The UK is heading for a 50% increase in the number of new colon cancer cases over the next 30 years, says an international team of scientists.
The forecast, in the European Journal of Cancer, is for 35,000 new cases a year by 2040, compared with 23,000 now.
Rising obesity is one reason - and if the UK reached US levels that could add another 2,000 to the total, they say.
The study used cancer data from seven countries to predict how cancer rates might change with an ageing population.
Each year there are 38,000 cases of bowel cancer, which can split into those in the colon and those lower down in the rectum. This latest study looked just at those in the colon itself.
Two of the biggest risk factors for colon cancer are physical inactivity and being overweight or obese.
Dr Andrew Renehan, from the University of Manchester, and one of the authors of the research, said that the computer models allowed researchers to predict what would happen to cancer rates in a variety of scenarios.
If UK trends in obesity and activity stayed as they are now, the predicted figure of approximately 35,000 cases a year is reached by 2040.
If obesity gets worse, following trends set in the US, where the problem has spiralled in recent years, then the annual figure is close to 37,000.
But if that does not happen, and there is a modest increase in activity, then that rise turns into a similar-sized fall.
And if the UK managed to match levels of physical exercise in The Netherlands - the best among the countries studied, that would prevent approximately 2,000 cases.
Dr Renehan said: "The predictive modelling is beginning to tease out the independent relevance of each of these factors in the prevention of colon cancer.
"We know that large numbers of colon cancer cases could be avoided by reducing exposure to risk factors."
Another researcher, Professor Jan-Willem Coebergh, from Erasmus University in The Netherlands, said: "We can safely say increasingly physical activity across Europe to the level already achieved in The Netherlands, where everybody cycles, would be of substantial benefit."
And a second study published at the same time appeared to show a small additional risk not just for obese people, but anyone carrying excess fat around their middle - even if their overall weight appears near-normal.
In the group studied, every additional inch of waistline from the slimmest to the biggest-waisted meant an extra 2% risk of bowel cancer, even when the BMI of the person was accounted for in the calculations.
The research, carried out by Imperial College London for the World Cancer Research Fund, combined the results of previous studies into the issue to produce more reliable evidence of the link.