(CNN) Cancers fueled by obesity are on the rise among young adults in the United States and appearing at increasingly younger ages, according to an analysis released Monday by the American Cancer Society.

The study, published in The Lancet Public Health, examined data on 12 obesity-related cancers between 1995 and 2014, as well as 18 common cancers not associated with weight. They found a disturbing trend among adults age 24 to 49.

"The risk of cancer is increasing in young adults for half of the obesity-related cancers, with the increase steeper in progressively younger ages," said co-author Ahmedin Jemal, who is the vice president of the Surveillance and Health Services Research Program for the American Cancer Society.

The risk, he said, was increasing in a stepwise manner in successively younger people.

"The findings from this study are a warning for increased burden of obesity-related cancer in older adults in the future," said Jemal, "potentially halting or reversing the progress achieved in reducing cancer mortality over the past several decades."

Cancers typically seen in the elderly

The six obesity-related cancers that showed startling increases among younger adults were colorectal, endometrial, gallbladder, kidney, pancreatic and multiple myeloma, a cancer of the bone marrow.

Most of these cancers have traditionally shown up in patients later in life, usually in their 60s and 70s.

Yet the study found some of the most significant increases were seen in the millennial age bracket, at a time when "overall cancer incidence is decreasing in males and stabilizing in females in the US," Jemal said.

Take pancreatic cancer for example, typically diagnosed in people over age 65. The analysis found the average annual increase for pancreatic cancer was 4.34% for ages 25 to 29, 2.47% in people aged 30 to 34, 1.31% for those in the 35 to 39 age bracket, and only 0.72% in those aged 40 to 44 years.

Overall, the risk of colorectal, endometrial, pancreatic and gallbladder cancers in millennials was about double the rate baby boomers had at the same age, the study found.

In contrast, rates in successive younger age brackets declined or stabilized in all but two of 18 non-obesity related cancers, including smoking-related and infection-related cancers. The two cancers not associated with obesity that rose in the younger age groups were gastrointestinal cancer and leukemia, a blood cancer.

"This study shows the incidence of cancer associated with obesity has been rising dramatically in groups of individuals born in more recent decades," said MD Anderson Cancer Center's Dr. George Chang, who was not associated with the analysis.

However, Chang warns against overgeneralizing on the basis of an epidemiological study.

"The study was not set up to establish causation," Chang said. "We know there are many factors that are associated with both obesity and cancer, such as lack of exercise and poor diet. How much each of those factors contribute to cancer is less clear."

Obesity is a global epidemic

Globally, obesity has reached "epidemic proportions," says the World Health Organization , which estimates that more than 1 billion adults are overweight, with at least 300 million of them considered clinically obese.

Millennials are on their way to being one of the heaviest generations on record. Research in the UK shows at least seven in 10 people born between the early 1980s and mid-1990s will likely be overweight or obese by their mid-30s and 40s. Only five in 10 baby boomers were obese at that same age.

In the United States, according to the National Health and Nutrition Examination Survey, more than 40% of Americans are obese, as are one in every six children ages 2 to 19.

study in the New England Journal of Medicine last year found almost 60% of the nation's children and teens will be obese by the age of 35 if the trend continues, with around half of the projected weight gain occurring during childhood.

That upward trajectory has experts concerned about associated medical conditions such as heart disease, diabetes and up to 13 types of cancer.

"Yet, I think the public in general doesn't even know that obesity is associated with cancer," said Case Western Reserve University oncologist Dr. Nathan Berger, who was not associated with the American Cancer Society study.

The link between fat and cancer

Fat cells, known as adiopose cells, do more than store excess calories in the body. They also release hormone-like fatty acids and proteins that affect metabolism, body weight and reproductive functions.

Science is actively trying to discover just how those hormones might contribute to certain cancers.

According to Berger, who runs a lab focused on obesity and cancer, one likely theory is that some of those hormones match receptors in certain cancers but not others, thus stimulating tumor growth in cancers that are a good fit.

Regardless of how it happens, science knows the association is real.

"We know in animal models that obesity accelerates the onset of cancer," said Berger. "And we know in people that obesity is associated with an increase in cancer and a worse prognosis for patients who have cancer. That's well established."

Chang, who is an oncology surgeon, says one of the reasons for a poor prognosis is the effect of excess weight on surgical outcomes. He points out that five of the six cancers on the rise in younger adults -- colorectal, uterine, gallbladder, kidney and pancreatic cancer -- are treated surgically.

"A patient who is obese will be at higher risk for complications and often times more severe complications from surgery than someone at an optimal weight," Chang said.

A wake-up call on obesity

What this research shows, Berger said, is an association between cancer and obesity in younger and younger ages. That, he said, should be a wake-up call for doctors, policy makers and the public, especially when it comes to childhood obesity.

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"We need to make the public aware that there's no time that it's OK to be obese," Berger said. "What that means is you can't say, 'Oh, what a cute, chubby little kid, but don't worry about it. They'll grow out of it.' Because what's happening is the little kid's obesity may already be affecting the carcinogenic process."

Chang agreed that urgent action is needed.

"Obesity during childhood actually is a major predictor of adult obesity," Chang said, "I think it highlights the importance of reducing the rates of obesity through better diet, better and more exercise. Those things should be the number one priority of any public health policy."