Why Obesity Occurs

Over 30% of Australians are obese¹ and the same level exists across most of the United States and Europe. Understanding the causes of this epidemic will reduce its associated problems of diabetes, hypertension, sleep apnoea, osteoarthritis and cancer.

For millennia humans were hunter gatherers, expending a lot of energy to obtain a meal and living in sustained periods of food shortage. This is the life that the body’s weight control mechanisms have adapted to over a long period of time. In the last 100 years, industrialisation of western countries has occurred leading to a rapid change in the environment in which we live. This new obesogenic environment is driven by access to cheap, highly palatable, calorie dense foods and has pushed society to a more sedentary working life. Although science is still far from understanding how the body controls weight it is obvious that due to genetic diversity some are more susceptible to weight gain and living in the obesogenic environment is far more hazardous to their health.

Multiple models have been proposed to try and explain the body’s weight control systems, the ones most often discussed are the set point and settling point models:

Set point- weight is pre-determined by our genes and any attempt to change weight will result in the body’s weight control mechanisms guiding a person back towards that set point through adjusting energy intake (food) and energy output (metabolic rate and activity levels).

Settling point- weight is determined by the balance of calories in and calories out. Eating less calories means body tissue is lost, as metabolically active tissue decreases, the body’s total energy output also decreases until it matches the low calories going in. When eating more calories, metabolically active body tissue is gained and so overall energy output increases until it matches the high calories going in.

All models to date have weaknesses that do not completely explain our personal experiences of weight control however the fact that the new obesogenic environment overwhelms whatever mechanisms the body has is evident to us all.

Besides genetics, there are other reasons why weight can not be lost with diet and exercise such as joint pain, insulin resistance and medications to name a few. These problems cause a vicious cycle to develop making further weight gain inevitable despite best attempts at a healthy lifestyle. Weight loss surgery breaks this cycle of weight gain causing most of a person’s excess weight to be lost and allowing long term success.

1. Australian Bureau of Statistics. National Health Survey- Overweight and Obesity 2019.