Weight loss and cancer: what a recent study found

Excess weight is connected with a range of health problems, cancer among them. Research has indicated that obesity is linked to at least 13 forms of cancer.

In certain parts of the world, including North America, Europe and the Middle East, obesity accounts for as much as 9% of all cancer cases. Within the next 25 years, it could even overtake other factors as the leading cause of cancer among UK women. Weight loss is therefore plainly an important health objective.

Yet a recent study delivered a surprising finding. Compared with people whose weight had remained stable, those who lost weight experienced a significantly higher cancer rate in the 12 months after the loss.

The research drew on information from almost 160,000 health professionals, followed for an average of 28 years. Until 2016, they provided updates on their health and lifestyle every two years, with researchers closely examining changes in body weight.

Participants were allocated to three categories according to how deliberate their weight loss appeared to be: "high" for people improving both their diet and exercise, "medium" for those making changes to either diet or exercise, and "low" where neither showed meaningful improvement.

Weight loss and cancer risk

One notable pattern emerged among the participants: those who lost over 10% of their body weight faced a significantly increased chance of receiving a cancer diagnosis during the following year.

The association did not apply equally across every type of cancer. Weight loss was strongly associated with cancers affecting the upper digestive tract - the gullet and stomach - as well as the liver, pancreas and bile ducts.

For these cancers, the likelihood rose substantially, ranging from three to more than seven times higher. The association was less pronounced for colorectal and lung cancer, while no apparent effect was seen on the probability of breast, prostate or women-only cancers.

Limits of the study

The study authors have stressed that their work has limitations. In particular, it depended on participants reporting their own weight, which may not always have been precise.

Participants were also assessed only once every two years, meaning some information may have been overlooked. In addition, the study population consisted mainly of US health professionals, who may be more cancer-aware and have better access to healthcare than the wider population.

Importantly, the greatest risk was seen in people making the smallest effort to lose weight - those placed in the "low intentionality for weight loss" group. On closer consideration, the findings may therefore reinforce existing knowledge: unexplained weight loss should be investigated.

Unintentional weight loss and obesity-related cancer

The new contribution of this study is that it identifies which cancers may underlie such unexpected weight loss. When a person loses weight for no obvious reason, doctors may place greater emphasis on investigating particular cancers, including those involving the stomach region or liver, to establish the cause.

Conversely, substantial evidence supports the benefits of intentional weight loss. For example, a study published in 2022 reported that people who made major lifestyle changes had a 16% lower risk of obesity-related cancers. A second study from the same year concluded that weight-loss surgery significantly lowered both the risk of developing cancer and the risk of dying from it.

It remains important, then, to recognise the established value of weight management and physical activity in cancer prevention.

Weight management is a complex area encompassing medicines, surgery and lifestyle changes. Greater understanding of the factors that genuinely shape wellbeing may allow future weight targets to be tailored more closely to individual health needs.

Research also indicates that being mildly overweight in later life may be less harmful than previously believed, and may even be associated with longer life. During a global obesity epidemic, such evidence can help refine approaches to both weight management and cancer prevention.

Gianmarco Contino, University of Birmingham

This article is republished from The Conversation under a Creative Commons licence. Read the original article.