Childhood obesity: the task of tackling cancer begins early
We’ve blogged a fair bit recently about the UK’s growing waistline, and how certain forms of cancer are more common among people who are overweight or obese. And we’ve discussed the theories about how obesity might cause cancer.
Lots of this has focused on preventing cancer by reducing obesity in adults. But obesity can come out to play earlier in life too.
Poor diet and not enough physical activity can leave children with an unhealthy body weight. The latest statistics reveal the alarming extent of this problem: one in five children in England is overweight or obese before they start primary school. By the time they leave, this increases to almost one in three.
But like embarrassing school photographs and much-loved teddy bears, these unhealthy habits often aren’t left behind in childhood. They persist; meaning obese children tend to carry their weight through to adulthood.
This is why it’s crucial we tackle children’s obesity — as well as adult obesity — to prevent cancer. Because obese children are more likely to grow into obese adults, and obese adults are more likely to develop cancer.
But there’s also another worrying angle to this story that — if true — would make the case for action on childhood obesity all the more urgent. This is that being obese as a child might directly affect a person’s risk of developing cancer as an adult, regardless of what they grow up to weigh. In other words, obesity in early years might be leading to biological changes that make cancer more likely later on.
There are a lot of uncertainties here, and it’s an emerging field — but it’s crucial we find out what’s going on. We’ll consider the evidence for this a little later. But first, let’s look briefly at what we do know.
Childhood obesity often rolls into adulthood
There’s been good evidence for decades that obese children are often still obese in later life. This effect can be seen in school-age children as well as in the very young. One study in Finland found children with the highest body mass index (BMI — a way to gauge if someone is a healthy weight for their height) between the ages of three and five were over three times more likely to be obese as adults.
More recently, an analysis that brought together the results of 15 previous studies found obese children and teenagers were around five times more likely to be obese as adults.
The studies included in the analysis recruited children through the mid to late twentieth century. And because childhood obesity levels have risen since the mid-1990s, the association between childhood obesity and adulthood obesity may be even bigger today.
The results also suggested teenage weight might be an even stronger predictor of adult obesity than childhood obesity. Because eight in 10 obese teenagers (80 per cent) were obese as adults, whereas only around half of obese children were still obese in adolescence, suggesting these children probably shed their excess fat during puberty.
So, we can be confident that excess weight in early life can often cling on into adulthood. And we know that excess weight in adulthood raises a person’s risk of cancer. Being overweight is a key preventable cause of cancer. And that’s why we think the Government needs to take action in tackling childhood obesity.
But could childhood obesity directly affect cancer risk in later life, independent of how much someone goes on to weigh as an adult?
A direct link
This is where the evidence gets thinner, and things aren’t so clear.
Some cancers have been studied more than others, and in the case of some cancers there are only single studies. It’s also very difficult to detangle the effects of obesity in childhood from the effects of obesity in adults. That’s because it’s hard to design studies to pin the risk of adult cancer solely on excess bodyweight in childhood, because changes in a person’s weight since may have affected their risk.
There’s also a toy box full of risk factors that come into play between childhood and adulthood, including our genes and our environment, as well as aspects of our daily lives, like what we eat, how much alcohol we drink, if we smoke, and how active we are. Together, these factors make the relationship between childhood obesity and adulthood cancer a tricky one to unpick.
But the beginnings of a link are starting to emerge.
Let’s take oesophageal cancer as an example.
In one study from last year, researchers looked at whether adults in Denmark who were heavier as children were more likely to develop a type of oesophageal cancer, called adenocarcinoma, as adults. The disease is more common in people who are overweight, but it’s not clear whether this risk can originate in early life.
By using archived BMI measurements of more than 250,000 children, and health records to see how many of them had gone on to develop oesophageal cancer as adults, the researchers found that adults with higher BMIs in childhood were at a higher risk of the disease in later life.
But there are question marks over whether their excess childhood weight itself was responsible for the extra risk. This is because the researchers weren’t able to account for how heavy the children became as adults, nor what lifestyles they went on to lead, such as what foods they ate, how much alcohol they drank, or if they smoked — all risk factors for oesophageal cancer.
So, from this study alone, we can’t be absolutely sure that a higher BMI as a child is directly linked to an increased risk of this type of cancer. Or if it’s indirectly associated because the person was always more likely to become obese as an adult. But it certainly warrants further investigation.
In another study published in 2012, Iranian researchers tested whether the body size people thought they were as children was linked to cases of a different type of oesophageal cancer, called squamous cell carcinoma, in later life.
By showing participants pictures of body shapes that corresponded to a BMI range, and asking them to recall the body shape they were most similar to at ages 15 and 30, the researchers could estimate how many adults were overweight or obese as children, to see if this was linked to whether they subsequently developed the disease.
