According to the International Agency for Research on Cancer (IARC), there is a high level of evidence to support that overweight and obesity are established risk factors for ten cancer sites, including colorectal cancer. In France, around 4,500 cases of colorectal cancer per year are attributable to excess weight. Understanding the mechanisms at play helps to understand why this factor weighs so much, and why it remains largely modifiable.
1. A bond established independently of food
It is important to distinguish overweight and diet: even taking into account eating habits, excess body fat and a sedentary lifestyle constitute independent risk factors. IARC considers that there is strong evidence that a sedentary lifestyle alone is associated with an increased risk of colorectal cancer, and that physical activity reduces this risk, independent of body weight.
In the United States, it is estimated that 35% of cases of colorectal cancer in men and 21% in women are attributable to obesity, proportions which illustrate the extent of the phenomenon on a population scale.
| Mechanism | What's happening | Consequence |
|---|---|---|
| Insulin resistance | Excess fat reduces insulin sensitivity | The pancreas produces more insulin, which stimulates cell proliferation |
| IGF-1 | Chronic hyperinsulinemia increases this factor growth | Promotes the multiplication of cells, including abnormal cells |
| Chronic inflammation | Adipose tissue releases pro-inflammatory molecules | Creates a breeding ground for tumor development |
| Imbalance hormonal | Change in estrogen and adipokine levels | Disrupts normal regulation of cell growth |
2. The biological mechanisms at play
Several metabolic pathways explain how excess adipose tissue promotes the development of cancer cells in the colon:These mechanisms are not specific to colorectal cancer: they are also involved in other cancers linked to excess weight, which explains why this risk factor so often comes up in the scientific literature on prevention.
3. The paradox of the sedentary athlete
One point is worth emphasizing: playing sports is not always enough to compensate for long hours spent sitting. Large-scale studies have highlighted what researchers call the phenomenon of the "sedentary athlete", a person who meets weekly recommendations for physical activity, but sits for ten hours or more the rest of the day, presents a degraded metabolic profile compared to someone who moves evenly throughout the day.
4. Concrete and cumulative action levers
The good news: unlike age or genetics, weight and level of physical activity are levers on which everyone can act, gradually:
- ●Reduce continuous sitting time: getting up every hour, even for a few minutes, improves measurable metabolic markers.
- ●Aim for 150 minutes of moderate activity per week: WHO recommendation, associated with a significant reduction in risk, without it being necessary to aim for intensive practice.
- ●Prioritize active travel: walking, cycling, stairs, these everyday actions add up and really count in the overall activity report.
- ●Get support if you are overweight: appropriate medical or dietary monitoring is more effective and more sustainable than an isolated approach.
⚠️ To remember from a medical point of view
The risk associated with being overweight is cumulative and progressive: there is no sudden threshold, but a dose-effect relationship between the body mass index (BMI) and the relative risk.
A recent study suggests that obesity could promote the occurrence of colorectal cancers before the age of 50, a trend currently being monitored closely by epidemiologists.
Weight loss in an overweight person reduces the risk, but gradually: the benefits are established over several years, which should encourage regularity rather than the search for quick results.
