INTRODUCTION

In 2008, there were an estimated 12.7 million cancer cases and 7.6 million cancer deaths worldwide, despite overwhelming evidence that many malignancies are preventable [1,2]. Survival rates are improving, but over half a million people die from cancer each year in the United States (US) alone. Cancer outranks cardiovascular disease as the number one cause of death in the US for those under the age of 85 [3].

Both screening and prevention can reduce mortality from cancer. Screening detects abnormalities before they are clinically apparent, allowing for intervention either before cancer develops or at an early stage, when treatment is most often effective. Prevention strategies focus on modifying environmental and lifestyle risk factors that promote cancer. It is estimated that 50 percent of cancer is preventable [4].

Multiple cancer risk factors have been identified; tobacco use, excess weight, poor diet, and inactivity account for two-thirds of all cancers in the US [5]. In one study, nine modifiable risks were identified as the cause of 35 percent of cancer deaths worldwide: smoking, alcohol use, diet low in fruit and vegetables, excess weight, inactivity, unsafe sex, urban air pollution, use of solid fuels, and contaminated injections in healthcare settings [6]. The International Agency for Research on Cancer (IARC) has identified and tabulated over 100 human carcinogens [7].

Lifestyle factors have been linked to a variety of malignancies, including the most common in the developed world: lung, colorectal, prostate, and breast cancer [8]. In a longitudinal study, participants who had all four lifestyle factors (never smoking, BMI <30, physical activity >3.5 hours weekly, prudent diet) had approximately one-third the risk of cancer compared to those who had none of these factors [9]. A comprehensive systematic review with a global focus conducted by the World Cancer Research Fund came to similar conclusions regarding dietary, weight, and activity factors [10]. A study of an index, based on recommendations from the World Cancer Research Fund and the American Institute of Cancer Research related to weight management, physical activity, plant and animal food consumption, breastfeeding in women, and alcohol intake, found a significant association of higher scores (better compliance) with risk reduction for total cancer and multiple specific cancers (colorectal, stomach, breast, endometrium, lung, kidney, liver, esophagus) but not for prostate, ovarian, pancreatic, or bladder cancer [11]. Despite a robust knowledge of what factors decrease cancer risk, implementation of cancer prevention lags [12].

This topic reviews the major modifiable cancer risk factors and briefly addresses chemoprevention. Lifestyle issues which are associated with an increased risk of cancer are also risk factors for other diseases, such as stroke, heart disease, and diabetes. (See "Overview of primary prevention of coronary heart disease and stroke" and "Prediction and prevention of type 2 diabetes mellitus".)