More than 135,500 cases of cancer a year in the UK could be prevented through lifestyle changes, according to new figures from a Cancer Research UK landmark study published today.
This equates to 37.7% of all cancers diagnosed each year in the UK - rising to 41.5% in Scotland.
The latest figures, calculated from 2015 cancer data, found that smoking remains the biggest preventable cause of cancer despite the continued decline in smoking rates.
Tobacco smoke caused around 32,200 cases of cancer in men (17.7% of all male cancer cases) and around 22,000 (12.4%) in women in 2015, according to the research published in the British Journal of Cancer.
Excess weight is the second biggest preventable cause of cancer. Around 22,800 (6.3%) cases of cancer a year are down to being overweight or obese. This amounts to around 13,200 (7.5%) cases of cancer in women and around 9,600 (5.2%) in men.
Obesity causes 13 types of cancer, including bowel, breast, womb and kidney. And the results suggest that more than 1 in 20 cancer cases could be prevented by maintaining a healthy weight.
The third biggest preventable cause of cancer is overexposure to UV radiation from the sun and sunbeds, which causes around 13,600 cases of melanoma skin cancer a year (3.8% of all cancer cases).
Other preventable causes of cancer include drinking alcohol, eating too little fibre (causing around 11,900 and around 11,700 cases respectively, which is 3.3% each) and outdoor air pollution. While air pollution is to blame for around 3,600 lung cancer cases a year (1% of all cancer cases), it still causes far fewer cases of lung cancer than tobacco.
Sir Harpal Kumar, Cancer Research UK's chief executive, said: "Leading a healthy life doesn't guarantee that a person won't get cancer, but it can stack the odds in your favour. These figures show that we each can take positive steps to help reduce our individual risk of the disease. "This research clearly demonstrates the impact of smoking and obesity on cancer risk. Prevention is the most cost-effective way of beating cancer and the UK Government could do much more to help people by making a healthy choice the easy choice."
Professor Linda Bauld, Cancer Research UK's prevention expert, said: "These new figures show that the battle to conquer smoking-related cancer is far from over. But the declining numbers of smokers show that prevention strategies are working.
"Obesity is a huge health threat right now, and it will only get worse if nothing is done. The UK Government must build on the successes of smoking prevention to reduce the number of weight-related cancers. Banning junk food TV adverts before the 9pm watershed is an important part of the comprehensive approach needed."
Janet Boak, 55, a grandmother of three from Carlisle, was diagnosed with womb cancer at 51 after she noticed spots of blood four years after her menopause. During surgery to remove two fibroids from her womb, cancer was found. Janet then had a full hysterectomy to remove her womb and cervix. Because the cancer was caught in the earliest stages and hadn't spread, she avoided any further treatment.
Volunteering to take part in some womb cancer research after her treatment, Janet was shocked to find out that the possible contributing factors for womb cancer included being obese and being inactive.
Janet said: "That was me. At almost 20st and wearing up to a size 30, I was huge and most of my weight was around my stomach. I couldn't remember the last time I did any exercise.
"Of course I'd known I needed to lose weight but I hadn't realised just how much I was putting my life at risk." Janet joined a local slimming group in 2015, changed her diet and started taking exercise.
She said: "The weight gradually fell off and now, over two years on, I've lost more than 6st and wear a size 16. I could barely keep up with my three young grandchildren before but now I'm always running around after them. I even ran Race for Life for Cancer Research UK last year - something I never imagined I would do.
"Getting cancer was terrifying but it was the wake-up call I needed. I don't know what the future holds but at least I know I'm doing all I can to stay healthy and happy."