This webpage presents information, evidence and judgements on exposures that increase or decrease the risk of weight gain, overweight and obesity.
Growing, global problem: obesity and cancer
Overweight and obesity, characterised by excess body fat, are widely considered to be one of the most pressing public health concerns of this century.
Over the past five decades, the global prevalence of people living with overweight and obesity has increased dramatically. At current estimates, 1.97 billion adults are living with overweight or obesity, with numbers projected to rise. Although the rate of increase has begun to slow in some high-income countries, the prevalence of obesity has tended to accelerate in low- and middle-income countries. These accelerations have occurred in tandem with considerable changes in food systems and dietary patterns, commonly termed the ‘nutrition transition’. Overweight and obesity is occurring at an ever earlier age, increasing lifetime exposure to the associated risks.
Increased risk of 12 cancers
Diet, Nutrition, Physical Activity and Cancer: a Global Perspective (the Third Expert Report), from World Cancer Research Fund International and the American Institute of Cancer Research, identifies 12 cancers causally linked to greater body fatness:
See also Body fatness and weight gain.
Three additional cancer sites were reviewed by the World Health Organization’s International Agency for Research on Cancer, which concluded that greater body fatness is a cause of thyroid cancer, multiple myeloma, and meningioma. In addition, being overweight or obese is associated with other comorbidities, including a higher risk of type 2 diabetes, high blood pressure, heart disease and stroke.
The increasing prevalence of overweight and obesity has global and national economic implications. These can be direct, through costs to social and healthcare systems, or indirect through increased absences from work or people living with obesity being unable to work. The costs of obesity are well characterised in high-income countries. However, this has been difficult to assess globally due to a lack of data from lower-income countries.
Balancing energy intake and expenditure
Maintenance of stable body weight in adulthood depends on the close matching of energy intake (through food and drink) and energy expenditure (through the body’s basic functions and physical activity) over the long term, called energy balance. Under normal circumstances energy balance is achieved through interaction between the body’s regulatory systems, including appetite, together with an important role for learning, memory and physical activity. These interactions can be influenced by a variety of factors, both internal (for example, genetic variation) and external (for example, changes in the composition of food and drink and the social circumstances in which they are consumed).
In addition to the findings in the Third Expert Report related to diet, nutrition and physical activity, other established influences on energy balance and body weight include:
Identical twin studies have identified many genetic variants that contribute to weight gain, principally by influencing appetite. However, mutations and chromosomal rearrangements known to cause obesity, such as congenital leptin deficiency, Prader-Willi Syndrome and Bardet-Biedl syndrome, are rare.
- epigenetics and maternal programming
The womb environment is an important determinant of fetal phenotype and disease risk in later life. Factors such as nutrition or infection influence the pattern of fetal gene expression and risk of weight gain, overweight and obesity. Infants of mothers who are obese tend to have greater fetal size and increased fat mass – both risk factors for obesity.
- gut microbiota
There is early but growing evidence that the bacteria residing in the colon – the microbiome – may be involved with the development of overweight and obesity, although the mechanisms are not fully established.
- psychosocial factors
Psychosocial factors that can influence body weight, including risk of overweight or obesity, include stress, discrimination, depressive mood and emotional eating disorders.
- environmental and policy factors
Overweight and obesity are complex issues, influenced by many factors outside of people’s direct personal control. Broadly, these are economic, social and environmental factors that operate at global, national and local levels. At a personal level these are experienced as the availability, affordability, awareness and acceptability of healthy diets and physical activity, relative to unhealthy diets and physical inactivity.
How the research was conducted
Due to the large number of studies covering a wide range of exposures, and because there are published reviews addressing relevant research questions, a pragmatic approach was taken based primarily on a ‘review of published reviews’.
Cancer Prevention Recommendations
Our Recommendations include maintaining a healthy weight, being physically active, eating a healthy diet, and limiting or avoiding alcohol consumption.
This collated and analysed the global scientific research on diet, nutrition, physical activity and risk of weight gain, overweight and obesity. The results were independently assessed by a panel of leading international scientists in order to draw conclusions about which of these factors increase or decrease the risk of gaining weight or becoming overweight or obese.
Where available, quantification of exposures in relation to outcomes has been reported as in the published reviews. However, due to the methods used (a ‘review of published reviews’), reliable summary estimates of quantified thresholds were not able to be calculated in this report.
Integration of the evidence
Each ‘singular’ exposure has been judged to show a relationship with weight maintenance or weight gain, based on either strong or limited evidence. However, our Expert Panel has greater confidence that any effects on energy balance can be ascribed to clusters of the individual exposures (including both strong and limited evidence). In part this is because such singular exposures often cluster together with other exposures that may have a similar effect, for example people who are physically active tend to have healthier lifestyles in other respects.
Increased aerobic physical activity, including walking, alongside consumption of foods containing dietary fibre, particularly wholegrains, fruit and vegetables, and higher adherence to a ‘Mediterranean type’ dietary pattern is more likely to decrease the risk of weight gain, overweight and obesity than modifying any given single exposure.
Conversely, increased sedentary time, including screen time, in combination with a ‘Western type’ diet, and consumption of sugar sweetened drinks, ‘fast foods’, and refined grains is more likely to increase the risk of weight gain, overweight and obesity than any exposure in isolation.
Our major findings on this exposure
There is strong evidence that:
- aerobic physical activity DECREASES the risk of weight gain, overweight and obesity. Walking protects against weight gain, overweight and obesity
- consuming foods containing dietary fibre DECREASES the risk of weight gain, overweight and obesity
- consuming a ‘Mediterranean type’ dietary pattern DECREASES the risk of weight gain, overweight and obesity
- having been breastfed DECREASES the risk of excess weight gain, overweight and obesity for children
- greater screen time INCREASES the risk of weight gain, overweight and obesity
- consuming sugar sweetened drinks INCREASES the risk of weight gain, overweight and obesity
- consuming ‘fast foods’ INCREASES the risk of weight gain, overweight and obesity
- consuming a ‘Western type’ diet INCREASES the risk of weight gain, overweight and obesity
There is limited evidence that:
- consuming wholegrains might DECREASE the risk of weight gain, overweight and obesity
- consuming fruit and vegetables might DECREASE the risk of weight gain, overweight and obesity
- breastfeeding (lactation) might DECREASE the risk of weight gain, overweight and obesity for the mother
- sedentary behaviours might INCREASE the risk of weight gain, overweight and obesity
- consuming refined grains might INCREASE the risk of weight gain, overweight and obesity
This webpage is a summary.
For much more, download the full chapter.