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Can losing weight reduce the risk of cancer?

by Ed Yong | Analysis

30 July 2009

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Losing weight can help to reduce the risk of cancer

Losing weight can help to reduce the risk of cancer

The link between body weight and cancer is now very well established. Overweight and obese people have higher risks of at least seven types of cancer, and both this list and the nation’s waistlines are growing all the time.

Large studies have told us that maintaining a healthy body weight, and stopping yourself from putting weight on, can help to reduce the risk of cancer. But what about the effect of losing some pounds if you already happen to be overweight or obese? It’s an important question given that at least 60 per cent of the UK currently falls within those categories.

Answering this question is no easy task. When studying people in real life, it’s difficult to separate people who lose weight voluntarily from those who shed it because they are already ill. On top of that, people often find it hard to lose weight successfully in the longer term and keep it off, as many of us know first-hand.

But some people take more drastic measures because of imminent dangers to their health. And by studying them, a team of Swedish scientists have found evidence that weight loss – albeit of an extreme kind – can help to protect against cancer.

The Swedish study

As part of the Swedish Obese Subjects Study, a group of scientist led by Lars Sjostrom observed the health of over 4,000 very obese people (with a Body Mass Index of at least 34).

Half of this group were treated with bariatric surgery – a term that covers a range of surgical techniques – from stomach stapling to surgically bypassing large parts of the gut – that are all designed to help severely obese people to lose weight. Meanwhile, the rest of the group received only lifestyle advice or no treatment at all.

After ten years, the study showed that the surgery patients lost an average of 20kg (just over 3 stone) – much more than the 1kg (2 pounds) or so lost by the group that had no surgery. That wasn’t the only thing that was lowered – their overall risk of cancer fell by a third too. However, this benefit seemed to only apply to women, who experienced a 42 per cent drop in their cancer risk. Among the men, there wasn’t any statistically significant effect.

The wider context

There’s now plenty of evidence that cancer is more common in people who are overweight or obese, or who gain weight over time. Studies all over the world have found consistent results, and we have a good idea of the biological routes through which body fat can affect cancer risk.

In contrast, the evidence that actually losing weight can reduce the risk is weaker, for the reasons we mentioned at the start of this post. For example, one study found that women who had deliberately lost at least 9kg (20 pounds) over a 7-year period had 11 per cent lower risk of cancer overall.

A few studies have looked at breast cancer. As an example, the US-based Nurses’ Health Study found that women who lose 10kg (22 pounds) since the menopause and kept the weight off had 57 per cent lower risk of breast cancer. And one study found that women whose weight had gone up and down by more than 4.5kg (10 pounds), more than 10 times over an 8-year period, had more than double the risk of kidney cancer than women whose weight was stable.

The result from actual clinical trials is less clear. One trial – the Women’s Health Initiative (WHI) – illustrates some of the difficulties. It found that women who were randomly assigned to a low-fat diet did not have lower risks of breast or bowel cancer than a control group who ate what they would normally do.

These results are regularly cited to dispute the link between obesity and cancer, but that’s a poor interpretation. It ignores the fact that after 3 years, only 57 per cent of the women were actually sticking to the low-fat diet and after 9 years, just one in five were still committed to it. After 6 years, women in the diet group were only 0.8kg lighter on average than the control group.

What does this mean?

As the WHI example shows, it is difficult to assess the effects of weight loss in a trial because people find it very difficult to lose weight. This is also why the Swedish study is so interesting – it’s a case where the people in the “intervention group” actually and demonstrably lost a lot of weight.

In this context, the new results are encouraging and add to the evidence linking obesity and cancer. The study does have weaknesses though.

For a start, unlike trials of a similar nature, it wasn’t randomised for ethical reasons. It was also quite small (a fact that the authors acknowledge) which means that there’s a small chance that the results are a statistical fluke. Nonetheless, it’s encouraging that two other studies of bariatric surgery, both published this year, found the same thing.

The study also found that only women who went through bariatric surgery saw a drop in their risk of cancer. This could simply be a statistical effect caused by small numbers of men (the trial included twice as many women as men).

You might also expect a stronger effect in women, given the cancers that are linked to body weight. Obesity accounts for about half of all womb cancer, obviously a female-only disease. It also influences the risk of breast cancer, which is vastly more common in women than men.

Both of these cancers are very sensitive to levels of the hormone oestrogen, which in turn depends on the amount of fat in an older woman’s body. It could be that the risk of these cancers responds very quickly to weight loss, while other obesity-related cancers that are more common in men (bowel and kidney, for example) take longer to be influenced.

Finally, the study doesn’t mean that radical procedures stomach stapling should become par for the course for cancer prevention, because of the risks involved in such serious operations. It does, however, provide strong evidence that losing weight can help to reduce the risk of cancer.