Individuals who lost more than 10% of their body weight had a significantly higher risk of being diagnosed with cancer in the following 12 months.

Excess weight is linked to many health issues, including cancer. Studies have shown that obesity is associated with at least 13 types of cancer.

And in some regions, such as North America, Europe and the Middle East, obesity contributes to up to 9% of all cancer cases and could even become the leading cause of cancer in UK women in the next 25 years.

Therefore, weight loss is clearly an important health goal.

However, a recent study, published in the JAMA Network, yielded an unexpected result. The research found that weight loss was associated with a significantly higher rate of cancer in the 12 months following weight loss, compared to people who did not lose weight.

The study used data from nearly 160,000 health professionals.

These individuals reported their health and lifestyle details every two years until 2016.

Researchers paid particular attention to changes in participants' weight.

They were divided into three groups based on how intentional their weight loss was: "high" for those who improved both their diet and physical activity, "moderate" for improvements in either diet or physical activity and "low" for no notable improvements in either.

The study provided a striking finding: individuals who lost more than 10% of their body weight had a significantly higher risk of being diagnosed with cancer in the following year.

Not all cancers were equally linked to weight loss.

The study found a strong connection between weight loss and cancers of the upper digestive system (throat and stomach), liver, pancreas and bile ducts.

For these types, the risk was much higher, with odds increasing from three to over seven times.

The connection was weaker for colorectal and lung cancer and did not seem to affect the likelihood of breast, prostate and female-specific cancers.

Study limitations

The study authors indicated certain limitations of the study. First, they relied on participants' self-reports of weight, which may not always be accurate.

Additionally, they checked participants only every two years, meaning some details could be missed.

Moreover, the study was primarily conducted on health professionals in the United States, who are more informed about cancer and have better access to healthcare.

Notably, the risk was highest among those who made the least effort to lose weight (those classified as "low intentionality for weight loss").

So, upon closer examination, this study could confirm what is already known: if you experience unintentional weight loss, it is important to medically investigate why.

"If you are losing weight without making changes to your diet and exercise routine, you should consult a doctor to find out what could be causing it. There are several conditions that can lead to unexpected weight loss. Doctors can assess whether there is something that requires further investigation", says Qiaoli Wang, a researcher at the Department of Clinical Science, Intervention and Technology, Karolinska Institute, the first author of the study, in a statement.

The new study helps highlight the cancers that could be the reason for this unexpected weight loss, say the authors.

This means that when someone loses weight without a clear reason, doctors may focus more on checking certain types of cancer, such as those in the stomach or liver, to find out what's going on.

On the other hand, there is plenty of evidence showing the benefits of intentional weight loss.

For example, a study published in 2022 found that people who made significant lifestyle changes had a 16% lower risk of obesity-related cancers.

Another study from the same year found that weight loss surgery greatly reduced the risk of developing and dying from cancer.

Therefore, it is important to remember the proven benefits of weight management and maintaining an active lifestyle to prevent cancer, say experts.

Weight management is a complex field that includes medications, surgeries and lifestyle changes, commented Gianmarco Contino, from the University of Birmingham, who did not participate in this research.

In the future, weight goals may be tailored to better suit individual health needs, thanks to a deeper understanding of what truly contributes to well-being.

Research suggests that being slightly overweight at an older age may not be as bad as once thought and may even be linked to a longer life.

As we face a global obesity epidemic, this information can help tailor weight management and cancer prevention approaches, according to the study authors.