Losing weight is generally seen as a good thing, but unintentional or unexpected weight loss in someone not trying to lose weight may be a sign of serious illness, researchers have found.

In fact, the international team led by researchers at Harvard University and the Dana-Farber Cancer Institute discovered that unintentional weight loss is associated with an increased risk of a cancer diagnosis within the next year. “People who are losing weight without trying to by making changes in their exercise routine or diet should see their doctor to consider possible causes,” Brian Wolpin, lead investigator on the study, said in a statement.

People who lost more than 10 percent of their body weight over two years were significantly more likely than those who did not to be diagnosed with cancer.

Many conditions can cause unintentional weight loss, Wolpin, director of the gastrointestinal cancer center at Dana-Farber, explained. A doctor can help determine if the issue needs further evaluation.

Data on diet and exercise from almost 158,000 people in the Nurses’ Health Study and the Health Professionals Follow-Up Study were included in the analysis. Women in the Nurses’ Health Study who were 40 years old or older were followed from June 1978 until June 30, 2016. Men in the Health Professionals Follow-Up Study who were 40 years old and older were followed from January 1988 until January 31, 2016.

People in the studies filled out a questionnaire every two years that included questions about their weight and level of physical activity. Every four years the questionnaire asked about dietary changes. These questions allowed Wolpin and his team to estimate how hard he or she had tried to lose weight.

Participants were divided into three groups: 1) Those who made dietary changes and exercised more were classified as having a high level of weight loss-promoting behavior; 2) those who changed either their diet or exercise habits had a medium level of behavior; and 3) those who changed neither their diet nor their exercise habits were characterized as having a low level of weight loss-promoting behavior.

People who lost more than 10 percent of their body weight over the last two years were significantly more likely than those who did not to be diagnosed with upper gastrointestinal tract, colorectal, blood and lung cancers within 12 months of their reported weight loss. Those who lost weight without changing their diet and exercise habits were more likely to subsequently be diagnosed with cancer.

“Unintentional weight loss could be a sign of a developing cancer. [It] could help diagnose the cancer earlier when there’s a chance for more effective treatment.”

“We wanted to distinguish healthy weight loss from unhealthy weight loss,” said Qaoli Wang, first author on the study and a research fellow at Dana-Farber.

Those diagnosed with early-stage cancers lost a similar amount of weight compared to those with later stage cancers. This finding suggests that weight loss prior to cancer diagnosis is not associated with the stage of disease at diagnosis. “Unintentional weight loss could be a sign of a developing cancer that could help diagnose the cancer earlier when there’s a chance for more effective treatment,” the researchers said, in a statement.

Weight in the current study was self-reported over decades, not measured objectively in a physician’s office. The study also considered all types of cancer, and the two populations included in the analysis were health professionals, and not representative of the U.S. population.

The study is published in JAMA, The Journal of the American Medical Association.