The longer someone remains overweight, the greater the danger.
Colorectal cancer was most common among those with significant BMI changes; 2,803 cases diagnosed during 21-year study period. Obesity causes chronic inflammation, gut dysbiosis, and metabolic changes that increase cancer susceptibility; ultra-processed foods drive rising obesity rates.
- 135,000 people tracked for 21 years; 2,803 colorectal cancer cases diagnosed
- Each BMI unit increase raises colorectal and gastrointestinal cancer risk by 2-4%
- Brazil: 60% of adults overweight, 26% obese; projected 41% obesity by 2035
- 46,000 new colorectal cancer cases annually in Brazil; 10% increase in recent years
A 21-year study of 135,000 people found that obesity and BMI increases significantly raise cancer risk, particularly colorectal and gastrointestinal cancers, with each BMI unit increase raising risk by 2-4%.
A two-decade study tracking more than 135,000 people has confirmed what researchers have long suspected: the relationship between weight and cancer risk is not simply about being overweight at any given moment. It is about how long you carry that weight, and how much your weight fluctuates over time. The findings, published in JAMA Network and drawn from data collected by the U.S. National Cancer Institute's Prostate, Lung, Colorectal and Ovarian Cancer Screening Trial, suggest that sustained obesity—and even the act of gaining weight across adulthood—substantially increases the likelihood of developing cancer, particularly in the gastrointestinal tract.
Over the 21-year observation period, 2,803 participants developed colorectal cancer, while another 2,285 were diagnosed with other gastrointestinal malignancies including cancers of the esophagus, liver, and pancreas. The data revealed a dose-response relationship: for every single-unit increase in Body Mass Index over a lifetime, the risk of colorectal cancer and other gastrointestinal cancers rose between 2 and 4 percent. This matters because it reframes obesity not as a static condition but as a cumulative exposure—the longer someone remains overweight, the greater the danger.
The implications are particularly urgent in Brazil, where the landscape of weight and health has shifted dramatically in recent decades. Current estimates show that 60 percent of Brazilian adults are overweight and 26 percent are obese, more than double the obesity rate from 2003. Among children, roughly 30 percent are already classified as overweight or obese, a troubling indicator of what lies ahead. If current trends persist, the country faces a projection of 41 percent of adults living with obesity by 2035. Against this backdrop, Brazil's National Cancer Institute estimates that nearly 46,000 new cases of colorectal cancer will be diagnosed annually in the coming years—a jump of more than 10 percent compared to projections from just three years earlier.
The mechanisms linking obesity to cancer remain incompletely understood, but researchers have identified several pathways. Chronic inflammation appears to play a central role; the obese body exists in a state of persistent inflammatory activation that may promote tumor growth. Changes to the gut microbiome, metabolic dysfunction, and immune system alterations all accompany obesity and may contribute to malignant transformation. Dr. Ana Paula Cardoso, an oncologist at Hospital Israelita Albert Einstein, emphasizes that much of this risk is driven by dietary patterns. Brazilians, like populations worldwide, have increasingly turned to ultra-processed foods—products high in sugar and calories, cheap and convenient, but nutritionally impoverished. These foods are not merely empty; they actively drive weight gain and the cascade of biological changes that follow.
One unexpected finding emerged from the study: regular use of aspirin, long known to have some protective effect against colorectal cancer, did not reduce the cancer risk associated with obesity. This suggests that obesity may fundamentally alter how the body responds to the drug's anti-inflammatory properties, potentially nullifying its preventive benefit. Cardoso notes that while aspirin's role in cancer prevention remains scientifically contested, the new evidence hints that obese individuals might require different dosing strategies or that obesity itself creates a biological environment resistant to the medication's protective mechanisms.
What distinguishes this research from earlier work is its emphasis on duration and trajectory. Obesity is not merely a risk factor; it is a risk factor that compounds with time. Someone who becomes obese in middle age and remains so faces greater danger than someone who was never obese. Someone whose weight climbs steadily across decades faces even greater danger still. This temporal dimension carries a message both sobering and, potentially, actionable: the window to intervene exists, but it narrows with each passing year of excess weight.
Cardoso and other public health experts argue that prevention must become a priority equal to treatment. Oncology centers are already strained; the resources required to treat the cancer epidemic that obesity is generating will be insufficient. The solution, they contend, lies not in better cancer drugs but in policies that make healthy eating accessible and affordable, that reduce the marketing and availability of ultra-processed foods, and that create environments where weight gain is not the path of least resistance. Obesity is also a risk factor for hypertension, diabetes, and numerous other chronic diseases. Every pound of prevention, in other words, prevents multiple diseases at once. The question now is whether Brazil and other nations will act on that knowledge before the projections become reality.
Citas Notables
The maintenance of obesity over time, along with weight changes and shifts in BMI categories, increases risk proportionally to the duration of exposure.— Dr. Ana Paula Cardoso, oncologist at Hospital Israelita Albert Einstein
Obesity is a known risk factor for many chronic diseases beyond cancer, including hypertension and diabetes, with significant impact on population health and healthcare costs.— Dr. Ana Paula Cardoso
La Conversación del Hearth Otra perspectiva de la historia
Why does the study focus so much on BMI changes over time rather than just current weight?
Because the body remembers. Twenty years of obesity is not the same as five years. The inflammation, the metabolic damage, the changes to your gut bacteria—these accumulate. A person who gained weight gradually and stayed heavy has been exposed to that risk for longer than someone who became obese recently.
But couldn't someone argue that what matters is just being obese now, regardless of history?
That's what earlier studies suggested. This one shows the picture is more complex. The risk scales with exposure time. It's like smoking—one year of smoking is bad, but twenty years is worse.
The aspirin finding is strange. Why would obesity block a drug's protective effect?
Aspirin works partly by reducing inflammation. But obesity itself is an inflammatory state. The theory is that obesity creates so much inflammation that aspirin can't overcome it, or the body's response to the drug is fundamentally altered. It's not that aspirin stops working—it's that obesity changes the terrain the drug is working in.
Is there any good news in this research?
Yes. The fact that risk increases gradually with BMI means there's a dose-response relationship. That means intervention at any point matters. Losing weight, stabilizing weight—these aren't futile gestures. They interrupt the accumulation of risk.
Why is Brazil specifically mentioned as facing 41 percent obesity by 2035?
Because it's already happening. Sixty percent of adults are overweight now. Children are following the same trajectory. The country is watching its future unfold in real time, and the question is whether it will act before that 41 percent becomes inevitable.