What causes obesity?

Reviewed by Professor Christine Venter

It's not clear what the exact cause of obesity is, but we do know that genetic, environmental, metabolic, cultural, behavioural and psychological factors are at the root.

In fact, obesity has many interrelated causes, with a history of yo-yo dieting (repeated dieting) and a combination of these factors being the most likely culprit.

There are other conditions that may cause weight gain and obesity, such as hypothyroidism, Cushing's syndrome, polycystic ovary syndrome and even certain medications, which can result in weight gain as a side effect.

Genetics 

Obesity is definitely influenced by genetics, although research has not been able to determine the specific metabolic defect in humans. Studies that have included twins, adoptees and families, confirm a strong genetic influence on fatness and the distribution of fat. These genetic factors also interact with environmental influences.

Dietary composition

The nutritional composition of the diet is an important risk factor for overweight and obesity.

· Dietary fat. Studies have shown an association between body fat and the fat content of your diet. It's generally accepted that the high-fat content of the average modern diet results in the consumption of more kilojoules than we realise

· Because high-fat diets are so energy dense (a high kilojoule/energy value per gram), they have an extensive effect on body weight. Fat also satisfies the appetite much less than carbohydrate or protein and is the last fuel in the body to be oxidised for energy. So it's easy to eat too much fat but difficult to burn it off.

· Carbohydrates and sugar. In general, diets high in sugar do not increase body fat any more than diets high in carbohydrates. There is an inverse relationship between energy from carbohydrate and energy from fat in the diet. As the proportion of energy from fat in the diet decreases, the proportion of energy from carbohydrate increases. This is also true for sugar

· Besides the different influence of carbohydrates and fat on appetite and food intake, the body more readily metabolises (burns) carbohydrates and store dietary fat. Body fat stores have the same composition as the fat eaten in the diet, but the energy needed to store dietary fat as body fat only needs about 3% of the kilojoules of those eaten. However, storing carbohydrate as body fat needs about 23% of the kilojoules eaten. This may explain why the body does not usually convert carbohydrates into fat.

Taste preferences

A preference for a sweet taste is instinctive, with sensory preferences being higher in children and declining by adulthood. But it's unlikely that the preference for fat is instinctive, however, several studies show that obese people do have a preference for high-fat foods nonetheless. A preference for both sugar and fat in combination can lead to a considerable proportion of energy being added to the diet, leading to excess energy intake.

Energy intake

Studies have revealed that most obese people tend to under-report the amount they eat. As discussed in last month's newsletter, for weight gain to occur, a persons energy intake must exceed his or her energy expenditure. Energy intake needs to be balanced with energy output.

Physical activity and obesity

Energy intake and fat intake alone cannot explain the epidemic of obesity. In affluent societies, very few people are engaged in physically arduous jobs and most domestic situations are now characterised by labour-saving devices.

Most adults, whether lean or obese, are inactive, while children are less physically active than previously. Adolescents are also showing a concerning trend towards a sedentary lifestyle. Television viewing and the use of cars are also a big part of this move towards an inactive life and with it obesity.

While physical activity does not offer a cure for obesity, a combination of diet and exercise is definitely recommended to help lose body fat while reducing the loss of lean body mass that occurs during dieting. Every effort should be made to increase physical activity because even small changes, such as using the stairs instead of the lift can make a significant difference to energy expenditure.

Besides a possible role in preventing obesity, physical activity has other health benefits:

· It increases levels of "good" cholesterol (HDL cholesterol)

· It improves the body's handling of dietary fat and enhances its ability to use glucose, reducing the risk of developing diabetes

· An adequate level of physical activity throughout life may help to reduce the rate of modern diseases, such as coronary heart disease and certain cancers

· It decreases blood pressure in hypertensive people..

There is no all-purpose diet that can be prescribed for all obese people. The energy required for weight loss is extremely individual and is determined on various factors such as age, gender, level of physical activity, medical conditions, dieting history etc.

The energy content and macronutrient composition of a reducing diet should be calculated by a health professional that is trained in the area of healthy fat loss.

(Reworked from article by Discovery)