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Therapeutic strategy for an obese patient
Evidence in bariatric surgery (part 1)
Evidence in bariatric surgery (part 2)
Major options in the treatment of obesity
Frequency, cost and social consequences of obesity
The connexion between overweight and mortality
The benefits of surgery in morbidly obese patients
Causes and mechanisms of obesity
Regulation of the appetite
Causes and mechanisms of obesity
Measuring obesity
Home > Generalities > Causes and mechanisms of obesity

Causes and mechanisms of obesity

Energy output:
The usual cause of obesity is an unbalance between energy intake and output. There are indeed intricate weight-regulating mechanisms that act upon the Body's energy stores. The latter can in effect be 'destored' when the body needs them, thereby compensating for a lack of energy intake compared to one's needs.

The daily energy output is made up of three entities:

>> Basic metabolism: this is the minimal output necessary for the body to remain alive. It depends essentially on the lean mass (which encompasses mostly the muscles) and represents 70% of the total energy output.

>> Physical activity: this of course varies greatly, from the sedentary person to the hard labourer. It accounts on average for 20% of the total energy output. Because of the excess weight an obese person carries around, even when exerting only moderate physical activity he will be using up more energy than someone with normal weight.
It is therefore easy to understand why physical activity alone will not enable an obese person to achieve maximum weight loss. However, if the main element of a correctly balanced diet is an overall decrease in food intake, sport will play a significant role as it will assist in maximising its effects. Sport has moreover been shown to be beneficial in the long term as a means to prevent a relapse.

>> Thermogenesis: this phenomenon results from the metabolism of food - absorption and storage - after meals.
It represents 10% of the total energy output. A certain amount of the calories absorbed during a meal are therefore immediately 'burnt off' by digestion. This explains why jumping a meal does not fit into sensible dieting recommendations.

Taking into account the above facts, and coupling these the notion that moderate but constant overeating can lead to a state of obesity, it is easy to understand the difficulties to obtain good therapeutic results with diets. In particular, it is known that only to maintain their lean body mass - in particular with regard to the muscular component - obese people are dependent on hypercalorific food rations.

The fact that the obese patient depends on a hypocalorific diet to survive is enough to inevitably fall into bad eating habits. An internationally renowned specialist has clearly explained the problem:

The concept of energy balance is easy to understand; it is its quantitative nature that is often disregarded. A typical obese patient will have increased body weight by some 20 kg over 10 years. This means a daily energy excess of 30-40 kcal (0.126-0.168 MJ) per day at the start of the development of obesity, which gradually has to be increased to maintain the enlarged body. A daily energy excess of this magnitude corresponds initially to less than half a sandwich or to the lack of low to moderate intensity exercise (eg, walking or climbing stairs) for half an hour.

(Per Björntorp. Obesity. The Lancet, 1997).

There is a true inequality for weight-gain in normal subjects, which can explain a feeling of unfairness. The famous experiences of Sims, carried on with inmates, are very relevant: some of them had a high-calorie diet (more than 10000 calories per day, whereas the normal intake is an average of 2200 calories). Only a minority of them could gain some weight (more than 6 kg), precisely the ones that had a family background of obesity, which is an acknowledged cause of obesity.
(Sims. Endocrine and Metabolic effects of Experimental Obesity in man, 1973).

Factors that encourage obesity
The above statements clearly show how obesity in itself is a complex phenomenon. Without getting too specific, the main factors that induce obesity can be roughly divided into the following main groups.

>> Factors related to Food.
These intervene at three levels.

- Excess intake (quantity): obviously food intake must vary according to a person's physical activity. A sedentary person will use up much less energy than a labourer. Too much food and not enough physical activity will inevitably lead to a person putting on excess weight.

- Imbalance in intake (quality): without going into too much detail, we know that fat (lipids) plays an important part in creating an imbalance. Excess intake of so-called fast-acting sugars (sweetened drinks, chocolate, etc) is also detrimental. In effect, these sugars are stored as fat and are then difficult to get rid off.

- Eating disorders: these include the absence of regular meal times, a problem characteristic of modern industrialised countries; and an overall poor lifestyle incompatible with regular meals. In a few cases there can be an alteration of the central nervous system regulating the appetite, but this is only true for a small minority of people.

>> Factors related to energy output
This relates to the state of being sedentary in the large sense of the term, and also to the change from an active to a sedentary lifestyle.

>> Genetic factors
There is certainly a genetic predispostition (linked to numerous genes, not just one) which makes an individual, or members of a same family more susceptible to becoming obese within a given environment. We should not, however, conclude that obesity is a genetic illness, except for very rare and specific cases such as the Prader-Willi syndrome which affects adolescents and combines obesity and mental deficiency.

>> Psychologic factors
As described in the chapter on the psychological aspects of obesity, there is no such thing as a typical psychological profile for an obese person. In other words, there is no specific trait of character or anatomy of the psychism that predisposes towards obesity. On the other hand, certain psychological states such as depression or stress can influence weight gain. Matters are further complicated by the fact that weight gain in turn can lead to a number of abnormal psychological traits which could wrongly be interpreted as the cause of obesity.

>> Social and cultural factors
It is wrong to infer that poverty predisposes towards obesity, however an inverse relationship between income and obesity can be observed in all western countries. This can be explained by different eating habits (more fat in the diet for the poorer classes of the population) and unequal rights to healthcare. Hardly one century ago obesity was considered as a sign of wealth and prosperity. Today the tendency is towards the opposite, where the rich can be seen ' taking care of their body '. It is in fact the life styles that have have been adopted today which exert a bad influence on eating habits : global reduction in physical activity due to improved means of transport which lead to people walking less, a reduction of sport activities in schools, an increase in fast-food restaurants and vendors of high calorific sweet drinks and foods, the disappearance of regular eating times, snacks eaten in front of the television, and more recently videos and an increased use of computers for recreation means.

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