Education And Debate

The runaway weight gain train: too many accelerators, not enough brakes

BMJ 2004; 329 doi: (Published 23 September 2004) Cite this as: BMJ 2004;329:736
  1. Boyd Swinburn, professor of population health (swinburn{at},
  2. Garry Egger, adjunct professor2
  1. 1 School of Exercise and Nutrition Sciences, Deakin University, 221 Burwood Highway, Melbourne, Victoria 3125, Australia
  2. 2 Southern Cross University, Military Road, East Lismore, New South Wales 2480, Australia
  1. Correspondence to: B Swinburn

    Obesity seems to be perpetuated by a series of vicious cycles, which, in combination with increasingly obesogenic environments, accelerate weight gain and represent a major challenge for weight management


    The chronic positive energy balance that leads to obesity is apparently relatively small.1 It is therefore paradoxical that obesity is so persistent and difficult to treat, because, in Western countries at least, the basic causes of obesity are readily apparent to everyone (eating too much and exercising too little). Obesity is associated with a substantial loss of quality of life and with social stigmatisation; awareness of the health consequences of obesity has never been greater. Even the body's physiological systems try to prevent weight gain by minimising the impact of energy imbalance on weight change.2

    To help explain this apparent paradox, we propose that numerous vicious cycles are acting as “accelerators” that maintain and even increase overweight. We liken the situation to a “runaway weight gain train” (figure), which already has high momentum from the downhill slope of obesogenic (obesity promoting) environments but is getting faster as the vicious cycles start acting as accelerators. The brakes, which, on the face of it, should be strong enough to slow down the train, turn out to be weak by comparison.


    The runaway weight gain train

    The downhill slope of obesogenic environments

    The obesogenic environments of the modern world have been well described.3 4 They make the unhealthy choices the easy, default choices, resulting in a high intake of energy dense foods and beverages and in reduced physical activity. Examples include the heavy promotion of fast food outlets, energy dense snacks, and high sugar drinks to children; the low cost and large serving sizes of such foods; and the transport systems and urban design that inhibit active transport and active recreation.

    In our runaway train model (figure), the twin tracks of “energy in” and “energy out” are on the downhill slope of the obesogenic environments, indicating that the default direction is downhill towards positive energy balance. Levelling out the obesogenic slope is fundamental to slowing down the momentum of the obesity epidemic. However, in some areas, such as “retro fitting” walking, cycling and public transport into a car oriented built environment, this will require a massive engineering effort and is likely to take a very long time.5

    Ineffective brakes

    Several “brakes” are acting against increasing weight gain. Possibly the strongest in Western societies is the social discrimination of being overweight or obese.6 Unlike other conditions related to a combination of inappropriate diet, lack of exercise, and genetic background—for example, hypertension and dyslipidaemia—obesity is visible to others and carries the social stigmata of sloth and gluttony. Interestingly, the rise of support groups for big people, the sanctions against discrimination on the basis of size, and the rise of the average body size into the overweight range may reduce the pressure on this brake.

    A further brake is the personal physical discomfort associated with obesity, which reduces quality of life substantially.7 Knowledge about the fundamental causes of weight gain (eating too much and exercising too little) is common, but this cognitive brake is often ineffectual. Some authors argue for a greater application of cognitive efforts to control weight gain,5 but knowledge alone seems to be a weak predictor of human behaviour, and educational strategies, at least at the population level, seem to have little effect on preventing weight gain.8

    Even physiology attempts to slow weight gain. The metabolic profile of an obese person is one of physiological brakes being fully applied to prevent further weight gain. These include reductions in appetite, increased fat oxidation, increased energy expenditure, insulin resistance, increased activity of the sympathetic nervous system, increased leptin concentrations, and so on.2 9 10 In response to weight loss, these physiological mechanisms are reversed (and more vigorous—for obvious survival reasons),9 10 so their role seems to be to dampen the impact of energy imbalances on subsequent weight change.

    While all these brakes seem to be substantive, they are clearly not strong enough to halt the increasing momentum of the weight gain train. This argues for the existence of a powerful set of accelerating forces in addition to the downhill obesogenic slope.

    Vicious cycles as accelerators

    A vicious cycle is a positive feedback loop in which the problem (in this case, weight gain) becomes self perpetuating. In almost all physiological systems, cycles are based on negative feedback relations, so the interaction of multiple systems ensures a dynamic stability. Positive feedback cycles, on the other hand, are destabilising and potentially accelerating, because an initial increase in one factor leads to changes in the system that feed back and further exaggerate the initial increase. Several vicious cycles, outlined in the table and explained below, seem to be operating in obesity, creating a potent driving force for perpetuation and even acceleration.

    Vicious cycles that potentially contribute as accelerators of weight gain

    View this table:

    Movement inertia cycle

    A heavy body weight is a disincentive for movement and physical activity, creating “movement inertia.” Although obese people expend more energy in performing a given level of physical activity than lighter people, they tend to do fewer of those energy demanding activities.11 These reductions in levels of physical activity are most apparent in people who are substantially overweight.12 Reducing levels of physical activity obviously then promotes further weight gain. The potential “cycle breakers” (analogous to a clutch in the train model) could be the use of motivational techniques, weight supportive exercise (such as pool based activities), and focusing on reducing energy intake in the first instance.

