London School of Hygiene & Tropical Medicine

A higher body mass index (BMI) increases the risk of developing 10 of the most common cancers*, the largest study of its kind on BMI and cancer, involving more than 5 million adults in the UK, published in The Lancet shows.

Researchers at the London School of Hygiene &Tropical Medicine working with the Farr Institute of Health Informatics estimate that over 12,000 cases of these 10 cancers each year are attributable to being overweight or obese, and calculate that if average BMI in the population continues to increase, there could be over 3500 extra cancers every year as a result.

Lead author Dr Krishnan Bhaskaran, National Institute for Health Research Postdoctoral Fellow at the London School of Hygiene & Tropical Medicine, said: "The number of people who are overweight or obese is rapidly increasing both in the UK and worldwide. It is well recognised that this is likely to cause more diabetes and cardiovascular disease. Our results show that if these trends continue, we can also expect to see substantially more cancers as a result."

Using data from general practitioner records in the UK's Clinical Practice Research Datalink (CPRD), the researchers identified 5.24 million individuals aged 16 and older who were cancer-free and had been followed for an average of 7.5 years. The risk of developing 22 of the most common cancers, which represent 90% of the cancers diagnosed in the UK, was measured according to BMI** after adjusting for individual factors such as age, sex, smoking status, and socioeconomic status.

A total of 166,955 people developed one of the 22 cancers studied over the follow-up period. BMI was associated with 17 out of the 22 specific types of cancer examined.

Each 5 kg/m² increase in BMI was clearly linked with higher risk of cancers of the uterus (62% increase), gallbladder (31%), kidney (25%), cervix (10%), thyroid (9%), and leukaemia (9%). Higher BMI also increased the overall risk of liver (19% increase), colon (10%), ovarian (9%), and breast cancers (5%), but the effects on these cancers varied by underlying BMI and by individual-level factors such as sex and menopausal status. Even within normal BMI ranges, higher BMI was associated with increased risk of some cancers.

There was some evidence that those with high BMI were at a slightly reduced risk of prostate cancer and premenopausal breast cancer.

Dr Bhaskaran explained: "There was a lot of variation in the effects of BMI on different cancers. For example, risk of cancer of the uterus increased substantially at higher body mass index; for other cancers, we saw more modest increases in risk, or no effect at all. For some cancers like breast cancer occurring in younger women before the menopause, there even seemed to be a lower risk at higher BMI. This variation tells us that BMI must affect cancer risk through a number of different processes, depending on the cancer type."

Based on the results, the researchers estimate that excess weight could account for 41% of uterine and 10% or more of gallbladder, kidney, liver, and colon cancers in the UK. They also estimate that a population-wide 1 kg/m² increase in average BMI (roughly an extra 3 to 4 kg, or 8 to 10 pounds, per adult), which would occur every 12 years or so based on recent trends, would result in an additional 3,790 cases of these 10 cancers in the UK each year.

The research was funded by the National Institute for Health Research, Wellcome Trust, and Medical Research Council.

* Uterus, gallbladder, kidney, cervix, thyroid, leukaemia, liver, colon, ovarian, and postmenopausal breast cancers.
** BMI is calculated by dividing a person's weight in kilogrammes by their height in metres squared. A BMI of between 18·5 and 24·9 kg/m² is considered ideal; overweight is defined as a BMI of between 25 and 29·9 kg/m²; and obese as a BMI of 30 kg/m² or higher.

Podcast

Krishnan discusses his research and findings

Publication

• Krishnan Bhaskaran, Ian Douglas, Harriet Forbes, Isabel dos-Santos-Silva, David A Leon, Liam Smeeth. Body-mass index and risk of 22 specific cancers: a population-based cohort study of 5·24 million UK adults. The Lancet. DOI: 10.1016/S0140-6736(14)60892-8