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The current theory that dominates medicine, science, media and most peoples opinions today is that overconsumption of calories causes obesity. If you overeat you get fat. Simple physics right? I propose that instead of this old premise, overeating is a consequence of our body putting on fat. This is a subtle but rather major difference. If some other factor is the cause of obesity then it will make no difference how much energy we consume because this is the resulting behaviour, not the cause. I am going to try and show evidence that obesity is caused by a defect in fat metabolism and homeostasis. The human body has many biological controls on how fat is stored and any change in these controls can have large consequences that results in obesity. 

 If overeating was the cause of obesity then by reducing the calories you eat you would lose weight. This of course is obvious to everyone. But what if it is wrong? It is well known, much to the medical communities frustration, that only 1% of diets work. Clearly, reducing calories has not resulted in a reduction in obesity. In fact, in many situations obese people have been shown to consume less or as much calories as their lean counterparts but still remain obese. A reduction in calories does not necessarily mean a reduction in weight. Only on extremely restrictive diets do obese people lose weight but they are found to lose muscle and fat from their organs and often still maintain a very high level of fat in their adipose tissue – they are simply emaciated fat people. 

However, some of the best examples to prove this point is through animal studies, which can directly test whether obesity is caused by eating too many calories. We simply ask ‘when animals become obese do they do this by eating more calories’? The examples I will give involve influences from genetics, neurobiology, hormones and natural circadian ryhthm’s.  

The first example I give looks at animal hibernation, which decouples food intake from weight gain. Hibernating squirrels will double their body weight during late summer in preparation for the hibernation in winter. This is remarkably controlled and unaffected by food intake. Squirrels in the laboratory will gain this weight even if calories are severely restricted. In addition, if the fat stores are surgically removed they will still gain that fat back very quickly despite limits on calories. This seasonal fat deposition is strongly genetically programmed and is robust to changes in food supplies. 

Investigators who have looked at weight regulation and reproduction have also shown this. Female rats who have had their ovaries surgically removed exhibit a reduction in the amount of the female sex hormone estrogen. Without estrogen the rats develop out of control appetites, severely reduce their physical activity, and quickly grow obese. Rats put on weight even if their diets are restricted to what they were eating before the surgery. When estrogen is infused back into the rats they lose the weight and adopt the normal eating and exercise behaviours exhibited prior to surgery.  Thus, calorie intake and physical activity were directly influenced by a change in hormone levels.  

Mice who have had lesions applied to their hypothalamus (a region of the brain that regulates fat metabolism) become severely obese and gain six times as much weight per calorie of food compared with normal mice. These mice became so lethargic that they barely move and develop voracious appetites. Again a change in the biological controls of fat storage results in changes in eating and physical behaviour – not the other way round.  

Genetic studies on obese strains of rats show that individuals that are placed on a restricted diet from birth onward, grow fatter by adulthood than their littermates who were allowed free reign over what they ate. Therefore, calories did not effect the obesity of these rats over their lifetime. Quite the reverse a reduction in calorie consumption seemed to be associated with increased obesity.  In addition, those rats on a restricted diet had 50% less muscle mass than normal rats and 30% less muscle mass than their counterparts who could eat what they wanted. Emaciated fat mice. 

It is clear from these animal studies that the intake of calories has no influence on weight gain. In some cases the animals would severely reduce their physical activity becoming very lethargic. However, it would be difficult to propose that a reduction in physical activity was the CAUSE for these changes but rather an associated behaviour. These studies certainly suggest that the cause of obesity is some error in fat metabolism or homeostasis that leads to increases in fat deposition and changes in behaviour (overeating, lethargy). 

In this final and human example I hope to put an end to the hypothesis that overeating is the cause of obesity. There is a rare condition known as lipodystrophy. In one case in 1913, a ten year old girl first loss fat from her face, then, over the next three years, this emaciation gradually extended down her trunk and arms. Obesity of the lower body began at fifteen and by the time she was 24 she had all the body fat localised in her lower waist. You can see the example below. 

This is clearly not a case of overeating. The most obvious explanation is fat was placed due to some genetic or hormonal condition. In fact, this is seen in the differences between the sexes. Males tend to store fat around the waist while females at the hips and this is large genetically and hormonally controlled. Although these are obvious examples they do beg the question – is fat deposition controlled by our biology and physiology or is it controlled by our behaviour. If it is not controlled by our behaviour then it is difficult to argue that obese humans are that way because they have no willpower, eat too much and do not exercise. Obesity is likely to be a defect in fat metabolism which results in overeating and lethargy. Asking obese people to change their eating patterns or to exercise more is equivalent to telling a seven foot man that they need to stop eating and exercise less in order to become shorter. 

Soon, I will talk about how I think obesity is controlled by our biology, how we get fat DESPITE calorie consumption and what we can do about it.

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