    Mechanical dysfunction cycle

    A related vicious cycle is one due to the mechanical problems associated with increased body weight. These include arthritis, arthralgia, low back pain, chest wall and diaphragm restriction, incontinence, obstructive sleep apnoea, oedema, and cellulitis.1315 These may result in pain and difficulty walking and moving, shortness of breath (often diagnosed as asthma), exercise and stress incontinence, daytime somnolence, fatigue, and so on, which further limit physical activity and increase weight gain. Some specific treatments for the mechanical dysfunction itself to break the cycle may be available, such as continuous positive airways pressure for obstructive sleep apnoea and anti-inflammatory drugs and surgery for orthopaedic problems.

    Psychological dysfunction cycle

    For some obese individuals, body dissatisfaction, the physical discomfort of obesity, and social stigmatisation may trigger or exacerbate depression, anxiety, and feelings of low self esteem and guilt. Some of the consequences of these psychological problems may increase energy intake (such as binge or comfort eating, excess alcohol intake) or reduce physical activity (such as lethargy and lack of motivation).16 In addition, some medications that might be used to manage the psychological condition may promote weight gain (for example, psychotropic drugs). Appropriate management of the psychological condition and the medication may be needed to help break the effects of this vicious cycle.

    Dieting cycle

    New diets (and recycled old diets) are constantly coming in and going out of fashion. They often have specific “magic” weight loss ingredients or a set of restrictive rules with “free” and “forbidden'” foods. Some of the diets are very hypocaloric and result in marked weight loss, and most are unsustainable. The metabolic response to a large weight loss is vigorous, and the subsequent weight that is regained is disproportionately higher in fat.17 In addition, the sense of failure (personal failure and “diet” failure) may contribute to the depression and psychological problems described in the previous cycle. For some people with so called food cravings or addictions, self deprivation of particular favourite foods, such as chocolate, may lead to a heightened desire for those foods18 and contribute to the failure of the diet.

    Low socioeconomic status cycle

    In developed countries, socioeconomic status is strongly associated with obesity, especially among women, with a higher prevalence of obesity in groups with a lower socioeconomic status.3 This relation seems to be bidirectional. Obesity leads to reduced opportunities for jobs, education, marriage, and social inclusion,19 and these may be more potent in women than in men. A lower income reduces the range of healthy choices, such as fresh fruit and vegetables, and expensive active recreational pursuits. Neighbourhoods with a low socioeconomic status are usually more obesogenic than neighbourhoods with a high status.20 A low income is also associated with higher levels of chronic stress, which may lead to comfort eating, excessive consumption of alcohol, and even the chronic activation of the hypothalamic-pituitary-adrenal axis,21 resulting ultimately in increased obesity and hence perpetuating the cycle. Improving equity of access to healthy choices and reducing the obesogenicity of poor neighbourhoods are important strategies to break this cycle.


    The combination of the described vicious cycles acting as accelerator wheels, a set of ineffective brakes, and increasingly obesogenic environments represents a major challenge for weight management. We have identified several vicious cycles that may be operating in obesity and acknowledge that there may even be more. Defining these potential vicious cycles is vital for several reasons. Firstly, they help explain the apparent paradox of continuing (and often increasing) obesity in the face of brakes such as strong social stigmatisation and knowledge of causes and solutions. Secondly, they may contribute to the increasing right skewness of the frequency distribution of body mass index as the population gains weight over time. In other words, the lean people gain a little weight but the already overweight and obese gain a lot of weight, creating an exaggerated interaction between individual and environment. Thirdly, they underline the variety and strength of the factors that people with established obesity have to struggle against in their attempts to lose weight. Fourthly, they point to opportunities for specific measures that can be taken at an individual or societal level that might help to diminish the driving forces that are promoting weight gain. Fifthly, they point to the need for more research into the nature of these vicious cycles and the effectiveness of strategies to break them. Lastly, all this makes it imperative to focus on prevention, especially in children. This is one train ride they won't want to take.

    Summary points

    Obesity seems to be perpetuated by a series of vicious cycles

    Obesity may result in movement inertia, mechanical dysfunction, psychological dysfunction, cyclical dieting, and socioeconomic disadvantage, all of which may promote further weight gain

    The “obesogenic” environment is also driving the obesity epidemic, whereas the “brakes” against weight gain (social, personal, cognitive, physiological) are weak by comparison

    Management of obesity should include the identification of active vicious cycles and implementation of strategies to break the cycle


    • The authors each have more than 15 years' experience in research and action on obesity at metabolic, clinical, and population levels. BS's obesity research includes metabolic studies at the National Institutes of Health, Phoenix, Arizona; clinical and public health interventions at the University of Auckland; and whole of community obesity prevention at Deakin University. He is a member of the Prevention Group of the International Obesity TaskForce. GE was the founder of GutBusters, a waist loss programme for men which treated thousands of overweight Australian men between 1991 and 2002. He was the lead author of the Australian Physical Activity Guidelines and the National Health and Medical Council's National Clinical Guidelines for Weight Control and Obesity Management.

    • Contributors Both authors contributed to defining the concepts included in the paper and both were involved in writing and editing. BS is guarantor.

    • Competing interests None declared.


    1. 1.
    2. 2.
    3. 3.
    4. 4.
    5. 5.
    6. 6.
    7. 7.
    8. 8.
    9. 9.
    10. 10.
    11. 11.
    12. 12.
    13. 13.
    14. 14.
    15. 15.
    16. 16.
    17. 17.
    18. 18.
    19. 19.
    20. 20.
    21. 21.
    View Abstract

    Log in

    Log in through your institution


    * For online subscription