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Unexplained weight loss can be one of the first signs of cancer, new study finds. Here's why.

Doctors say that unintentional weight loss of more than 5% of body weight within six to 12 months is concerning.
Doctors say that unintentional weight loss of more than 5% of body weight within six to 12 months is concerning. (Getty Images)

Losing weight without trying can be an early sign of cancer, according to a new study. The research raises questions about significant weight loss without changes to diet, exercise or lifestyle, as well as when people should seek medical care. Experts stress that unexplained weight loss doesn't always signal that something is off with your health — but it can be an indicator. Here's what you need to know about the latest findings, as well as when to speak to your doctor about your weight loss.

What the study says

The study, which was published in JAMA, found that recent weight loss was linked with a significantly higher rate of being diagnosed with cancer over the next 12 months, compared with people without recent weight loss.

What are the key findings

For the study, researchers from several health organizations and cancer centers analyzed data from nearly 160,000 people in the Nurses’ Health Study, a longitudinal study that looks for risk factors for major chronic diseases. The researchers found that people who lost more than 10% of their body weight were diagnosed with cancer within a year at a rate of 1,362 per 100,000 person-years compared to 860 per 100,000 person-years who didn't recently lose weight. (Person-years combines the number of years someone is in a study with the length of the study.)

People with cancer of the upper gastrointestinal tract — cancer of the esophagus, stomach, liver, biliary tracts or pancreas — were the most likely to have significant weight loss before their diagnosis. The researchers also found that people with breast and brain cancer and melanoma had no significant weight loss beforehand.

Overall, the risk of being diagnosed with cancer after significant unexplained weight loss was low — it was 3.2%. But compared with the 1.3% of people diagnosed with cancer who didn't have significant weight loss, it was notable.

What experts think

Experts say that the findings back up what they see in the field. "This is a well-known phenomenon with cancer," Dr. Wael Harb, a hematologist and medical oncologist at MemorialCare Cancer Institute at Orange Coast and Saddleback Medical Centers in Orange County, Calif., tells Yahoo Life. "We always have to think about cancer as a potential cause when we see these patients."

In fact, in 2020, Cancer.net found that 40% of people reported unexplained weight loss when they were first diagnosed with cancer. "It has been known for centuries that weight loss is one sign of a possible future cancer diagnosis," Dr. Steven Clinton, an oncologist with the Ohio State University Comprehensive Cancer Center — James Cancer Hospital and Solove Research Institute, tells Yahoo Life. In fact, he says unexplained weight loss was seen a lot more in the past in cancer patients before diagnostic tools helped find cancer earlier.

There are a few reasons someone might have weight loss with cancer. "In general, individuals with cancer may experience a variety of symptoms, including pain, weakness, fatigue — all of which may lead to a decreased interest in eating," Jaylin Kellogg, a nutrition support dietitian at Fox Chase Cancer Center, tells Yahoo Life. Some patients might also have nausea, trouble chewing or swallowing, or have a hard time absorbing nutrients, which can lead to a reduced appetite and weight loss, she says. "Some cancers even cause the body to burn more calories at rest than it usually would, which can contribute to unintentional weight loss," Kellogg says.

Why it matters

The study reiterates the importance of talking to your doctor if you have unexplained weight loss, Dr. Pamela Hodul, a gastrointestinal surgeon at Moffitt Cancer Center, tells Yahoo Life. "Unintentional weight loss of more than 5% of your body weight within six to 12 months is concerning and should prompt a visit to your doctor," she says. But Hodul cautions weight loss doesn't automatically mean you have cancer. "There are several metabolic causes for weight loss in individuals, and cancer is only one potential explanation," she says.

It's also important to be mindful of this if you're on a weight loss medication, Harb says. He recommends getting a full physical before going on weight loss medication to ensure you're healthy before you start. Hodul also suggests being on the lookout for sharp drops in weight. "Safe weight loss has a predictable course, while cancer-related weight loss may present with an increase in loss of weight beyond that which is expected," she says.

Harb stresses that there are many other health conditions that can cause significant unexplained weight loss, including thyroid and endocrine problems. "You shouldn't jump to conclusions if you experience this," he says. "But you need to have a thorough physical to see if there are other symptoms or signs that can point out why this is happening."