Around a third of the adults who developed oesophageal cancer had been obese at the age of 15. What’s interesting is that women who were obese at age 15, but not at 30, were three times more likely to develop this type of cancer as an adult. This is important — it suggests that once a woman’s risk of the disease increased at the age of 15, it remained higher — even if she went on to lose weight.
But we need to tread carefully because, unlike for oesophageal adenocarcinoma, obesity isn’t a known risk factor for oesophageal squamous cell carcinoma. Smoking and drinking alcohol are, though. And the researchers accounted for people’s smoking history. But because hardly any of the people included drank alcohol, they didn’t include the information they collected on drinking in their calculations.
So the evidence available makes it hard to know whether body weight really did have an effect on cancer risk. But studies like this can help scientists design future research that can more closely test these potential links.
What about other types of cancer?
It’s not just oesophageal cancer that researchers have been paying attention to. A handful of studies have linked excess body weight in early life to an increased risk of bowel cancer for some people in adulthood.
And we’ve found a few other studies looking at other types of cancer too — including endometrial cancer, non-Hodgkin lymphoma, thyroid cancer, liver cancer, and one type of kidney cancer in adulthood.
But remember, not all of these studies took into account the adults’ BMI. And this is a crucial part in helping us understand if obesity in childhood can, on its own, predict a person’s future risk of cancer independent of how much they weigh as an adult. What’s more, asking people to recall their childhood body size isn’t the most reliable or accurate way of recording or measuring weight, because people may incorrectly remember their past. And BMI itself isn’t the best way of measuring weight in children and teenagers. Doctors use charts specific to children’s gender and age to measure young people’s BMI, but many studies don’t use them.
On top of this, BMI isn’t a great measure of abdominal obesity — the fat around our bellies that can increase our risk of cancer. This is because BMI doesn’t distinguish between muscles and fat mass. So a person’s BMI can be high when they’re muscular but not overweight. While this does mean solely relying on BMI as an indicator of whether someone is carrying, or carried, excess body fat, should be approached with caution, BMI remains a reliable measure of healthy weight in a population.
So although this research might be a warning sign, it’s too early to say there’s a definite link. Further studies will be needed — perhaps integrating a better measure of childhood obesity and more accurate adult tracking — before a clearer link might emerge.
A protective factor?
Not all research is painting children’s obesity as a potential cause of cancer in later life. A heavier body weight in early life could be having the opposite effect in women who’ve had their menopause, by reducing their risk of breast cancer.
Again, further studies will be needed to pin down what’s going on here. Especially as we know that being overweight after her menopause increases a woman’s risk of the disease.
So how do we weigh up the evidence?
It’s crystal clear that overweight children are more likely to become overweight adults, increasing their risk of cancer. This link is based on solid evidence.
It’s also looking possible that obesity in childhood might increase the risk of certain forms of cancer later on. And — conversely — even offer women protection against breast cancer in later years. But what we don’t know is whether, as well as making children more likely to grow into obese adults, obesity in childhood can preset future cancer risk, causing lasting damage.
Although the evidence for childhood obesity directly increasing a person’s risk of cancer in later life is growing, we need to handle it with caution. Because even though it may suggest obesity is associated with certain cancers in adult life it doesn’t mean obesity is necessarily the cause. It may mean that people in the available studies ate poor diets as kids, or weren’t very active when they were young, each of which can raise the risk of cancer.
What’s more, not all obese adults were obese as children or teenagers. And obese children can shed their puppy fat. So we can’t say only obese children — who tend to grow into obese adults — might be at increased risk of cancer, because obese adults who were healthy weights as kids can develop cancer too. While we know this means targeting obesity in pre-pubescent children isn’t a magic bullet that will fix the obesity problem — and subsequently help prevent adult cancers — it’s definitely a good place to start.
And to keep making progress, we need lots more studies, and lots more data, to see if we can untangle the effect of adult weight and other risk factors when it comes to cancer risk. Ideally, studies would follow a group of people from childhood through to late adulthood. But this isn’t exactly practical — for one thing it would need studies lasting 60 years or so. So researchers will have to come up with other ways of finding out.
But regardless of whether children’s obesity leads to cancer in later life directly, or indirectly, it’s clear there’s a problem. And that’s why it’s important that everyone — from individuals to the Government — do their bit to help reduce levels of childhood obesity.
If parents encourage their children to lead a healthy lifestyle, they can help them keep a healthy body weight later on in life. If government develops policies on matters related to diet and exercise, it can help prevent children from gaining too much weight in the first place.
And that can only be a good thing.
Thea Cunningham is a health information officer at Cancer Research UK
Originally published at scienceblog.cancerresearchuk.org on February 19, 2016.