugenie March 14, 2018

Cancer research has just had a massive advertising campaign telling us that obesity causes cancer, yet the blog post above seems to contradict this – there’s little evidence that weight loss reduces cancer risk.

So why did you run such a massive advertising campaign on such a poor evidence base? Obesity may be correlated with increased risk of cancer, but that doesn’t mean that obesity causes cancer. The two are not the same. If obesity actually caused cancer to any significant extent, we’d expect to see much stronger evidence that intentional weight loss reduced cancer risk.

Muriel Fisher March 9, 2018

I found this reportedly interesting as my mother died of ovarian cancer 4years ago. Both myself and my sister are both very over weight. Since Christmas I have been at the gym at least 3 times a week and cutting down on what I eat has helped me to start losing weight. So it’s great to know all the above and to know. I’m giving myself a better chance when it comes to cancer. Hopefully it will spur me on!!!!!

Mrs l millard March 7, 2018

Really good and useful information the more we know the better.

Bipasa Datta March 5, 2018

Excellent. Videos are fun and effective to watch

mozeqatim March 1, 2018

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